Gas vacuolation in Oscillatoria rubescens decreased with increased nitrogen limitation and increased with transitions from nitrogen to inorganic carbon limitation. Gas vacuoles consist of protein vesicles that can accumulate in carbon- limited but not in unenriched nitrogen-limited cells. Nitrogen limitation is a factor in the formation of deep population maxima; carbon limitation can promote surface blooms.

Alkaline phosphatase activity (APA) was investigated monthly for 4 years in the eutrophic Lake Nantua, a lake colonized by a large population of Cyanobacteria. Total enzymatic activity as well as specific activities (the ratios between APA and biomass expressed as dry weight, chlorophyll a, cell phosphorus) varied strongly but they followed a similar pattern during each year. The data were processed using correspondence analysis. Specific APA was never related to depth but highest activities were always associated with low particulate phosphorus and nitrogen, low dissolved inorganic phosphorus (DIP) concentrations, low chlorophyll a to filament number ratio and zero nitrate in the waters, indicating P and N limiting conditions. However a high N/P ratio, close to Redfield optimum also occurred at these conditions. Low activities were associated only with high chlorophyll a to filament number ratio.The results suggest that, during summer P-depletion and as long as the N/P ratio is close or above an optimum value, the DIP enzymatically regenerated from DOP pool by phosphatases could temporarily contribute to the algal phosphorus supply.

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A new fixation method was developed for the Nuclear Magnetic Resonance (NMR) study of natural phytoplankton samples collected in situ . To test NMR reliability, a Chlorella continuous culture was used in a phosphorus deficiency recovery experiment. The method was then applied to natural metalimnetic cyanobacterial plankton . The maximum Entropy Method was used to enhance the generally poor signal to noise ratio resulting from the low amount of available material and NMR sensitivity . Suggestions are made on how to improve reliability .