I conducted my investigations focusing on microbial degradability of proteinaceous materials in seawater for the better understanding of biogeochemical cycles in marine environments.

Bacteria are not only the consumer of protein but also the important source of it. I measured carbon, nitrogen, and protein contents of natural bacterial assemblages in various marine environments. On the average, oceanic bacteria contained 12.4 fg C, 2.1 fg N and 8.0 fg of protein per cell, whereas coastal bacteria contained 30.2 fg C, 5.8 fg N and 18.4 fg of protein. Protein contribution was similar between the oceanic and coastal bacteria: it accounts 32% and 49% of total C and N, respectively, indicating that protein is the largest constituent of bacterial nitrogen.

About 40% of cell protein was located in membrane of Vibrio and Synechococcus strains. Coastal bacterial assemblages degraded it 2.8-fold more slowly than the soluble protein, and twice more slowly than the purified membrane protein. It reveals that membrane protein is sterically protected by membrane matrix against bacterial enzymatic hydrolysis and it is a possible source of dissolved protein accumulated in seawater. The comparison of microbial degradability between colloidal (10-100 nm) DOM (COM) and low-molecular-weight (LMW; <10 nm) DOM indicated that coastal bacterial assemblages consumed proteins in LMW-DOM more intensively than that in COM, suggesting that association with macromolecules decreases microbial degradability of proteins.

L-leucine-4-methyl-coumarinylamid hydrochloride (Leu-MCA) is the most commonly used peptide-like analog in aquatic environments. For the fist time, my study demonstrated the possible correlation between bacterial activity of hydrolyzing of Leu-MCA (LAPase) and that of purified proteins, which indicates that LAPase represents proteases. LAPase and beta-glucosidase activities in the subarctic Pacific revealed a systematic shift in vertical pattern of the their ratio along the east-west axis. Bacterial cell specific LAPase activity in the upper 100 m was positively correlated with the concentration of dissolved zinc in seawater, suggesting that the deficiency of dissolved zinc reduced bacterial production of LAPase and facilitated high export of proteinaceous material to the mesopelagic zone in the eastern subarctic Pacific.

These findings are crucial to understand carbon and nitrogen cycles in marine environments, because proteinaceous material is the largest identified pool in seawater.