International Journal of Aquaculture 2025, Vol.15, No.4 http://www.aquapublisher.com/index.php/ija © 2025 AquaPublisher, registered at the publishing platform that is operated by Sophia Publishing Group, founded in British Columbia of Canada. All Rights Reserved. Publisher
International Journal of Aquaculture 2025, Vol.15, No.4 http://www.aquapublisher.com/index.php/ija © 2025 AquaPublisher, registered at the publishing platform that is operated by Sophia Publishing Group, founded in British Columbia of Canada. All Rights Reserved. Publisher Aqua Publisher Editedby Editorial Team of International Journal of Aquaculture Email: edit@ija.aquapublisher.com Website: http://www.aquapublisher.com/index.php/ija Address: 11388 Stevenston Hwy, PO Box 96016, Richmond, V7A 5J5, British Columbia Canada International Journal of Aquaculture (ISSN 1927-5773) is an open access, peer reviewed journal published online by AquaPublisher. The journal publishes all the latest and outstanding research articles, letters and reviews in all working and studying within varied areas of aquaculture, containing the latest developments and techniques for practice in aquaculture; information about the entire area of applied aquaculture, including breeding and genetics, physiology, aquaculture-environment, hatchery design and management, utilization of primary and secondary resources in aquaculture, production and harvest, the biology and culture of aquaculturally important and emerging species. All the articles published in International Journal of Aquaculture are Open Access, and are distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. AquaPublisher uses CrossCheck service to identify academic plagiarism through the world’s leading plagiarism prevention tool, iParadigms, and to protect the original authors' copyrights. Aqua Publisher is an international Open Access publisher specializing in the field of marine science and aquaculture registered at the publishing platform that is operated by Sophia Publishing Group (SPG), founded in British Columbia of Canada.
International Journal of Aquaculture (online), 2025, Vol. 15, No. 4 ISSN 1927-6648 http://aquapublisher.com/index.php/ija © 2025 AquaPublisher, registered at the publishing platform that is operated by Sophia Publishing Group, founded in British Columbia of Canada. All Rights Reserved. Latest Content Mechanisms of Algal Toxin Production: From Genes to Environmental Triggers Bing Wang, Qikun Huang International Journal of Aquaculture, 2025, Vol. 15, No. 4, 165-174 Habitat Degradation and Restoration in Aquatic Ecosystems: Implications for Fish Populations Wenzhong Huang 1, Yeping Han 2 International Journal of Aquaculture, 2025, Vol. 15, No. 4, 175-183 Molecular Basis and Regulatory Network of Sex Determination in Groupers Liting Wang, Manman Li International Journal of Aquaculture, 2025, Vol. 15, No. 4, 184-196 Epigenetic Regulation of Growth and Stress Response in Oysters Guilin Wang, Rudi Mai International Journal of Aquaculture, 2025, Vol. 15, No. 4, 197-207 Advancements in Cultivation and Post-Harvest Handling of Eleocharis dulcis Yue Zhu, Jinni Wu International Journal of Aquaculture, 2025, Vol. 15, No. 4, 208-220
International Journal of Aquaculture, 2025, Vol.15, No.4, 165-174 http://www.aquapublisher.com/index.php/ija 165 Research Insight Open Access Mechanisms of Algal Toxin Production: From Genes to Environmental Triggers Bing Wang, Qikun Huang Tropical Microbial Resources Research Center, Cuixi Academy of Biotechnology, Zhuji, 311800, Zhejiang, China Corresponding author: qikun.huang@cuixi.org International Journal of Aquaculture, 2025, Vol.15, No.4 doi: 10.5376/ija.2025.15.0016 Received: 12 May, 2025 Accepted: 25 Jun., 2025 Published: 10 Jul., 2025 Copyright © 2025 Wang and Huang, This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Preferred citation for this article: Wang B., and Huang Q.K., 2025, Mechanisms of algal toxin production: from genes to environmental triggers, International Journal of Aquaculture, 15(4): 165-174 (doi: 10.5376/ija.2025.15.0016) Abstract Algae toxins are toxic secondary metabolites produced by algae during water blooming, with a variety of chemical structure types and toxicological effects. With the eutrophication of water bodies and the intensification of climate change, harmful algae blooms in freshwater and marine ecosystems around the world occur frequently and releases a large number of toxins, posing a serious threat to ecosystems and human health. In order to reveal the internal mechanism of algatoxin production, researchers started from genetic and environmental factors and carried out multiomic studies such as genome, transcriptome, metabolomic and ecological experiments. This study reviews the classification, chemical structure and mechanism of action of common algae toxins, explores the ecological function of toxins in algae and the food chain migration process; summarizes the discovery and characteristics of genes and gene clusters related to algae toxin biosynthesis; analyzes the impact of environmental factors such as nutrients, light, temperature, hydrodynamics and biological interactions on the production of toxins, as well as the latest progress in toxin monitoring and prediction based on molecular markers, remote sensing and big data. Finally, a prospect is proposed for the insufficient progress of the current research, in order to provide a theoretical basis for algatoxin risk assessment, ecological management and prevention and control strategies. Keywords Algae toxins; Gene clusters; Molecular regulation; Environmental factors; Monitoring and prediction 1 Introduction Algae toxins are toxic secondary metabolites produced by toxin-producing algae (mainly cyanobacteria, dinoflagellate, diatoms, etc.) in water. They can be divided into several major categories such as hepatic toxins (such as microcystins), neurotoxins (such as paralytic shellfish toxin Saxitoxin, anatoxin, amnesia shellfish toxin Domoic acid), and diarrheal toxins (such as oxalic acid, Okadaic acid) and Okadaic toxins. These toxins have complex molecular structures and diverse toxicological mechanisms (such as microcystis toxins can inhibit protein phosphatases, while marine toxins often act on ion channels) (Zhou et al., 2021; Thomas et al., 2024). Harmful Algal Blooms (HABs) are a rapid proliferation phenomenon of algae populations. They often break out under eutrophication conditions. They are accompanied by the release of a large number of algal toxins, resulting in hypoxia, reduced transparency in water, and poisoning events in aquatic organisms and humans through the food chain (Danil et al., 2021). For example, HABs can cause the water to reduce dissolved oxygen, hinder plant photosynthesis, and cause a large number of deaths such as fish and shellfish. At the same time, toxins are enriched through food webs, posing a serious threat to the safety of human drinking water and seafood. According to statistics, there are many food poisoning incidents caused by algatoxins every year around the world, which poses an ongoing challenge to public health. Therefore, in-depth research on the production mechanism of algae toxins, from gene regulation to environmental triggers, is of great significance to understanding the formation rules of harmful algae blooms and formulating pollution control strategies. This study analyzes the ecological function of toxins in algae and the food chain migration process; introduces the impact of horizontal transfer such as transposons on the diffusion of toxin production capacity; elaborates on the molecular regulatory mechanisms at the level of gene regulation and epimodal modification, as well as major signaling pathways and regulatory factors; combines the latest progress in toxin monitoring and prediction of related technologies, aiming to improve the risk assessment and ecological management capabilities of algatoxins.
