DIRECT DETERMINATION OF CARBON AND NITROGEN CONTENTS OF NATURAL BACTERIAL ASSEMBLAGES IN MARINE ENVIRONMENTS

In order to better estimate bacterial biomass in marine environments, we developed a novel technique for direct measurement of carbon and ni trogen contents of natural bacterial assemblages, Bacterial cells were separated from phytoplankton and detritus with glass fiber and membra ne filters (pore size, 0.8 mu m) and then concentrated by tangential f low filtration. The concentrate was used for the determination of amou nts of organic carbon and nitrogen by a high-temperature catalytic oxi dation method, and after it was stained with 4',6-diamidino-2-phenylin dole, cell abundance was determined by epifluorescence microscopy. We found that the average contents of carbon and nitrogen for oceanic bac terial assemblages were 12.4 +/- 6.3 and 2.1 +/- 1.1 fg cell(-1) (mean a standard deviation; n = 6), respectively, Corresponding values for coastal bacterial assemblages were 30.2 +/- 12.3 fg of C cell(-1) and 5.8 +/- 1.5 fg of N cell(-1) (n = 5), significantly higher than those for oceanic bacteria (two-tailed Student's t test; P < 0.03), There wa s no significant difference (P > 0.2) in the bacterial C:N ratio (atom atom(-1)) between oceanic (6.8 +/- 1.2) and coastal (5.9 +/- 1.1) ass emblages. Our estimates support the previous proposition that bacteria contribute substantially to total biomass in marine environments, but they also suggest that the use of a single conversion factor for dive rse marine environments can lead to large errors in assessing the role of bacteria in food webs and biogeochemical cycles. The use of a fact or, 20 fg of C cell(-1), which has been widely adopted in recent studi es may result in the overestimation (by as much as 330%) of bacterial biomass in open oceans and in the underestimation (by as much as 40%) of bacterial biomass in coastal environments.