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 (< or = 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.

FIG. 1.

Contribution of the SAR11 clade to bacterial production in the Northwest Atlantic Ocean. A) The abundance of SAR11 bacteria in the leucine-assimilating community versus the abundance of SAR11 bacteria in the total prokaryote community (DAPI-positive cells). B) The percentage of total silver grain area associated with SAR11 bacteria versus the abundance of SAR11 bacteria in the total prokaryote community. Error bars indicate 95% confidence intervals based on 580 to 4,000 bacteria per sample; 1:1 lines bisect the graphs. Sta., station.

FIG. 2.

Contribution of SAR11 bacteria to glucose, amino acid, and protein assimilation in the Northwest Atlantic Ocean. The abundance of SAR11 bacteria in the glucose-, amino acid-, and protein-assimilating community versus the abundance of SAR11 bacteria in the total prokaryote community (A, C, E). The percentage of total silver grain area associated with SAR11 bacteria and the abundance of SAR11 bacteria in the total prokaryote community (B, D, F). Error bars represent 95% confidence intervals based on 580 to 4,000 bacteria per sample; 1:1 lines bisect the graphs. Sta., station.

FIG. 3.

Percentage of SAR11 bacteria (A) and total prokaryotes (B) assimilating amino acids, glucose, and protein at four stations in the Northwest Atlantic Ocean. Error bars represent 95% confidence intervals. Sta., station.