International Journal of Aquaculture, 2025, Vol.15, No.4, 165-174 http://www.aquapublisher.com/index.php/ija 166 2 Types and Biological Functions of Algae Toxins 2.1 Chemical structure and mechanism of common algae toxins There are many types of typical algae toxins, and there are obvious differences in their chemical structure and toxicological mechanism. For example, the microcystis toxins (MCs) produced by cyanobacteria are cyclic peptides, catalyzed by non-ribosomal peptide synthetase (NRPS) and polyketone synthetase (PKS). They are generally circulating peptides composed of 7 amino acids, which have a strong inhibitory effect on protein phosphatases 1 and 2A, resulting in increased risk of hepatocyte damage and liver cancer (Shishido et al., 2013). Neurotoxins include Anatoxin-a and paralytic shellfish toxin (PSP, mainly represented by Saxitoxin, STX), etc. They are mostly small-molecular alkaloids that cause neurotoxicity by blocking sodium channels or affecting the release of neurotransmitters. Amnesic shellfish toxins (polycarboxylic acids with glycol structure) produced by seaweed can overactivate glutamate receptors, leading to neuroexcitation toxicity (Maguire et al., 2018); Okada toxins and other diarrheal toxins (such as OA) are polyethers or fatty acid derivatives that cause diarrhea and cell damage by inhibiting protein phosphatase. 2.2 The ecological function of toxins in algae: defense and competition In addition to the occasional metabolites produced by algae, toxins often have important ecological significance. Existing research suggests that algatoxins may be used as a chemical defense substance for algae to inhibit competitive algae or to repel herbivorous organisms, thereby improving the competitiveness of toxin-producing populations. For example, some cyanobacteria secrete microcystis toxins or verbena toxins under nutritional deficiency or other stress conditions to affect the growth of coexisting populations; the production of anaphylactic toxins and paralytic shell toxins is also believed to reduce biological predation or competitors (Figure 1) (Teneva et al., 2023). In addition, toxins may also be related to stress tolerance in algae: studies have found that microcystis toxin plays a role in antioxidant stress and can help algae resist highlights or heavy metal stress; similarly, some dinoflagellate toxins are reported to be associated with population survival strategies under low temperature or low nutritional conditions. Algatoxins may play the role of signaling molecules, allelopathic substances or stress defense agents in the algae population ecology, so that toxin-producing algae have an advantage in resource competition and environmental stress. In addition, the accumulation of toxins through the food chain can weaken the growth and reproduction of algae predators or higher consumers, and also have an indirect effect on maintaining the stability of toxin-producing algae populations (Li, 2014). 2.3 Accumulation and transmission of toxins in the food chain The transmission characteristics of algatoxins in the food chain are an important manifestation of their ecological harm. Toxins produced by algae can be enriched biologically through feeding of benthic organisms such as zooplankton and shellfish, and are further transmitted to fish, birds and even humans along the food chain. Studies have shown that microcystis and other algatoxins are often detected in fish and shellfish. After being ingested, they can "amplify" the toxicity in the food chain. For example, after eating toxic shellfish, top predators or humans can experience symptoms of food poisoning (Kershaw et al., 2021). At the same time, atmospheric aerosols are also a way to spread toxins. Tide explosions and waves can spray toxins into the air, and it will also cause poisoning after being inhaled by the respiratory tract. Therefore, algatoxins not only threaten native plankton and benthic invertebrates, but also affect higher trophic levels through the feeding chain, becoming the focus of attention of the entire ecosystem and even public health. 3 Genes and Gene Clusters Related to Algae Toxin Synthesis 3.1 Discovery and identification methods of genes related to toxin synthesis With the development of molecular biology technology, toxin biosynthesis genes of toxin-producing algae are constantly being discovered. Traditional methods include designing primers using conserved sequences of known toxin synthase genes and detecting gene presence in unknown strains or environmental samples by PCR. For example, amplification of the microcystis toxin synthetase mcyE gene, the anaC gene of anaC gene and the paralytic castis toxin synthetase sxtA gene by conventional PCR can effectively detect and distinguish different types of toxin cyanobacteria (Ribeiro et al., 2020; Moraes et al., 2023). In recent years, high-throughput
