Microbial degradation of peptidoglycan in seawater

A constituent of the bacterial cell wall, peptidoglycan, has been suggested to be a significant fraction of marine dissolved organic matter, but little is known about its turnover. We measured hydrolysis and remineralization rates of peptidoglycan in surface waters by using radiolabeled peptidoglycan that was extracted from a Gram-negative bacterium. Polysaccharide (N-acetyl-glucosamine and N-acetyl-muramic acids) and peptide (D-glutamate) components of peptidoglycan were specifically radiolabeled. Purity of the preparations and specificity of labeling were ascertained by high-performance liquid chromatography. First-order kinetic constants of peptidoglycan remineralization were 2–21 times lower than those of proteins. The turnover time of peptidoglycan was estimated to be 10– 167 d, indicating its semilabile nature. The two main components of peptidoglycan are degraded differently; the remineralization rate of the peptide component was three times greater than that of the polysaccharide moiety. Chemically modified, low-molecular-weight material was produced during the degradation of the polysaccharide component, but not during degradation of protein. These results indicate that peptidoglycan is less degradable than proteins in marine environments and are consistent with observations that D/L-isomer ratios of amino acids increase during early diagenesis. A substantial fraction of marine dissolved organic matter (DOM), one of the largest reservoirs of organic carbon on earth (Hedges 2002), might be derived from bacteria (reviewed by Nagata 2000; see also Ogawa et al. 2001); concentrations of, especially, bacterial cell wall (peptidoglycan; McCarthy et al. 1998) and outer membrane components (porin; Tanoue et al. 1995) are relatively high in high-molecularweight DOM recovered from surface and deep waters. Cell wall and membrane constituents might be inherently harder to degrade because of their biochemical composition (McCarthy et al. 1998) or because of the matrix surrounding these macromolecules (Nagata and Kirchman 1992, 1997; Borch and Kirchman 1999). However, few studies have examined whether cell wall and membrane components are more refractory than other cellular constituents, such as protein, that contribute to the DOM pool (Nagata et al. 1998). The hypothesis that the bacterial cell wall component, peptidoglycan, is less degradable than proteins is supported by two lines of indirect evidence: (1) concentrations of Damino acids, fundamental components of peptidoglycan, are high relative to L-amino acids in DOM (Lee and Bada 1977; 1 To whom correspondence should be addressed. Present address: Center for Ecological Research, Kyoto University, Kamitanakami Hiranocho, Otsu, Shiga 520-2113 Japan ([email protected]. ac.jp). 2 Present address: Alfred-Wegener-Institute for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany.