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

We investigated the effects of sunlight on bacterial 3 H-leucine (Leu) and 3 H-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 sun- light 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 stimu- lation 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 ambi- ent solar irradiance. Between 21° S and 4° N, the δ 13 C of dissolved inorganic carbon (δ 13 CDIC) 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 wide- spread direct dependence on solar irradiance, especially longer wavelengths, for bacterial produc- tion in surface water of the southeastern Pacific Ocean. Additionally, our data suggest that latitudinal trends in δ 13 CDIC are strongly associated with trends in solar UVB and bacterial production in upwelling waters, with implications for carbon cycling in tropical and subtropical waters.