We measured the elemental composition of single cells from six strains of marine Prochlorococcus and two strains of marine Synechococcus by X-ray microanalysis in the transmission electron microscope (TEM), which allows measurements of all major elements in cells without any fixation or staining. The mean carbon: volume ratios ranged from 136 to 280 fg mum(-3) in Prochlorococcus and 138 to 290 fg mum(-3) in Synechococcus. Using mean values of elemental content of C: N: O: P: S, we obtained molar ratios of 143-214: 15-24: 15-41: 1:0.58-1.64, and 65-293: 7-36: 11-36: 1 :0.31-1.54 for strains of Prochlorococcus and Synechococcus, respectively. The diffusible ions Mg++ (89-490 mmol L-1) and Na+ (230-660 mmol L-1) dominated in Prochlorococcus. In Synechococcus Na+ (90-480 mmol L-1) dominated. For all samples the range of K+ content was 23-130 mmol L-1. Elemental composition varied widely in relation to strains and growth media. However, C: P and N: P ratios were above the Redfield ratio in all Prochlorococcus strains and one of the Synechococcus strains (WH 8103). This likely reflected the low P content of these cells. This low P requirement would be clearly advantageous in the oligotrophic conditions occupied by these organisms. The high relative carbon content, compared to what has been found for heterotrophic bacteria using similar methods, is suggested to confer a competitive advantage to photosynthetic over heterotrophic bacteria in ecosystems where both functional groups are mineral nutrient limited
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