In this study, we test the applicability of the Droop/Caperon internal stores model to describe the growth and decline of the globally abundant marine cyanobacterium Prochlorococcus in batch culture as a function of internal and external inorganic and organic carbon and nitrogen. A rigorous parameter fitting exercise, constrained by the measured cell density, ammonium and inorganic carbon concentrations, revealed many different combinations of parameter values that provided equally good model‐data fit. Introducing data on C : N ratio from the published literature provided additional constraints which could only be satisfied with a structural change to the model. The simplest addition that satisfied these constraints and improved quantitative overall model‐data agreement was to explicitly represent excretion, which was predicted to account for 0.03–8.5% of the daily primary productivity; somewhat lower than previous estimates based on radiotracer incorporation. We argue that this significant and biologically meaningful process should not be neglected in models of marine phytoplankton.

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Toxic cyanobacterial blooms are a growing threat to freshwater bodies worldwide. In order for a toxic bloom to occur, a population of cells with the genetic capacity to produce toxins must be present together with the appropriate environmental conditions. In this study, we investigated the distribution patterns and phylogeny of potentially-toxic Microcystis (indicated by the presence and/or phylogeny of the mcyD and mcyA genes). Samples were collected from the water column of almost 60 water bodies across widely differing gradients of environmental conditions and land use in Israel. Potentially, toxic populations were common but not ubiquitous, detected in ~65% of the studied sites. Local environmental factors, including phosphorus and ammonia concentrations and pH, as well as regional conditions such as the distance from built areas and nature reserves, were correlated with the distribution of the mcyD gene. A specific phylogenetic clade of Microcystis, defined using the sequence of the mcyA gene, was preferentially associated with aquaculture facilities but not irrigation reservoirs. Our results reveal important environmental, geospatial, and land use parameters affecting the geographic distribution of toxinogenic Microcystis, suggesting non-random dispersal of these globally abundant toxic cyanobacteria.

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