International Journal of Aquaculture, 2025, Vol.15, No.4, 165-174 http://www.aquapublisher.com/index.php/ija 167 sequencing and transcriptome analysis have played an important role in the discovery of toxin genes. American scientists analyzed marine diatom toxicity-producing gene clusters (such as the dodolalic acid synthesis gene cluster dabA-dabD of pseudo-rhombus algae) through comparative transcriptome analysis; similarly, genome sequencing revealed the composition of multiple toxin synthesis gene clusters in dinoflagellate and cyanobacteria (Lorenzi et al., 2019). In addition, metabolomics and isotope labeling techniques are also used to correlate specific metabolites with candidate genes, providing auxiliary evidence for the analysis of toxin synthesis pathways. Figure 1 Morphology of Chlamydomonas asymmetrica (A), Dunaliella salina (B), and Scenedesmus obtusiusculus (C), after exposure to 1 µg/mL MC-LR and CYL for 15, 24, 48, and 72 h. Control: non-treated cell cultures. Bar = 10 µm (Adopted from Teneva et al., 2023) 3.2 Structural characteristics of key toxin synthesis gene clusters Toxin synthesis genes in toxin-producing algae usually exist in clusters, encoding enzymes required for the complete biosynthetic pathway. Taking the cyanobacteria microcystis toxin as an example, the mcy gene cluster of M. aeruginosa PCC7806 spans about 55 kb, including a total of 10 open reading frames of mcyA-mcyJ, encoding biosynthetic enzymes such as multimodal polyketone synthetase (PKS) and non-ribosomal peptide synthetase (NRPS) (Rhee et al., 2012). The cluster is closely arranged in two directions of operon structures transcribed, providing a complete enzyme system for the synthesis of a complex peptide loop. Similarly, the gene cluster (ndaA–ndaF) of the cyclogenic toxin Nodularin is highly homologous to the mcy cluster and shares some of the core enzymes. The synthesized gene cluster of dinoflagellate paralytic cadoxin (PSTs) contains more than 20 genes, encoding enzymes such as PKS and aminotransferases required to synthesize STX; the oxalic acid
International Journal of Aquaculture, 2025, Vol.15, No.4, 165-174 http://www.aquapublisher.com/index.php/ija 168 (DabA-dabD) gene cluster was found in some marine diatom genomes, indicating that non-cyanobacteria can also perform complex toxin biosynthesis. The common characteristics of these gene clusters are: the gene arrangement is compact, mostly in tandem structures, usually spanning tens of kb or even hundreds of kb intervals, the core encodes a complex biosynthetic enzyme, and there are often modified enzymes or regulatory elements on the side. This structural characteristic ensures efficient coordination of the toxin biosynthesis process and provides a basis for engineering replication and functional research. 3.3 Gene horizontal transfer and diffusion of toxin synthesis ability Studies have shown that toxin synthesis gene clusters have strong mobility and mostly transfer horizontally between different species or strains. Genomic analysis found that transposase or integrase genes associated with multiple toxin-producing gene clusters existed next to each other. For example, mcy, NDA (cyclic cytoxin) and SXT (numbing) gene clusters can all detect relevant sequences on certain plasmids or transposal elements, suggesting that these large gene clusters may mediate transmission in populations through transposons (Popin et al., 2021). This horizontal transfer phenomenon allows relative populations that do not have the ability to produce toxins to change into toxic types by obtaining gene clusters, causing the rapid spread of toxin production capacity. In recent years, there has also been evidence that genomic rearrangements and gene deletions/repetitions of gene clusters are one of the sources of strain diversity. These studies show that toxin gene clusters are not rigid and unchanged, but are constantly spread and optimized through horizontal gene transfer and structural remodeling during evolution, so that the toxin synthesis capacity can be dynamically distributed in algae populations (Chen et al., 2024). 4 Molecular Regulation Mechanisms of Toxin Production 4.1 Transcriptional regulatory factors and signal transduction pathways The expression of toxin biosynthetic gene clusters is regulated by a variety of transcription factors and signaling pathways. Taking the cyanobacterium microcystis toxin as an example, it is known that the nitrogen fixation regulator NtcA can directly bind to the promoter region of the mcy gene cluster to couple toxin synthesis with nitrogen metabolism. In addition, global regulators Fur (hepcidin) and Sigma factors may also regulate mcy cluster transcription by sensing metal or light changes. In dinoflagellate, although the specific transcription factor recognition mechanism is not clear, studies suggest that signals such as cell cycle, light intensity and nitrogen and phosphorus nutrient status can affect the expression of toxin genes. In terms of signaling, phosphorylation cascades, second messengers (such as circular AMPs), etc. may be involved in the interaction of nuclear factors and toxin genes (Zhu et al., 2016). Overall, the transcriptional regulatory network is complex and multi-level, involving environmental signal perception and fine regulation of transduction into target gene promoters, but the detailed mechanism is still being explored. 4.2 Effects of epigenetic modification on toxin synthesis In addition to direct regulation of transcription factors, epigenetic mechanisms (such as DNA methylation, histone modification) may also participate in the regulation of toxin synthesis. In recent years, some scholars have explored the methylation pattern of the Microcystis genome through single-molecule real-time (SMRT) sequencing and other methods, and found that some regulatory genes have different methylation levels in the poison-producing strains, suggesting that they may affect gene expression activity (Stern et al., 2024). However, there are currently few reports on epigenetic regulation of algae toxin synthesis gene clusters. In the future, chromatin immunoprecipitation sequencing (ChIP-seq) and genome-wide methylomics can be used to evaluate the regulatory effect of methylation, acetylation and other modifications on mcy, sxt, and Ana cluster promoters to reveal the role of epigenetics in the regulation of toxin production (Popin et al., 2021). 4.3 Integration analysis of metabolic networks related to toxin synthesis Toxin synthesis requires a large amount of precursors and energy, so its yield is often associated with the overall metabolic state of algae. Multiomics data integration analysis has been used to study the interconnection of toxin synthesis and other metabolic pathways. Through joint metabolomic-tratome analysis, it was found that conditions
