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Aquatic Microbial Ecology

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AME 47:153-162 (2007)  -  doi:10.3354/ame047153

Responses of heterotrophic bacteria to solar irradiance in the eastern Pacific Ocean

J. Dean Pakulski1, Amy Baldwin1, Amanda L. Dean2, Sarah Durkin1, Deneb Karentz3, Cheryl A. Kelley4, Kerry Scott1, Howard J. Spero5, Steven W. Wilhelm2, Raid Amin6, Wade H. Jeffrey1,*

1Center for Environmental Diagnostics and Bioremediation, University of West Florida, Building 58, 11000 University Parkway, Pensacola, Florida 32514, USA
2Department of Microbiology, M409 WLS, University of Tennessee, Knoxville, Tennessee 37996, USA
3Department of Biology, 2130 Fulton Street, University of San Francisco, San Francisco, California 94117-1080, USA
4Department of Geological Sciences, 101 Geological Science Building, University of Missouri-Columbia, Columbia, Missouri 65211, USA
5Department of Geology, One Shields Avenue, University of California, Davis, California 95616-8605, USA
6Department of Statistics, University of West Florida, Building 38, 11000 University Parkway, Pensacola, Florida 32514, USA
*Corresponding author. Email:

ABSTRACT: We investigated the effects of sunlight on bacterial 3H-leucine (Leu) and 3H-thymidine (TdR) incorporation at 12 locations, from 41°S to 4°N in the eastern Pacific Ocean, during July and August 2000. Surface water samples amended with Leu and TdR were incubated under ambient sunlight using optical filters corresponding to the following wavebands: UVB+UVA+PAR, UVA+PAR, long-wavelength UVA+PAR, and PAR. Incorporation rates of Leu and TdR in dark controls were statistically compared to rates in the light treatments to determine the effect of solar irradiance on bacterial production at each station. We observed robust photo-stimulation of TdR incorporation with UVA+PAR and long-UVA+PAR treatments at 17°S and in all light treatments at 13°S. PAR stimulation of Leu incorporation occurred over much of the south to north survey. Bacterial community structure analyses indicated the presence of 4 communities that exhibited unique responses to ambient solar irradiance. Between 21°S and 4°N, the δ13C of dissolved inorganic carbon (δ13CDIC) was significantly and inversely correlated with UVB, UVA, PAR, and dark Leu incorporation, but not to sea surface temperature, or concentrations of nitrate and chlorophyll. Our results demonstrate widespread direct dependence on solar irradiance, especially longer wavelengths, for bacterial production in surface water of the southeastern Pacific Ocean. Additionally, our data suggest that latitudinal trends in δ13CDIC are strongly associated with trends in solar UVB and bacterial production in upwelling waters, with implications for carbon cycling in tropical and subtropical waters.

KEY WORDS: Microbial diversity · Pacific Ocean · Ultraviolet radiation · Bacterioplankton

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