Stable carbon isotopic composition of amino sugars in heterotrophic bacteria and phytoplankton: Implications for assessment of marine organic matter degradation

Compound-specific isotope analysis has opened up a new realm for resolving the sources and transformation processes of marine organic matter. However, the stable carbon isotope patterns of amino sugars remain unknown. We examined delta 13C of amino sugars in marine phytoplankton and heterotrophic bacteria, and the variations in amino sugar delta 13C during 66-d planktonic organic matter degradation experiments, to investigate the metabolic sources and transformations of amino sugars by bacterial reworking. The delta 13C values of glucosamine (GlcN) and galactosamine (GalN) were comparable in heterotrophic bacteria (difference Delta delta 13CGlcN-GalN = 0.4-4.0 parts per thousand) but pronouncedly different in phytoplankton (Delta delta 13CGlcN-GalN = 4.3-16.6 parts per thousand), suggesting similar synthesis pathways of GlcN and GalN in bacteria that differed from phytoplankton. Compared to GlcN and GalN, bacteria preferentially use isotopically light organic compounds for muramic acid (MurA) synthesis. During simulated microbial degradation of organic matter, the delta 13C difference between GlcN and GalN decreased from 5.8 parts per thousand on the initial day to 1 parts per thousand at a late stage in the experiment, but the difference between GlcN and MurA remained at 5.3 parts per thousand. This difference is consistent with the pattern in cultured phytoplankton (average Delta delta 13CGlcN-GalN = 5.9 parts per thousand +/- 1.4 parts per thousand) and heterotrophic bacteria (average Delta delta 13CGlcN-MurA = 4.6 parts per thousand +/- 3.4 parts per thousand), indicating enhanced bacterial resynthesis as degradation proceeded. Based on the difference in delta 13C among GlcN, GalN, and MurA, we propose a novel index of variation in amino sugar delta 13C, representing amino sugar resynthesis, to describe the diagenetic state of organic matter. Together, these findings suggest that amino sugar delta 13C can be used as a new tool to track heterotrophic processes of marine organic matter.