Rex R. Malmstrom

Contribution of SAR11 Bacteria to Dissolved Dimethylsulfoniopropionate and Amino Acid Uptake in the North Atlantic Ocean

ABSTRACT SAR11 bacteria are abundant in marine environments, often accounting for 35% of total prokaryotes in the surface ocean, but little is known about their involvement in marine biogeochemical cycles. Previous studies reported that SAR11 bacteria are very small and potentially have few ribosomes, indicating that SAR11 bacteria could have low metabolic activities and could play a smaller role in the flux of dissolved organic matter than suggested by their abundance. To determine the ecological activity of SAR11 bacteria, we used a combination of microautoradiography and fluorescence in situ hybridization (Micro-FISH) to measure assimilation of 3H-amino acids and [35S]dimethylsulfoniopropionate (DMSP) by SAR11 bacteria in the coastal North Atlantic Ocean and the Sargasso Sea. We found that SAR11 bacteria were often abundant in surface waters, accounting for 25% of all prokaryotes on average. SAR11 bacteria were typically as large as, if not larger than, other prokaryotes. Additionally, more than half of SAR11 bacteria assimilated dissolved amino acids and DMSP, whereas about 40% of other prokaryotes assimilated these compounds. Due to their high abundance and activity, SAR11 bacteria were responsible for about 50% of amino acid assimilation and 30% of DMSP assimilation in surface waters. The contribution of SAR11 bacteria to amino acid assimilation was greater than would be expected based on their overall abundance, implying that SAR11 bacteria outcompete other prokaryotes for these labile compounds. These data suggest that SAR11 bacteria are highly active and play a significant role in C, N, and S cycling in the ocean.

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.

Assimilation of Polysaccharides and Glucose by Major Bacterial Groups in the Delaware Estuary

ABSTRACT The contribution of major bacterial groups to the assimilation of extracellular polymeric substances (EPS) and glucose in the Delaware Estuary was assessed using microautoradiography and fluorescence in situ hybridization. Bacterial groups contributed to EPS and glucose assimilation in part according to their distribution in the estuary. Abundance of the phylogenetic groups explained 35% and 55% of the variation in EPS and glucose assimilation, respectively. Actinobacteria contributed 70% to glucose assimilation in freshwater, while Alphaproteobacteria assimilated 60% of this compound in saline water. In contrast, various bacterial groups dominated the assimilation of EPS. Actinobacteria and Betaproteobacteria contributed the most in the freshwater section, whereas Cytophaga-like bacteria and Alpha- and Gammaproteobacteria participated in EPS assimilation in the lower part of the estuary. In addition, we examined the fraction of bacteria in each group that assimilated glucose or EPS. Overall, the fraction of bacteria in all groups that assimilated glucose was higher than the fraction that assimilated EPS (15 to 30% versus 5 to 20%, respectively). We found no correlation between the relative abundance of a group in the estuary and the fraction of bacteria actively assimilating glucose or EPS; the more active groups were often less abundant. Our results imply that the bacterial community in the Delaware Estuary is not controlled solely by “bottom-up” factors such as dissolved organic matter.