Biomass Production and Assimilation of Dissolved Organic Matter by SAR11 Bacteria in the Northwest Atlantic Ocean

Abstract

Members of the SAR11 clade often dominate the composition of marine microbial communities, yet their contribution to biomass production and the flux of dissolved organic matter (DOM) is unclear. In addition, little is known about the specific components of the DOM pool utilized by SAR11 bacteria. To better understand the role of SAR11 bacteria in the flux of DOM, we examined the assimilation of leucine (a measure of biomass production), as well as free amino acids, protein, and glucose, by SAR11 bacteria in the Northwest Atlantic Ocean. We found that when SAR11 bacteria were >25% of total prokaryotes, they accounted for about 30 to 50% of leucine incorporation, suggesting that SAR11 bacteria were major contributors to bacterial biomass production and the DOM flux. Specific growth rates of SAR11 bacteria either equaled or exceeded growth rates for the total prokaryotic community. In addition, SAR11 bacteria were typically responsible for a greater portion of amino acid assimilation (34 to 61%) and glucose assimilation (45 to 57%) than of protein assimilation (<34%). These data suggest that SAR11 bacteria do not utilize various components of the DOM pool equally and may be more important to the flux of low-molecular-weight monomers than to that of high-molecular-weight polymers.The SAR11 clade is one of the most abundant bacterial phylogenetic groups in the ocean. About 25% of the 16S rRNA gene sequences retrieved from uncultured marine bacteria belong to the SAR11 bacteria (13), and representatives of the SAR11 clade have been found in clone libraries from around the world (28). Investigations using fluorescence in situ hybridization (FISH) confirm that SAR11 bacteria often make up 25 to 35% of the total prokaryotic community in the surface waters of both coastal and open-ocean systems (22,24). The high abundance and global distribution of SAR11 bacteria suggest that they mediate a significant fraction of the dissolved organic matter (DOM) flux in the ocean. As with most marine bacterial groups, however, little is known about the contribution of SAR11 bacteria to the flux of DOM.The uptake of some DOM components by SAR11 bacteria was examined previously in the Gulf of Maine and the Sargasso Sea. Malmstrom el al. (22) found that the SAR11 clade dominated the assimilation of dissolved free amino acids and dimethylsulfoniopropionate, supporting the hypothesis that SAR11 bacteria mediate a large portion of the DOM flux. However, the assimilation of high-molecular-weight (HMW) DOM, which is a major source of C for bacterial communities (2), was not examined. In the Delaware Bay, Cottrell and Kirchman (5) found differences between the assimilation of low-molecular-weight (LMW) monomers and that of HMW polymers by the alpha-proteobacteria. The SAR11 bacteria, which are an abundant subgroup of the alpha-proteobacteria (13), may also contribute differently to the fluxes of LMW and HMW components of the DOM pool. To better understand their contribution to the flux of DOM, the assimilation of LMW monomers and HMW polymers ...

Expression patterns reveal niche diversification in a marine microbial assemblage

“…Finally, %RP patterns were compared against available literature values for taxon-specific in situ growth rates (Yokakawa et al, 2004;Malmstrom et al, 2005;Allers et al, 2007;Teira et al, 2009;Ferrerra et al, 2011), and results showed that rankings of taxa based on %RP tracked well with rankings based on measures of in situ growth rates ( Figure 1d). Among the 200 top-recruiting reference bins, the percent of sequences identified as ribosomal proteins (%RP hits) within a genome ranged from 0.05% (Candidatus Pelagibacter ubique HTCC1002) to 20.5% (Chryseobacterium gleum ATCC 35910), and showed distinct phylogenetic patterns ( Figure 1c).…”

Patterns and mechanisms of genetic and phenotypic differentiation in marine microbes

“…This has shown that many such phylotypes occur in both open ocean and coastal environments, but apparently not every phylotype is found everywhere (at least not in the same proportions; Giovannoni & Rappé 2000). For example, it was from clone libraries that the first members of the SAR11 group were identified; these, now named Pelagibacter, comprised at least 16% of total cells in the mesopelagic and up to 50% of bacterioplankton in the surface ocean (Morris et al 2002;Malmstrom et al 2005). Moreover, it has generally been found that easily cultivable phylotypes are frequently not the most abundant members of bacterioplankton, although abundance may in some cases underestimate importance owing to higher turnover rates (Worden et al 2006).…”

Temporal dynamics of Prochlorococcus ecotypes in the Atlantic and Pacific oceans

“…For example, one SAR11 subgroup reaches peak abundance in surface waters during the summer, whereas another subgroup peaks in the winter . SAR11 bacteria are the most abundant heterotrophs at BATS and are major consumers of dissolve organic compounds (Morris et al, 2002;Malmstrom et al, 2005), whereas Prochlorococcus is the most abundant photoautotroph and a substantial source of dissolved organics (DuRand et al, 2001;Bertilsson et al, 2005). In addition, SAR11 bacteria and Prochlorococcus are both major consumers of small compounds like amino acids and dimethylsulfoniopropionate (DMSP) (Zubkov et al, 2003;Malmstrom et al, 2004;Vila-Costa et al, 2006;Michelou et al, 2007), which are significant sources of C, N and S to marine microbial communities.…”