Bioreactivity of peptidoglycan in seawater

The components of bacterial peptidoglycan (PG), D-amino acids and muramic acid, have been identified as constituents of marine dissolved organic matter (DOM), suggesting that PG is a possible component of the recalcitrant DOM. However, little is known about the bioreactivity of PG directly released from bacterial cells. We conducted an incubation experiment on marine bacteria and examined the degradation processes of PG and protein released from bacterial cells using C-13 as a tracer. We used D-Ala for an indicator of PG, and L-Ala and L-Val for protein. Most PG released from bacterial cells degraded immediately, but a small portion remained at the end of the incubation experiment (240 d), accounting for 1.1% of maximum particulate PG in bacterial cells. Protein was more bioreactive than PG by one order of magnitude. The D:L ratio of Ala released from bacterial cells increased as the diagenetic stage progressed, indicating that this ratio is a useful indicator of bioavailability for dissolved organic compounds. The recalcitrant bulk organic carbon released from bacterial cells accounted for 1.8-4.8% of the bacterial organic cellular carbon. Our results suggest that PG is more stable than protein, but more bioreactive than bulk dissolved organic carbon (DOC).