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.
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