DIFFERENTIAL METABOLISM OF DIMETHYLSULFONIOPROPIONATE AND ACRYLATE INSALINE AND BRACKISH INTERTIDAL SEDIMENTS

In anoxic Spartina alterniflora-dominated sediments along a naturally occuring salinity gradient (the Cooper River estuary, South Carolina, U.S.A.), dimethylsulfoniopropionate (DMSP) was metabolized to dimethyl sulfide (DMS) and acrylate by sediment microbes. The race of DMSP deg radation and acrylate mineralization by sediment microbes was similar at all sites along this 25-km transect. However, sediments amended wit h acrylate (or DMSP) showed significantly higher rates of N-2 fixation (measured as acetylene reduction activity) (ARA) in the saline sedime nts downstream than brackish sediments. These results are consistent w ith the fact that acrylate stimulated the rates of both denitrificatio n and CO2, production in the saline sediments at the mouth of the rive r more than tenfold over rates in brackish sediments. Enrichment exper iments indicate that microbes capable of using DMSP or acrylate were n ot present in upstream sediments despite the fact that microbial bioma ss, percent organic matter, and both glucose-stimulated ARA and denitr ification were highest upstream. It appears that acrylate utilizing, N -2 fixing, and denitrifying populations are insignificant in the lower salinity sediments of the estuary. These results may reflect the avai lability of DMSP, which averaged 10.3 nmol g wet wt(-1) of saline sedi ments and levels less than our detection limit (1 mu M) in brackish se diments.