Analysis of changes over time in microcystin content of nitrogen‐limited Microcystis aeruginosa (Kützing) Lemmermann batch cultures (strain MASHOl‐A19) showed that net microcystin production was limited to the phase of growth when cell concentration was increasing. The net microcystin production rate decreased as the specific cell division rate (µ_c) decreased, but, more importantly, the specific toxin production rate (µ_MCYST) decreased at an identical rate to that of µ_c when the culture became nitrate‐limited. The actual size of the microcystin pool (total culture microcystin concentration) increased while cells were dividing, then remained constant or decreased only slightly during the stationary and death phases, even when the cultures were severely nitrate‐starved. These findings demonstrate conclusively that the processes of cell division and microcystin production are tightly coupled under nitrogen‐limited cell division. Our findings suggest that microcystin production is controlled by environmental effects on the rate of cell division, not through any direct effect on the metabolic pathways of toxin production. Reevaluation of data presented by others shows this to be the case for two other cyanobacterial species producing nine different microcystins over a wide range of environmental variables. We believe these relationships now provide a unifying view of environmental control of microcystin production in hepatotoxic cyanobacteria. We conclude that there is a direct linear correlation between cell division and microcystin production rates in all microcystin‐producing cyanobacteria regardless of the environmental factor that is limiting cell division. We also conclude that microcystin is not a secondary metabolite, as is currently thought, but that it displays many of the attributes of essential intracellular nitrogenous compounds in toxigenic cyanobacteria.

Internal pressurization and convective through‐flow are demonstrated to be common attributes of wetland plants with cylindrical culms or linear leaves. Eight of 14 species tested produced static internal gas pressure differentials of 200–1,300 Pa relative to ambient and internal convective airflows of 0.2 to >10 cm³ min⁻¹ culm⁻¹, depending on species. Four species produced internal static pressure differentials of <100 Pa. Two species did not pressurize. The driving forces are gradients in temperature and water vapor between the internal gas spaces of the plants and the ambient atmosphere (thermal transpiration and humidity‐induced pressurization).A clear diel variation in pressurization and convective flow was observed; rates were highest in the afternoon and lowest at night, responding to ambient changes in light, temperature, and humidity. The resistance to airflow at the stem‐rhizome junction was very high for some species, resulting in a low ability to convert internal pressurization into convective airflow through the rhizomes. Species with a high potential for internal pressurization and a low internal resistance to convective flow seem to have a competitive advantage over species that rely exclusively on diffusive gas transport, which allows them to grow in deeper waters.

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