Effects of climate change on metabolite accumulation in freshwater and marine cyanobacteria

Global climate change and anthropogenic nutrient inputs are responsible for increased frequency of cyanobacterial blooms that potentially contain 55 classes of bioactive metabolites. This study investigated the effects of CO₂ availability and concomittant pH levels on two cyanobacteria that produce microcystins: a marine cf. Synechocystis sp. and a freshwater Microcystis aeruginosa. Cyanobacterial strains were semi-continuously cultured in mesotrophic growth media at pH 7.5, 7.8, 8.2, and 8.5 via a combination of CO₂ addition and control of alkalinity. The cell concentration between treatments was not significantly different and nutrient availability was not limited. Concentration of most known cyanobacterial bioactive metabolites in both cyanobacterial strains increased as CO₂ increased. At pH 7.8, bioactive metabolite intracellular concentration in M. aeruginosa and Synechocystis was 1.5 and 1.2 times greater than the other three treatments, respectively. Intracellular concentration of microginin in M. aeruginosa at pH 7.5 was reduced by 90% compared to the other three treatments. Intracellular concentration of microcyclamide-bistratamide B was lower in M. aeruginosa and higher in Synechocystis at elevated CO₂ concentration. M. aeruginosa products were more diverse metabolites than Synechocystis. The diversity of accumulated metabolites in M. aeruginosa increased as CO₂ increased, whereas the metabolite diversity in Synechocystis decreased as pH decreased. Overall, intracellular concentration of bioactive metabolites was higher at greater CO₂ concentrations; marine and freshwater cyanobacteria had different allocation products when exposed to differing CO₂ environments.