Interactive effects of micronutrients and phosphorus speciation on the growth and toxin production of freshwater cyanobacteria

Freshwater cyanobacterial blooms, especially those dominated by Microcystis spp., represent a significant manifestation of eutrophication, posing threats to the environment and human health through the release of harmful toxins. While previous studies have extensively explored the role of macronutrients—primarily nitrogen and phosphorus—in bloom formation, mounting evidence suggests that micronutrients such as copper, iron, zinc, and manganese also exert critical regulatory influences on cyanobacterial physiology, including photosynthesis, enzymatic activity, and toxin synthesis. Notably, these micronutrients interact with phosphorus at multiple biochemical levels, affecting its speciation, bioavailability, and assimilation pathways in freshwater systems. This review synthesizes recent advances in understanding the interactive effects between micronutrients and phosphorus speciation on the growth and microcystin production of freshwater cyanobacteria. Emphasis is placed on the dual regulatory roles of micronutrients under concentration gradients, the synergistic and antagonistic mechanisms influencing toxin biosynthesis, and the modulation of these processes under varying environmental conditions such as temperature and pH. Despite these insights, knowledge gaps remain, particularly regarding multi-nutrient interactions under ecologically realistic scenarios and the role of different phosphorus fractions. Future research should prioritize integrated mesocosm-scale experiments and predictive modeling frameworks to bridge the gap between laboratory findings and field conditions. By highlighting the importance of micronutrient–macronutrient coupling, this review provides theoretical and practical guidance for refining bloom prediction models and developing targeted nutrient management strategies for mitigating harmful algal blooms.