OVERESTIMATION OF HETEROTROPHIC BACTERIA IN THE SARGASSO-SEA - DIRECTEVIDENCE BY FLOW AND IMAGING CYTOMETRY

Accurate measurements of bacterial biomass in the ocean are needed for modeling marine microbial food webs and global biogeochemical cycling . We present direct evidence that previous estimates of heterotrophic bacteria biomass in the oligotrophic ocean are confounded by the prese nce of the abundant photosynthetic procaryote, Prochlorococcus. The ch lorophyll autofluorescence of these photosynthetic bacterial cells is very faint and fades rapidly under epifluorescence microscopy. Detecti on and enumeration of these cells thus far has almost exclusively been by flow cytometry. Using a cooled, charge-coupled device (CCD) camera we were able to image these cells for direct biovolume measurements. A double-exposed image of DAPI-stained Prochlorococcus cells shows tha t they are indistinguishable from heterotrophic bacteria in standard s lide preparations. At two Sargasso Sea stations Prochlorococcus could cause an overestimation of surface (top 150 m) integrated heterotrophi c bacterial biovolume (biomass) of 18 and 22% determined by standard m icroscope methods. At the subsurface chlorophyll maximum Prochlorococc us was 33 and 43% of the heterotrophic bacterial biovolume (biomass) a t these stations. Prochlorococcus cell size increased from 0.05 mu m(3 ) in the surface mixed layer to about 0.2 mu m(3) below 100 m, confirm ing previous interpretations of flow cytometric light scatter measurem ents. Shifting biomass from the heterotrophic bacteria pool to the pri mary producer compartment has significant implications for ecosystem s tructure and trophic transfer in marine food webs.