International Journal of Aquaculture, 2025, Vol.15, No.4, 165-174 http://www.aquapublisher.com/index.php/ija 169 rich in carbon sources are often conducive to the synthesis of carbon-rich toxins such as microcysticus toxins (Zhang et al., 2024); the nitrogen metabolism pathway is directly related to the supply of precursors for toxin production. When nutrients are unbalanced, excess energy and carbon sources may turn to toxin synthesis to maintain cellular homeostasis. Overall metabolic network analysis also reveals the cross-regulation of amino acid, lipid metabolism and toxin biosynthesis. Combining the metabolic pathway model helps to understand how algae balance growth and toxin production in resource allocation, providing a basis for dynamic regulatory models (Rawls et al., 2019). 4.4 Case analysis: case study on the regulation of dinoflagellate toxins by nitrite signal Some studies have focused on the effects of specific signaling molecules on toxin production, such as the role of nitrite signals in the inorganic nitrogen form in dinoflagellate toxin synthesis. A study reported that supplementing different concentrations of nitrite in certain dinoflagellates with paralytic shellfish production changes the expression of the SXT gene cluster and toxin yield. This implies that different nitrogen sources or intermediate products act as signaling molecules to regulate toxin synthesis through nitrogen metabolism-related regulatory factors (Abassi et al., 2023). However, there are still few cases in this area, and more experiments are needed to verify them in combination with gene expression analysis. For example, using transcriptome technology to compare gene expression profiles before and after nitrite treatment can clarify changes in relevant signaling pathways and transcription factors such as NtcA or GlnB proteins, thereby revealing how nitrite affects the transcription of toxin synthesis gene clusters. 5 Effects of Environmental Inducers on Algae Toxin Production 5.1 Changes in nutrient concentration and proportion Nutrients (nitrogen, phosphorus) are key environmental factors that affect algae growth and toxin production. Studies have shown that under conditions of adequate and balanced nutrients, toxin-producing algae generally prefers rapid growth rather than large accumulation of toxins; while when nutrients are unbalanced (especially N or P-limited), algae tend to increase toxin synthesis as a coping strategy. Zeng Ling et al. (2018) pointed out that under low nitrogen or low phosphorus conditions, the individual toxin content of dinoflagellate Prorocentrum lima is significantly increased, and the effect of phosphorus restriction on toxin accumulation is often greater than that of nitrogen restriction (Figure 2) (Wan et al., 2023). This phenomenon may be because growth is inhibited when nutrients are restricted and excess carbon resources are redistributed for toxin synthesis. Furthermore, for nitrogen-rich toxins (such as microcystistoxins), their synthesis is inhibited when the nitrogen supply is insufficient, while it is relatively promoted when phosphorus is limited (Brandenburg et al., 2020). Overall, changes in nutrient concentration and N: P ratio significantly regulate algatoxin yield and type by affecting algae metabolism and energy distribution. 5.2 Light, temperature and hydrodynamic conditions Light intensity and light cycle are important factors that affect the growth of photosynthetic toxin-producing algae. Generally speaking, moderate increase in light can enhance the photosynthesis and metabolic activity of the algae, thereby increasing the rate of toxin synthesis; but excessive or violent fluctuations may also lead to light inhibition and reduce toxin production. Temperature also significantly affects the production of algatoxins: each toxin-producing algae has its optimal toxin-producing temperature range, and toxin synthesis decreases when it is out of range. Warm and warm conditions often promote the reproduction and accumulation of toxins of cyanobacteria and dinoflagellate, which is also the main reason for the frequent occurrence of algae blooms and the increase in toxin levels in summer. In terms of hydrodynamic conditions, steady water bodies often promote the accumulation of phytoplankton algae, while strong mixing or disturbance can disintegrate the algae population and release intracellular toxins. But mixing can also disperse algae to different depths of the water column to change the light environment, thereby indirectly affecting toxin synthesis (Pavlidou et al., 2020). For example, circulating water sometimes produces high concentrations of dissolved toxins at night. Therefore, physical factors such as light, temperature and water fluidity comprehensively regulate the physiological state and nutritional acquisition of algae, thereby affecting the intensity and timing of toxin production.
