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

Abstract

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.

Basin-scale variability of phytoplankton biomass, production and growth in the Atlantic Ocean

“…However, the measured zooplankton grazing rates and abundances are consistent with slow rather than fast phytoplankton growth (Jackson, 1980). The observation that the composition of the particulate organic matter (POM) in the open ocean roughly follows the Red"eld ratios has been proposed as evidence for the lack of nutrient limitation of phytoplankton growth , although the interpretation of POM compositional data in nutrient-depleted areas is greatly confounded by the large contribution of detritus and bacterial biomass (Fuhrman et al, 1989;Malone et al, 1993). Following Goldman's view, the oligotrophic regions could be regarded as highly balanced, quasi steady-state systems where phytoplankton are growing at near maximal rates ( '1 d\ ).…”

Isotope fractionation associated with ammonium uptake by a marine bacterium

“…In the open ocean, bacterial biomass equals or exceeds phytoplankton biomass (Fuhrman et al 1989). Bacteria also process on the order of 50% of primary production (Cole et al 1988).…”

Phenology of high-elevation pelagic bacteria: the roles of meteorologic variability, catchment inputs and thermal stratification in structuring communities

“…Prokaryotes and other microorganisms have an important function in many fundamental biogeochemical processes (Fuhrman et al, 1989;Conrad, 1996;Cotner and Biddanda, 2002), and microbial community structure is linked to the rates and dynamics of these processes (Cottrell and Kirchman, 2000;Covert and Moran, 2001;Carlson et al, 2002). Ecological theory guides research into the relationships between community structure and function, but the overwhelming genotypic and phenotypic diversity of microorganisms in natural environments confounds efforts to ascribe macroorganismal concepts of community assembly to the microbial world (Finlay, 2002;Prosser et al, 2007).…”