Dominance of bacterial biomass in the Sargasso Sea and its ecological implications

Abstract

Here we report that depth profiles of planktonic nonphotosynthetic bacteria (BACT), cyanobacteria (CYANO), and both photosynthetic (PNAN) and heterotrophic (HNAN) nanoplankton (i.e. eukaryotes 70 and > 80% of the euphotic zone microbial C and N, respectively, and > 90% of the biological surface area; CYANO, PNAN, and HNAN biovolume 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.