MEPS 285:3-9 (2005)  -  doi:10.3354/meps285003

ABSTRACT: Unicellular N₂-fixing cyanobacteria from tropical marine oligotrophic environments have been proposed to be major contributors to the global N cycle but still remain poorly characterized. These organisms are likely to be limited by phosphorus availability in situ. The aim of this study was to identify growth kinetics of isolates from the tropical North Atlantic and subtropical North Pacific in relation to phosphorus and temperature in continuous cultures. Cells from the Atlantic measured 2.5 µm in diameter (A-2.5). Genetically identical isolates from the Pacific showed 2 diameters depending on P-media concentrations (small: 3 µm, 1 µM PO₄ [P-3] and large: 7 µm, 4 µM PO₄ [P-7]). All 3 isolates were highly stenothermal, and optimal growth temperatures ranged between 26 and 30°C. Small cells (A-2.5 and P-3) had lower half-saturation constants (K_s) for PO₄ than large cells (P-7) (0.06 to 0.21 µM vs. 0.20 to 0.25 µM). Maximum growth rates and N:P ratios increased with temperature for all isolates; N:P ratios were close to Redfield ratios (N:P = 16) when isolates approached maximum growth rates. N₂-fixation activity did not vary between growth rates, but did increase with temperature; rates were consistently lower than previously published rates for the same isolates under non-P-limiting conditions. From these studies, we conclude that both Atlantic and Pacific unicellular cyanobacteria that have the capacity to fix N₂ have a limited temperature range for growth and that smaller sized isolates could be better adapted for conditions of phosphorus limitation.

KEY WORDS: Growth kinetics · Phosphorus · Temperature · Unicellular cyanobacteria · N₂ fixation