Degradation of Collidal Organic Matter in Seawater

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In order to better understand the carbon cycle in the upper oceans, we examined degradation by natural bacterioplankton of membrane and soluble proteins prepared from the marine bacterium, Vibrio alginolyticus, radiolabeled with ^3H-or ^<14>C leucine. First order kinetic constants indicated that proteins in crude mambrane extract are degraded at significantly slower rates than the soluble proteins. Proteins determined to be intimately associated with the membrane were not degraded over two days, while a substantial fraction of soluble proteins was degraded during the same period. The data are consistent with a model that membrane and cell wall materials severely restrict access of bacterial proteases to membrane proteins. Our data support the hypothesis that macromolecular organic complexes play a role in temporary storage of dissolved organic carbon (DOC) in seawater. The results are also consistent with recent finding that bacterial membrane protein accumulates in seawater. The formation of a large pool of refractory DOC in seawater may influence carbon cycling in the ocean and changes in atmospheric concentrations of carbon dioxide, the most important greenhouse gas on our planet.