1. Department of Microbiology, 220 Nash Hall, Oregon State University, Corvallis, Oregon 97331, USA
2. Woods Hole Oceanographic Institution, Redfield 3-22, MS no. 32, Woods Hole, Massachusetts 02543, USA

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

Abstract

Sulphur is a universally required cell nutrient found in two amino acids and other small organic molecules. All aerobic marine bacteria are known to use assimilatory sulphate reduction to supply sulphur for biosynthesis, although many can assimilate sulphur from organic compounds that contain reduced sulphur atoms^{1, 2, 3}. An analysis of three complete 'Candidatus Pelagibacter ubique' genomes, and public ocean metagenomic data sets, suggested that members of the ubiquitous and abundant SAR11 alphaproteobacterial clade are deficient in assimilatory sulphate reduction genes. Here we show that SAR11 requires exogenous sources of reduced sulphur, such as methionine or 3-dimethylsulphoniopropionate (DMSP) for growth. Titrations of the algal osmolyte DMSP in seawater medium containing all other macronutrients in excess showed that 1.5  10⁸ SAR11 cells are produced per nanomole of DMSP. Although it has been shown that other marine alphaproteobacteria use sulphur from DMSP in preference to sulphate^{1, 2}, our results indicate that 'Cand. P. ubique' relies exclusively on reduced sulphur compounds that originate from other plankton.

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