Cellular microcystin content in N-limited Microcystis aeruginosa can be predicted from growth rate

Cell quotas of microcystin (Q(MCYST); femtomoles of MCYST per cell), protei n, and chlorophyll a (Chl a), cell dry weight, and cell volume were measure d over a range of growth rates in N-limited chemostat cultures of the toxic cyanobacterium Microcystis aeruginosa MASH 01-A19. There was a positive li near relationship between Q(MCYST) and specific growth rate (mu), from whic h we propose a generalized model that enables Q(MCYST) at any nutrient-limi ted growth rate to be predicted based on a single batch culture experiment, The model predicts Q(MCYST) from mu, mu (max) (maximum specific growth rat e), Q(MCYST) (maximum cell quota), and Q(MCYST) (minimum cell quota). Under the conditions examined in this study, we predict a Q(MCYST) of 0.129 fmol cell(-1) at mu (max) and a Q(MCYST) of 0.050 fmol cell(-1) at mu = 0, Net MCYST production rate (R-MCYST) asymptotes to zero at mu = 0 and reaches a maximum of 0.155 fmol cell(-1) day(-1) at mu (max). MCYST/dry weight ratio (milligrams per gram [dry weight]) increased linearly with mu, whereas the MCYST/protein ratio reached a maximum at intermediate mu. In contrast, the MCYST/Chl a ratio remained constant. Cell volume correlated negatively with mu, leading to an increase in intracellular MCYST concentration at high IJ Taken together, our results show that fast-growing cells of N-limited M. a eruginosa are smaller, are of lower mass, and have a higher intracellular M CYST quota and concentration than slow-growing cells, The data also highlig ht the importance of determining cell MCYST quotas, as potentially confusin g interpretations can arise from determining MCYST content as a ratio to ot her cell components.