1. ¹Molecular and Cellular Biology Program, Oregon State University, Corvallis, OR, USA
2. ²Department of Microbiology, Oregon State University, Corvallis, OR, USA
3. ³Pacific Northwest National Laboratory, Richland, WA, USA
4. ⁴Department of Chemistry, Oregon State University, Corvallis, OR, USA
5. ⁵Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA, USA

Correspondence: SJ Giovanonni, Department of Microbiology, Oregon State University, 220 Nash Hall, Corvallis, OR 97331, USA. E-mail: steve.giovannoni@oregonstate.edu

Received 26 May 2008; Revised 4 August 2008; Accepted 4 August 2008; Published online 4 September 2008.

Abstract

The northwestern Sargasso Sea undergoes annual cycles of productivity with increased production in spring corresponding to periods of upwelling, and oligotrophy in summer and autumn, when the water column becomes highly stratified. The biological productivity of this region is reduced during stratified periods as a result of low concentrations of phosphorus and nitrogen in the euphotic zone. To better understand the mechanisms of microbial survival in this oligotrophic environment, we used capillary liquid chromatography (LC)-tandem mass spectrometry to detect microbial proteins in surface samples collected in September 2005. A total of 2215 peptides that mapped to 236 SAR11 proteins, 1911 peptides that mapped to 402 Prochlorococcus proteins and 2407 peptides that mapped to 404 Synechococcus proteins were detected. Mass spectra from SAR11 periplasmic substrate-binding proteins accounted for a disproportionately large fraction of the peptides detected, consistent with observations that these extremely small cells devote a large proportion of their volume to periplasm. Abundances were highest for periplasmic substrate-binding proteins for phosphate, amino acids, phosphonate, sugars and spermidine. Proteins implicated in the prevention of oxidative damage and protein refolding were also abundant. Our findings support the view that competition for multiple nutrients in oligotrophic systems is extreme, but nutrient flux is sufficient to sustain microbial community activity.