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Appl. Environ. Microbiol. doi:10.1128/AEM.00212-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Light-stimulated bacterial production and amino acid assimilation by cyanobacteria and other microbes in the North Atlantic Ocean

College of Marine and Earth Studies, University of Delaware, Lewes, DE 19958, USA

^* To whom correspondence should be addressed. Email: kirchman{at}udel.edu.

	Abstract

We examined the contribution of photoheterotrophic microbes - those capable of light mediated assimilation of organic compounds - to bacterial production and amino acid assimilation along a transect from Florida to Iceland in May 28 - July 9, 2005. Bacterial production (leucine incorporation at 20 nM final concentration) was on average 30% higher in light than in dark-incubated samples, but the effect varied greatly (3% to 60%). To further characterize this light effect, we examined the abundance of potential photoheterotrophs and measured their contribution to bacterial production and amino acid assimilation (0.5 nM addition) using flow cytometry. Prochlorococcus and Synechococcus were abundant in surface waters where light-dependent leucine incorporation was observed whereas aerobic anoxygenic phototrophic bacteria were abundant but did not correlate with the light effect. The per-cell assimilation rates of Prochlorococcus and Synechococcus were comparable to or higher than those of other prokaryotes, especially in the light. Picoeukaryotes also took up leucine (20 nM) and other amino acids (0.5 nM), but rates normalized to biovolume were much lower than those of prokaryotes. Prochlorococcus was responsible for 80% of light-stimulated bacterial production and amino acid assimilation in surface waters south of the Azores, while Synechococcus accounted for on average 12% of total assimilation. However, nearly 40% of the light-stimulated leucine assimilation was not accounted for by these groups, suggesting that assimilation by other microbes is also affected by light. Our results clarify the contribution of cyanobacteria to photoheterotrophy and highlight the potential role of other photoheterotrophs in biomass production and dissolved organic matter assimilation.

This article has been cited by other articles:

• Kirchman, D. L. (2008). New light on an important microbe in the ocean. Proc. Natl. Acad. Sci. USA 105: 8487-8488 [Full Text]