Despite the recently discovered importance of heterotrophic microorganisms in the structure and function of marine planktonic systems, little is known about the relative significance of these organisms in open-ocean oligotrophic environments. Here we report that depth profiles of planktonic nonphotosynthetic bacteria (BACT), cyanobacteria (CYANO), and both photosynthetic (PNAN) and heterotrophic (HNAN) nanoplankton (i.e. eukaryotes C 2 0 pm diam.) to 2600 m in the Sargasso Sea in summer and fall (the least productive times of year) indicate that BACT biomass dominates these microorganisms at all depths. From the microscopic counts and our 'best guess' conversion factors, we determined that BACT contain >70 and >80 '5, of the euphotic zone microbial C and N, respectively, and >90 YO of the biological surface area, C Y A N O , P N A N , and HNAN b~ovolume each make up 7 to 17 % of the total. Even calculations based on maximum probable phytoplankton biomass (from chlorophyll) and minimum probable BACT biomass still showed domination by BACT in the euphotic zone. An important practical implication is that conventional measurements of particulate organic C and N collected on glass fiber filters may be missing most of the biomass. Independent bacterial heterotrophic production estimates based on tritiated thymidine incorporation and clearance by nanoplanktonic grazers suggest bacterial division rates of about once per 5 to 15 d. Interpretation of these results suggest that BACT consume significant amounts of C in this system, probably released from phytoplankton dlrectly or via herbivores. These results imply a fundamental difference between oligotrophic and eutrophic waters with respect to biomass distribution and that BACT are major particulate reservoirs of C and N in the sea.

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ABSTRACT. The potential for viral lysis of marine bacteria in seawater enriched with the virus size fraction from seawater was investigated in seawater samples from Long Island Sound, USA, the eastern Pacific Ocean and the Caribbean Sea. Ultrafiltration was used to concentrate material from seawater in the >0.05 pm to ~0 . 2 2 pm size fract~on. Electron microscopy counts of virus-like particles in the high molecular weight concentrate (HMWC) correlated closely to epifluorescence microscopy counts of <0.22 pm DAPI-positive, DNase-resistant particles of the HMWC. Acridine orange direct counts of bacterial abundances significantly declined (often to 1/2 or less of controls) when seawater was incubated with 4-to 16-fold enrichments of HMWC. Microwave treatment of the HMWC before addition to seawater virtually eliminated the declines in bacterial abundance. The combined evidence of the size range of particles, the heat lability of the HMWC and the presence of <0.22 pm DAPI-fluorescent, DNase-resistant particles by epifluorescence microscopy and abundant virus particles by electron microscopy suggests that a dominant bacterial mortality agent in the seawater concentrate was bacteriophage, although we could not rule out an effect of high molecular weight proteins.