International Journal of Aquaculture, 2025, Vol.15, No.4, 165-174 http://www.aquapublisher.com/index.php/ija 170 Figure 2 Light microscopy (LM) and confocal microscopy (CLSM) images of P. lima cells in P-limited conditions and a control. Representative confocal microscope images of P. lima cells showing oil bodies with green fluorescence are displayed (Adopted from Wan et al., 2023) 5.3 Microbial community and symbiosis/competitive relationship The interaction between algae and symbiotic or competitive microorganisms is also an important factor in affecting toxin production. Certain concomitant bacteria promote algae growth and toxin synthesis, such as indirectly affecting toxin levels by providing vitamins or breaking down nutrients. Conversely, inhibitory or competitive microbial communities may reduce the growth rate of toxin production by producing antagonistic substances or competing nutrients. In natural water bodies, pathogenic microorganisms or special bacteria can also degrade toxins and alleviate toxin accumulation (Zeng et al., 2020). Clusters of bacterial genes that have the ability to degrade microcystis toxins (such as mlrA-D) have been found to break the toxin into a nontoxic component. Therefore, water body microbial community structure, bacterial and algae symbiosis and microbial competition may all significantly regulate the production and decomposition process of algatoxins. 5.4 Case analysis: research on the relationship between nutrient salt and toxin level in freshwater lakes In many freshwater nutritious lakes (such as Taihu Lake in China, Dianchi Lake, etc.), research has found that nutrient load is highly correlated with microcystis toxin content. Specifically, the input of excess nitrogen and phosphorus nutrients promotes the outbreak of microcysticus, but long-term high nitrogen often makes phosphorus in water a limiting factor, and the concentration of cytotoxin tends to increase when phosphorus is relatively scarce. Monitoring data show that in these lakes, when the temperature rises in spring and summer accompanied by the peak of nitrogen and phosphorus input, the microcystis community proliferates rapidly and produces a large number of MCs; after the nutrients are consumed in autumn, the MCs released by the rupture of the algae maintain a high level of water toxicity (Schampera and Hellweger, 2024). Therefore, lake nutrition regulation strategies (such as reducing exogenous nitrogen and phosphorus load) are crucial to control algatoxin levels, and fine adjustments to nutrient structure will also affect the final toxin output (Lawson and Young, 2025). These research cases highlight the key role of nutrition management in ecological restoration and toxin prevention.
International Journal of Aquaculture, 2025, Vol.15, No.4, 165-174 http://www.aquapublisher.com/index.php/ija 171 6 Multi-Level Regulatory Model For Toxin Production 6.1 Gene-environmental interaction model Toxin production is the result of the interaction between genes' intrinsic abilities and external environmental conditions. Existing research proposes a gene-environment interaction regulation model, which believes that the expression of gene clusters is driven by environmental signal input, and gene products (toxins) may also feedback to regulate growth and metabolism. The microcystis toxin mcy gene cluster is affected by the nitrogen regulator NtcA, which reflects the genome's response to exogenous nutritional status; at the same time, MCs are believed to be able to affect the iron metabolism and signaling pathways of microcystis itself (indirectly regulate gene expression), forming a positive and negative feedback circle (Wei et al., 2024). When external conditions change (nutrition, temperature, light intensity, etc.), the gene expression model is quickly adjusted to adapt to the new environment. This type of interaction model emphasizes that simple gene cluster annotation is not enough to predict toxin yield, and environmental variables need to be integrated into the model to more accurately describe the space-time dynamics of toxins. 6.2 Dynamic regulation of toxin synthesis and metabolic energy distribution In algae cells, toxin biosynthesis requires a large amount of energy and precursor substances, so the dynamic regulation of toxin synthesis is closely related to the distribution of cell metabolic energy. Research shows that when cell growth rate slows down (such as entering a stable period), the immediate yield of toxins can remain unchanged or relatively increased, resulting in an increase in cell toxicity (Salvador et al., 2016). This is because during the growth restricted phase, excess energy is redistributed to toxin biosynthesis. On the contrary, during the rapid growth period, energy is mainly used for cell reproduction, and the amount of newly synthesized toxins is low. This energy distribution model illustrates the relationship between growth kinetics and toxin concentration, and also provides a theoretical basis for understanding the laws of toxin accumulation at different growth stages. A dynamic metabolic model constructed incorporating multiomics data can be used to simulate how algae regulate energy flow to balance growth and toxin production under different environmental conditions. 6.3 Comprehensive regulatory network revealed by multiomics data In recent years, the joint analysis of multipleomics (genome, transcriptome, proteome, metabolomic, epigenetic group, etc.) has made breakthroughs in algatoxin research. By integrating these data, a complete toxin synthesis regulatory network map can be drawn. Single-cell sequencing technology can identify gene modules with the greatest differences in expression between toxic and avirulent strains; proteome-metabolomic analysis reveals changes in toxin precursor supply and transporters; epilogue data supplement the modification information at the transcriptional regulation level. Combining these high-dimensional data can establish a hierarchical network model from gene-transcription-protein-metabolism, identify key regulatory nodes and pathways, and provide multi-scale explanations for predicting toxin production. This type of comprehensive regulatory network research is becoming a hot topic in the future, and it is expected to describe the full dynamics of toxin production at the single cell level (Erwin et al., 2023; Li et al., 2024). 