Letters to Nature

Nature 420, 806-810 (19 December 2002) | doi:10.1038/nature01240; Received 31 May 2002; Accepted 7 October 2002

SAR11 clade dominates ocean surface bacterioplankton communities

Robert M. Morris1, Michael S. Rappé1, Stephanie A. Connon1, Kevin L. Vergin1, William A. Siebold1, Craig A. Carlson2 & Stephen J. Giovannoni1

  1. Department of Microbiology, Oregon State University, Corvallis, Oregon 97331, USA
  2. Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California 93106-9610, USA

Correspondence to: Stephen J. Giovannoni1 Correspondence and requests for materials should be addressed to S.J.G. (e-mail: Email: steve.giovannoni@orst.edu).

The most abundant class of bacterial ribosomal RNA genes detected in seawater DNA by gene cloning belongs to SAR11—an alpha-proteobacterial clade1. Other than indications of their prevalence in seawater, little is known about these organisms. Here we report quantitative measurements of the cellular abundance of the SAR11 clade in northwestern Sargasso Sea waters to 3,000 m and in Oregon coastal surface waters. On average, the SAR11 clade accounts for a third of the cells present in surface waters and nearly a fifth of the cells present in the mesopelagic zone. In some regions, members of the SAR11 clade represent as much as 50% of the total surface microbial community and 25% of the subeuphotic microbial community. By extrapolation, we estimate that globally there are 2.4 times 1028 SAR11 cells in the oceans, half of which are located in the euphotic zone. Although the biogeochemical role of the SAR11 clade remains uncertain, these data support the conclusion that this microbial group is among the most successful organisms on Earth.