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Appl. Environ. Microbiol. doi:10.1128/AEM.02559-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The SAR92 clade: An abundant coastal clade of culturable marine bacteria possessing proteorhodopsin

Oregon State University, Department of Microbiology, Nash Hall 220, Corvallis, OR 97331; Division of Life and Marine Sciences, Inha University, Incheon 402-751, Republic of Korea

^* To whom correspondence should be addressed. Email: stinglu{at}science.oregonstate.edu.

	Abstract

Proteorhodopsin (PR) is a protein that is abundant in marine bacterioplankton. PR is hypothesized to a light-dependent proton thus creating a proton gradient that can be used for energy production without electron transport. Currently, the only culture published that possesses PR is the highly abundant Alphaproteobacterium Candidatus Pelagibacter ubique (SAR11 clade), but, surprisingly, its growth in batch culture was not enhanced by light. Here, we present the first cultured Gammaproteobacterium that possesses a PR gene. Genome sequencing and analysis of HTCC2207 showed that the PR gene is present as a lone transcriptional unit directly followed by an operon containing genes that are presumably involved in the synthesis of retinal, the chromophore of PR. Half-time decay times of different PR intermediates in native HTCC2207 cells ranged between 2 and 15 ms, the absorbance maximum of PR was determined to be 528 nm.

Proteorhodopsin was identified in three additional strains using a specific PCR assay on other cultured members of the SAR92 clade. Phylogenetic analysis of the PR genes determined that they form a deeply-rooting cluster not closely related to any PR genes recovered so far. Fluorescence in situ hybridization (FISH) and RNA blots show that the SAR92 clade reaches up to 10% of the total bacterial population in surface waters close to the Oregon coast and decreases over depth and distance from the shore. Although being carbon-limited in the applied media, also these cultures do not grow at higher growth rate nor have better growth yields when incubated under light.

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