Church¹, Matthew J., Hugh W. Ducklow¹, and David M. Karl²

¹School of Marine Science, The College of William and Mary, Gloucester Point, VA 23062, Tel: 831-459-3874, Fax: 831-459-4882, E-mail: mattc@vims.edu and ²School of Ocean and Earth Sciences, University of Hawaii, Honolulu, HI 96822

Light-enhanced bacterial production at Station ALOHA in the oligotrophic North Pacific Ocean

Carbon and energy fluxes in the sea are determined in large part by the activities of planktonic microorganisms, particularly throughout the open ocean ecosystems that dominate the planet. Recently characterized phototrophic marine bacterioplankton have the potential to significantly impact carbon and energy fluxes in the oceans; however, no quantitative information exists regarding the impact of sunlight on heterotrophic production in the sea. In this study we conducted time-series measurements of heterotrophic bacterial production over a two year period at the Station ALOHA in the oligotrophic North Pacific Ocean. Our results demonstrate a consistent photoenhancement of heterotrophic protein production rates. Light-stimulated bacterial production comprised a significant flux of carbon in the upper ocean increasing secondary production rates by 1.4-1.8-fold throughout the photic zone, and resulting in carbon fluxes equivalent to 6% of the daytime primary production rate and fluxes approximately equal to the total particulate carbon export from the upper ocean. Experiments aimed at evaluating the response of heterotrophic protein production to irradiance revealed that rates were sensitive to low-light intensities, typically saturating below 0.2 mmol quanta m^-2 s^-1. Our results suggest that mixotrophy and photoheterotrophy may be important carbon fluxes in the upper ocean of the oligotrophic North Pacific gyre.