7 Technological Progress in Monitoring and Predicting Algae Toxin Production 7.1 Real-time monitoring method based on molecular markers With the analysis of toxin synthesis gene clusters, real-time monitoring based on gene markers has become an effective means to quickly detect the poison-producing populations in algae flowers. Traditional ecological monitoring methods are difficult to quickly distinguish between toxin-producing and non-toxin-producing algae strains, while molecular technologies such as PCR and qPCR can detect the abundance of toxin synthetic genes (such as mcyE, anaC, sxtA, etc.) to evaluate the potential toxicity risks in real time. In addition, high-throughput sequencing technologies (such as 18S/16S sequencing or functional gene-based metagenomics) can quickly obtain microbial community structure and toxin gene diversity in water bodies, providing a basis for early warning. In recent years, environmental DNA (eDNA) detection methods have also been developed, which can directly amplify and quantify the scattered DNA in water samples without distinguishing between cells or DNA regions,
International Journal of Aquaculture, 2025, Vol.15, No.4, 165-174 http://www.aquapublisher.com/index.php/ija 172 so as to achieve sensitive monitoring of pollutants (Liu and Han, 2025). These molecular marker methods have high sensitivity, high specificity and automation characteristics, and are suitable for rapid early warning of algatoxin events. 7.2 Remote sensing and big data-driven toxin prediction model Satellite remote sensing technology can indirectly monitor the occurrence of algae blooms and estimate the concentration of chlorophyll by detecting changes in the water color spectrum, but it is still difficult to judge the specific toxin type. In recent years, methods combining machine learning and big data analysis have been used to construct toxin prediction models. Some studies use multi-source data (meteorology, nutrients, historical algae bloom event records, water quality parameters, etc.) to predict potential algae bloom outbreaks and toxin concentration trends. Integrated models such as deep neural networks and random forests have made progress in experiments, which can screen out the key environmental factors driving toxin accumulation and make dynamic predictions (a study combined with hydrodynamic model and pollution source distribution to achieve real-time forecasts of exogenous nitrogen and phosphorus input in lakes and microcystic toxin concentrations). Looking ahead, combining remote sensing high-frequency monitoring with machine learning models can achieve more timely and accurate toxin risk warnings and provide support for ecological management of large-scale waters such as reservoirs and lakes. 7.3 New high sensitivity analysis technology for toxin detection At the laboratory analysis level, with the advancement of instrument technology, algatoxin detection has entered the era of high sensitivity. Liquid chromatography-mass spectrometry (LC-MS/MS) technology can quantify multiple algatoxin isomers simultaneously, with sensitivity reaching the nanogram level, and has become the current detection standard; immunoassay methods (ELISA, colloidal gold) are widely used for rapid on-site screening due to their simple operation. In addition, new nanosensors and molecular probes are also under development, enabling direct dilution-free measurement and low-cost detection. More and more research focuses on real-time detection devices in the field, such as multi-functional instruments based on microfluidic chips, or portable devices that can be read through mobile phones, which will provide a more convenient means to respond to algatoxin events in a timely manner. 8 Conclusions and Prospects At present, significant progress has been made in the research on the mechanism of algae toxin production both at the genetic level and in environmental response. Researchers have identified a variety of toxin-synthesis gene clusters and their core regulatory factors, such as NtcA-mediated nitrogen response pathways; combined with molecular and ecological evidence, a comprehensive regulatory network of gene-metabolism-environment has been gradually constructed. However, there are also shortcomings in the research: many key regulatory elements (such as feedback regulation of nonstructural proteins and metabolic intermediates) have not been clarified; the role of epigenetic regulation and genomic plasticity in toxin production needs to be further revealed; the mechanism of environmental factors affecting each factor is complex, and the interaction effects between factors still lack quantitative description. In addition, most current monitoring and prediction methods rely on empirical models, and prediction accuracy and timeliness need to be improved. The study of algatoxin production mechanism has important implications for ecological management. Revealing the association between toxin synthesis and environmental stress and nutritional fluctuations can provide early warning indicators for algae burn prevention and control, such as reducing toxin risk by controlling nitrogen and phosphorus input structure. The improvement of the genetic marker monitoring system will make early warning more targeted. The focus of future research includes: applying gene editing and synthetic biology tools to verify the functions of key regulatory factors; developing multi-scale and multi-parameter dynamic models to integrate gene information and environmental variables; deepening the application of multiomics in ecological niche and evolutionary perspectives, and understanding the origin and adaptability of toxin gene clusters. In addition, a comprehensive
