Production process of refractory marine dissolved organic matter by marine bacteria ; Experimental analyses using bacteria-specific D-amino acids.

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Subjects in this study were to clear the biogeochemical process of production of marine dissolved organic matter(DOM), particularly that of refractory fraction(R-DOM), and to discuss the significance of DOM and /or R-DOM as an organic carbon reservoir on earth and its possible relation to global carbon change and recycle. An incubation experiment was carried out for the analyses of contribution of marine bacteria to DOM production, especially to R-DOM production, by tracing concentrations and composition of dissolved organic constituents, particularly those of bacteria-specific D-amino acids in culture samples, using combined methods of ^<13>C-tracer, GC, and GC/MS.Glucose(20mgC) labeled with ^<13>C and seawater(2l) of Tokyo Bay containing natural bacterial populations were added as a substrate and bacterial seeds to artificial seawater(18l) in polycarbonate bottle(20l). And then the solution was incubated for over 90days in the dark at 27℃. Aliquots of sample were recovered at interva … More ls 1-30days and filtered through a glass-fiber filter (Whatman, GF/F) so as to apply the filtrate to chemical analysis of DOM.Rapid increasing in concentration of dissolved amino acids, especially of alanine, and high values in ratio of D-alanine to L-alanine were found in early stage(1-4days) of incubation while the concentration of DOC and its initial ^<13>C/^<12>C ratio(^<13>C-atom%) strictly decreased, indicating most probable conversion (and/or consumption) of ^<13>C-glucose to other organic molecules and CO_2 by bacteria. It was obvious that DO^<13>C, ^<13>C-amino acids and D-amino acids, particularly ^<13>C-alanine and D-,L-alanine, were specifically produced by bacteria from ^<13>C-glucose during incubation. It was also revealed that no less than 1% of ^<13>C-DOC and D-alanine produced in early stage of incubation(up to 4days) remained stable even after 90days incubation, respectively.In previous study of Hama et al. 2004, about 5% of organic carbon produced initially by phytoplankton photosynthesis was observed to remain stable after 60days incubation in the dark, which was consisted of DOC and accounted for more over 60% of all organic carbon remained, having a relatively low molecular weight and more biorefractory natures.Form these results, it is obviously shown that major parts of marine R-DOM are produced by marine bacteria through their specific conversion from photosynthetic products of phytoplankton. It should be also enable to estimate a magnitude of contribution of bacteria to R-DOM production as a function of carbon isolation in concerning with the environmental importance of global carbon change and recycle. Less