Specific global responses to N and Fe nutrition in toxic and non-toxic Microcystis aeruginosa

The bloom-forming cyanobacteria species Microcystis aeruginosa includes toxic and non-toxic (microcystin-producing) strains. Certain stress conditions stimulate synthesis of microcystin (MCYST) and enhance the binding of the MCYST molecule to proteins. In this quantitative proteomic study, we compared the response of a wild-type toxic strain PCC 7806, an mcyH(-) knockout non-toxic strain, and a naturally occurring non-toxic strain, PCC 7005, after 8 days in low iron (Fe) and nitrogen (N) starvation in order to assess the benefit of MCYST synthesis in non-optimal conditions. Fe limitation increased MCYST synthesis and caused an accumulation of phycobilisome proteins and the ferric iron transporter FutA only in the toxic PCC 7806 but not the non-toxic strains. In N starvation, photosynthetic, C and N metabolism proteins were more abundant in the non-toxic strains, as were chaperones and proteases. Significant interaction between nutrient availability and toxicity existed for thioredoxin peroxidase and several thioredoxin-regulated proteins. We propose a competition of MCYST for binding sites in thioredoxin-regulated proteins during oxidative stress (low Fe) but not in growth-limiting conditions (low N). This then leads to differences in the regulation of C:N metabolism in toxic and non-toxic M.aeruginosa in nutrient-replete and nutrient-limited conditions.