International Journal of Aquaculture, 2025, Vol.15, No.4, 165-174 http://www.aquapublisher.com/index.php/ija 173 toxin monitoring and early warning platform combining machine learning and Internet of Things technology will also become a research hotspot. Through the above efforts, it is expected to achieve a full-chain management strategy from basic research to application level, providing reliable support for responding to the increasingly frequent harmful algae blooms and toxin hazards under the background of global climate change. Acknowledgements Thank you to all reviewers for their meticulous review, and also thank the members of the research team and technicians for their support in experimental design and data analysis. Conflict of Interest Disclosure The authors confirm that the study was conducted without any commercial or financial relationships and could be interpreted as a potential conflict of interest. References Abassi S., Kim H., Bui Q., and Ki J., 2023, Effects of nitrate on the saxitoxins biosynthesis revealed by sxt genes in the toxic dinoflagellate Alexandrium pacificum(group IV), Harmful Algae, 127: 102473. https://doi.org/10.1016/j.hal.2023.102473 Brandenburg K., Siebers L., Keuskamp J., Jephcott T., and Van De Waal D.B., 2020, Effects of nutrient limitation on the synthesis of N-rich phytoplankton toxins: a meta-analysis, Toxins, 12(4): 221. https://doi.org/10.3390/toxins12040221 Chen Y., Jiang Y., He Z., Gao J., Li R., and Yu G., 2024, First report of PST-producing Microseira wollei from China reveals its novel toxin profile, Harmful Algae, 137: 102655. https://doi.org/10.1016/j.hal.2024.102655 Danil K., Berman M., Frame E., Preti A., Fire S., Leighfield T., Carretta J., Carter M., and Lefebvre K., 2021, Marine algal toxins and their vectors in southern California cetaceans, Harmful Algae, 103: 102000. https://doi.org/10.1016/J.HAL.2021.102000 Erwin S., Fletcher J.R., Sweeney D.C., Theriot C., and Lanzas C., 2023, Distilling mechanistic models from multi-omics data, BioRxiv, 2023: 09. https://doi.org/10.1101/2023.09.06.556597 Huang W.Z., 2024, Phylogenetic insights into cassava’s domestication: unraveling genetic origins and evolutionary trajectories, International Journal of Molecular Evolution and Biodiversity, 14(3): 120-132. https://doi.org/10.5376/ijmeb.2024.14.0015 Kershaw J.L., Jensen S.K., McConnell B., Fraser S., Cummings C., Lacaze J., Hermann G., Bresnan E., Dean K., Turner A., Davidson K., and Hall A., 2021, Toxins from harmful algae in fish from Scottish coastal waters, Harmful Algae, 105: 102068. https://doi.org/10.1016/j.hal.2021.102068 Lawson G., Young J., Aanderud Z., Jones E., Bratsman S., Daniels J., Malmfeldt M., Baker M., Abbott B., Daly S., Paerl H., Carling G., Brown B., Lee R., and Wood R., 2025, Nutrient limitation and seasonality associated with phytoplankton communities and cyanotoxin production in a large hypereutrophic lake, Harmful Algae, 143: 102809. https://doi.org/10.1016/j.hal.2025.102809 Li Y., Yang H., Fu B., Kaneko G., Li H., Tian J., Wang G., Wei M., Xie J., and Yu E., 2024, Integration of multi-omics histological and biochemical analysis reveals the toxic responses of Nile Tilapia liver to chronic microcystin-LR Exposure, Toxins, 16(3): 149. https://doi.org/10.3390/toxins16030149 Liu Z., and Han Y.P., 2025, Phylogenetic relationships and evolutionary history of major algal lineages: a comprehensive review, International Journal of Marine Science, 15(2): 107-117. https://doi.org/10.5376/ijms.2025.15.0010 Lorenzi A., Chia M., Lopes F., Silva G., Edwards R., and Bittencourt-Oliveira M., 2019, Cyanobacterial biodiversity of semiarid public drinking water supply reservoirs assessed via next-generation DNA sequencing technology, Journal of Microbiology, 57: 450-460. https://doi.org/10.1007/s12275-019-8349-7 Maguire I., Fitzgerald J., Heery B., Nwankire C., O’Kennedy R., Ducrée J., and Regan F., 2018, Novel microfluidic analytical sensing platform for the simultaneous detection of three algal toxins in water, ACS Omega, 3: 6624-6634. https://doi.org/10.1021/acsomega.8b00240 Moraes M.A.B., De Abreu R.M., Podduturi R.O.G., Jørgensen N., and Calijuri M., 2023, Prediction of cyanotoxin episodes in freshwater: a case study on microcystin and saxitoxin in the lobo reservoir são paulo state brazil, Environments, 10(8): 143. https://doi.org/10.3390/environments10080143 Popin R., Alvarenga D., Castelo-Branco R., Fewer D., and Sivonen K., 2021, Mining of cyanobacterial genomes indicates natural product biosynthetic gene clusters located in conjugative plasmids, Frontiers in Microbiology, 12: 684565. https://doi.org/10.3389/fmicb.2021.684565
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International Journal of Aquaculture, 2025, Vol.15, No.4, 175-183 http://www.aquapublisher.com/index.php/ija 175 Review and Progress Open Access Habitat Degradation and Restoration in Aquatic Ecosystems: Implications for Fish Populations Wenzhong Huang1, Yeping Han2 1 Biomass Research Center, Hainan Institute of Tropical Agricultural Resouces, Sanya, 572025, Hainan, China 2 Institute of Life Science, Jiyang College of Zhejiang A&F University, Zhuji, 311800, Zhejiang, China Corresponding author: yeping.han@jicat.org International Journal of Aquaculture, 2025, Vol.15, No.4 doi: 10.5376/ija.2025.15.0017 Received: 20 May, 2025 Accepted: 29 Jun., 2025 Published: 24 Jul., 2025 Copyright © 2025 Huang and Han, This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Preferred citation for this article: Huang W.Z., and Han Y.P., 2025, Habitat degradation and restoration in aquatic ecosystems: implications for fish populations, International Journal of Aquaculture, 15(4): 175-183 (doi: 10.5376/ija.2025.15.0017) Abstract Aquatic ecosystems provide rich biodiversity and ecological service functions, but also face serious threats of degradation. This study reviews the main habitat types and characteristics of aquatic ecosystems, analyzes the main causes of habitat degradation, such as water conservancy engineering construction, pollution eutrophication, and overfishing, and discusses the impact of habitat degradation on fish population diversity, quantity and life cycle. At the same time, this study reviews a series of aquatic habitat restoration methods such as water conservancy engineering transformation, ecological restoration and ecological management, and discusses the positive role of these recovery measures on the restoration of fish population diversity, resource volume and ecological function. Based on the current research status and future trends, management suggestions such as strengthening comprehensive watershed management, promoting technological innovation and international cooperation were put forward, aiming to provide theoretical support and practical guidance for aquatic ecosystem restoration and fish protection. Keywords Aquatic ecosystems; Habitat degradation; Ecological restoration; Fish populations; Biodiversity 1 Introduction Aquatic ecosystems include various water ecological types such as oceans, lakes, rivers, wetlands, etc., and are an important carrier of global biodiversity. Although freshwater bodies account for only 0.8% of the earth's surface, they are home to about 15 000 freshwater fish species (nearly 50% of the global fish species) (Geist and Hawkins, 2016). A large number of studies have shown that the health of