A novel inhibitory interaction between dimethylsulfoniopropionate (DMSP) and the denitrification pathway

Dimethylsulfoniopropionate (DMSP) is an abundant organic sulfur compound in marine algae and denitrification influences nitrogen availability to primary producers, the key regulators of coastal eutrophication. In this study, we tested the effect of DMSP on the nitrous oxide (N 2O) reduction step of denitrification in sediments and biofilms from the Douro and Ave estuaries (NW Portugal) and in pure cultures of a denitrifying bacterium, Ruegeria pomeroyi. N 2O accumulation rates were monitored in sediment slurries and bacterial cell suspensions amended with DMSP concentrations ranging from 0 to 5 mM. In these treatments N 2O accumulation rates increased linearly with DMSP concentration (R 2 from 0. 89 to 0. 99, p < 0. 001), suggesting an inhibitory effect of DMSP on the nitrous oxide reductase activity. The addition of DMSP to sediments and bacterial culture resulted in accumulation of dimethylsulfide (DMS) as well as N 2O. However, no direct inhibition on N 2O reductase activity by DMS was observed. Natural concentrations of DMSP in the different estuarine sites were found to be linearly correlated to natural N 2O effluxes (R 2 = 0. 64, p < 0. 001), suggesting that DMSP may negatively affect N 2O reductase in situ. This newly identified interaction between DMSP and N 2O emissions may have a significant ecological role as the inhibition of the nitrous oxide reduction enhances nitrogen loss via N 2O. Since N 2O is a powerful greenhouse gas, the results from our study may be important for evaluating climate change scenarios.