Elemental composition of single cells of various strains of marine Prochlorococcus and Synechococcus using X-ray microanalysis

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 mm23 in Prochlorococcus and 138 to 290 fg mm23 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 L21) and Na (230–660 mmol L21) dominated in Prochlorococcus. In Synechococcus Na (90–480 mmol L21) dominated. For all samples the range of K content was 23–130 mmol L21. 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. Marine cyanobacteria of the genera Synechococcus and Prochlorococcus comprise the prokaryotic component of the oxygenic photosynthetic picoplankton. These ,2-mm sized cells contribute around half of the carbon fixed in marine systems and hence are of particular ecological significance with regard to global C cycling (see Partensky et al. 1999a). Since the first reports describing the presence of Synechococcus and Prochlorococcus in the euphotic zone (Johnson and Sieburth 1979; Waterbury et al. 1979; Chisholm et al. 1988), much has been learned of the physiology and molecular ecology of these organisms (see Carr and Mann 1994; Partensky et al. 1999b; Moore et al. 2002; West and Scanlan 2002; Scanlan 2003). However, one important area that has largely escaped investigation, despite its potential ecophysiological significance, has been the C : N : P stoichiometry of these organisms. Indeed, relatively few reports on elemental composition of marine bacteria are available, and for Synechococcus and Prochlorococcus the only reported data are