aquatic ecosystems is closely related to the survival of fish populations. In recent years, due to intensified human activities, aquatic habitats have suffered severe damage, and fish populations and diversity are facing a crisis of sharp decline. On the one hand, anthropogenic interference such as pollutant emissions, eutrophication, water conservancy engineering and overfishing has led to a decline in the quality of aquatic habitats, inhibiting the reproduction and growth of fish (Bennett, 2024); on the other hand, the protection and restoration of aquatic habitats are considered to be a key way to maintain and improve the health of fish populations (Theis et al., 2024). This study aims to systematically sort out the characteristics of different habitat types in aquatic ecosystems, causes of habitat degradation and their impact on fish populations, and review the current commonly used habitat restoration technologies and their positive effects on fish resource recovery. Finally, it proposes future research directions and management suggestions to provide reference for aquatic ecological protection and sustainable use of fish resources. 2 Aquatic Ecosystem Habitat Types and Characteristics 2.1 River ecosystem River ecosystems (moving water ecosystems) are one of the most active habitats on the earth, and their main features are continuous flow of water, sufficient oxygen, water temperature and water quality are significantly affected by upstream incoming water. The river presents obvious gradients of hydrological, chemical and
International Journal of Aquaculture, 2025, Vol.15, No.4, 175-183 http://www.aquapublisher.com/index.php/ija 176 biological composition from the source to the estuary, forming a rich habitat structure (such as torrents, slow flows, drift belts and beach dam areas, etc.). This complex environmental structure provides fish with diverse living space and food resources (O’Mara et al., 2024). For example, free-flowing natural rivers have nurtured nearly 50% of the world's fish population, and many of them are migratory and require connected habitats in different sections to complete the spawning and foraging processes. River ecosystems usually have strong self-purification capabilities, but are also susceptible to upstream runoff, erosion, and recharge water quality (Wan et al., 2025). Fish often have ecological behaviors of river migration in rivers, and their population structure and distribution are highly sensitive to hydrological changes in the basin and habitat connectivity. Therefore, maintaining the natural flow and connectivity of the river is crucial to maintaining the diversity of river fish. 2.2 Lake and wetland ecosystems Lake and wetland ecosystems belong to static or semi-static environments, which are characterized by slow water flow, stable vertical structure of water body and large water volume, which are prone to water temperature stratification and nutrient deposition. Lakes are mostly inland water bodies with rich nutrients, with strong occlusion and high ecosystem productivity, but they are also prone to eutrophication and algae blooms. Wetlands (including swamps, estuary swamps, floodplain, etc.) are between land and water, with interlaced ecological characteristics, and are important places for fish reproduction, nurturing and overwintering (Cutler et al., 2024). Lakes and wetlands provide fish with a rich food chain basis (such as phytoplankton and invertebrates) and are key habitats for many freshwater fish. Compared with rivers, lake wetlands often have more complex species communities and ecological corridors, but are also more susceptible to runoff input and internal circulation (Bai et al., 2022). The study found that fish diversity in lakes and wetlands is often restricted by factors such as lake size, connectivity, and nutritional status. For example, large lakes such as Poyang Lake and Dongting Lake in the middle and lower reaches of the Yangtze River in China attract a large number of migratory and settled fish to reproduce every year due to their relatively stable water levels and abundant aquatic plants (Maileht et al., 2024). Protecting lake wetlands requires comprehensive consideration of the overall water quality improvement of lake basins and the restoration of tidal flats and wetland vegetation to support the needs of fish throughout their life cycle. 2.3 Marine and coastal ecosystems Marine ecosystems include offshore, far sea, coral reefs and mangroves and are the largest aquatic habitat system in the world. Compared with freshwater systems, the ocean has a wider spatial scale and physical environmental heterogeneity, such as constant salinity but drastic temperature changes with latitude, and the current transport of matter (Griffin et al., 2025). Coastal ecosystems such as coral reefs, algae reefs and mangroves provide complex reef structures and juvenile habitats for numerous marine fish, supporting rich species diversity (Ghimayen et al., 2024). Marine habitats are usually relatively productive, with marine fish populations having high mobility and large-scale migration behavior. Due to human development of marine resources, marine ecosystems are also facing threats such as overfishing, marine pollution and climate change. In general, the habitat characteristics of marine ecosystems are vast areas, large depths, and abundant species, but due to marine biology and physics laws (such as light depth and nutrient distribution), fish have clear niche stratification and seasonal distribution laws in different sea areas. 3 Main Causes of Habitat Degradation 3.1 Water conservancy project construction and habitat breaking Water conservancy projects such as dams, reservoirs, and waterway improvement built by humans have changed the natural hydrological dynamics of the original rivers, causing habitat fragmentation and significant changes in the physical environment. The dam storage has slowed down the river flow rate, a large amount of sediment and nutrients deposited in the upstream reservoir area, and the sand content and nutrient input of the water in the downstream river section have sharply decreased, destroying the basement conditions and floating food chain required for fish to lay eggs (Dudgeon, 2024). After the basin connectivity is cut off, migratory fish have difficulty returning to the spawning area through obstacles, and their life cycle is seriously disrupted. Statistics found that
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