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Aquatic Microbial Ecology

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AME 14:29-37 (1998)  -  doi:10.3354/ame014029

Degradation by bacteria of membrane and soluble protein in seawater

Toshi Nagata1,*, Rumi Fukuda1, Isao Koike1, Kazuhiro Kogure1, David L. Kirchman2

1Ocean Research Institute, University of Tokyo, 1-15-1 Minami-dai, Nakano, Tokyo 164, Japan
2College of Marine Studies, University of Delaware, Lewes, Delaware 19958, USA

In order to test the hypothesis that bacterial membrane protein is more slowly degraded than soluble protein in seawater, we examined degradation by natural bacterioplankton of membrane and soluble proteins prepared from the marine bacterium Vibrio alginolyticus radiolabeled with 3H- or 14C-leucine. First order kinetic constants indicated that proteins in crude membrane extract are degraded at significantly slower rates (1/2 to 1/6) than the soluble proteins. Proteins determined to be intimately associated with the membrane were not degraded during the initial 45 h, while a substantial fraction of soluble proteins was degraded during the same period. The data are consistent with a model in which membrane and cell wall materials severely restrict access of bacterial proteases to membrane proteins. After prolonged incubation, however, membrane proteins started to be degraded, suggesting that proteins protected by membrane components were made available for degradation after ectoenzymatic destruction of membrane components. Our data support the hypothesis that macromolecular organic complexes play a role in temporary storage of dissolved organic matter (DOM) in seawater and that complementary hydrolysis by different ectoenzymes produced by diverse bacterioplankton is important in determining rates and patterns of DOM degradation in the sea.

Protein degradation · Marine bacteria · Dissolved organic matter

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