Variability of microcystin cell quota: A small model explains dynamics and equilibria

Laboratory experiments revealed a relationship between microcystin (MCYST) production and cell division. In order to evaluate this relationship, a model with two differential equations describing population growth and MCYST production was applied. It is demonstrated that MCYST production and expected maximum values of MCYST cell quota are generally well explained by this approach. Furthermore, the relevance of the two model parameters MCYST production coefficient p and MCYST depletion rate dM was analysed. It was found that even low values of the depletion rate (only 0.02 d−1 of actual MCYST) support the experimentally observable variability of MCYST cell quota. Nevertheless, the correlation between growth rate and MCYST production rate remained to be significant. The maximum of observable MCYST cell quota is chiefly influenced by the growth rate and MCYST production coefficient. MCYST cell quota of the inoculum was identified as an additional factor influencing MCYST cell quota of a growing population, but this effect is of minor importance and only of relevance during early exponential growth. The model allows for a dynamic estimation of MCYST production in batch and continuous cultures as well. Future research about the ecological role of MCYSTs should not stick on correlations between cell quota and environmental factors only, but instead focus on factors, which control the production coefficient p.