Cyanobacterial harmful algal blooms (cHABs) are characterized by the formation of toxins that can impact animal health, cause water quality issues, and recreational hazards. Microcystis, a common genus of cyanobacteria, produces the potent protein phosphatase inhibitor microcystins. Microcystins are nitrogen-rich and have an associated metabolic cost for production. Some outstanding questions in the study of cHABs is why are microcystins produced, what are the benefits of toxin formation, and why only some Microcystis strains produce microcystins? We examined a potential biochemical role for microcystins in the cyanobacterial photosystem regulation in response to various light conditions. Single-celled culture strains of toxic and non- toxic Microcystis aeruginosa were grown under different light irradiances. High-light conditions caused light stress based on decreased photosynthetic efficiency. Cells responded over 2-3 days by decreasing their chlorophyll and phycobilisome content per cell in unison. Looking at a natural system over a diurnal cycle of changing light intensities, Microcystis responded to high-light environments via vertical migration deeper in the water column to avoid light stress. In one culture and in situ, there were no changes in microcystins concentration per cell, but one strain showed decreased microcystins under low-light. Therefore, microcystins protein phosphatase activity may not be involved with phosphorylation/dephosphorylation under high-light. High-light environments are also associated reactive oxygen species (ROS) production. We investigated if microcystins were linked to ROS sensitivity by examining how three chlorophytes and seven cyanobacteria responded to the ROS compound hydrogen peroxide (H2O2). There was no evidence that toxic cyanobacteria were more sensitive to ROS than non-toxic cyanobacteria or chlorophytes. Addition of H2O2 did not change the microcystins concentration per cell. While these experiments did not elucidate the biochemical function of microcystins in Microcystis, they provided valuable information for water quality managers. cHAB monitoring programs must carefully consider vertical migration away from the surface during high-light conditions. The use of H2O2 as a control mechanism may not selectively remove toxic cyanobacteria. Despite visual similarities, Microcystis is composed of diverse species with a wide-range of responses to light and ROS. Care must be taken when applying conclusions using a limited number of strains to broader populations
