Distribution and turnover of dissolved DMSP and its relationship with bacterial production and dimethylsulfide in the Gulf of Mexico

We measured the distribution of particulate and dissolved pools of the phytoplankton osmolyte dimethylsulfoniopropionate (DMSP) in the euphotic zone at a series of shelf (<40 m total water depth) and oceanic (>500 m depth) stations in the northern Gulf of Mexico. We also measured turnover rates of the dissolved DMSP pools (DMSPd) with tracer additions of S-35-DMSPd and short-term (<1 h) incubations, with the aim of examining the relationship between DMSPd turnover and bacterial production. Particulate DMSP concentrations were relatively low (<25 nM) throughout the study area with about twofold higher mean concentration at the shelf sites (15 nM) compared with the oligotrophic oceanic sites (7 nM). DMSPd concentrations averaged 3.0 nM in shelf waters and 1.3 nM in oceanic waters. Concentrations of dimethylsulfide (DMS), a degradation product of DMSP, also were low throughout the Gulf, averaging 2.0 nM for all depths sampled and 2.5 nM in surface waters. Microbial assemblages metabolized S-35-DMSPd with the sulfur being incorporated into biomass, volatile compounds (DMS and methanethiol), and other dissolved products. DMSPd turnover was relatively slow (mean of 3.8 nM d(-1)) in oligotrophic oceanic waters and averaged 10-fold higher (39 nM d(-1)) in mesotrophic shelf waters. DMS concentrations ranged from 0.2 to 5.1 nM in oceanic waters and appeared to be weakly related to DMSP turnover. In contrast, DMS concentrations in shelf waters fell within a narrow range (0.8-2.8 nM) and showed no relationship at ail with DMSPd turnover. DMSPd turnover rates were high enough to sustain the measured concentrations and estimated turnover of DMS, even if the conversion efficiency of DMSPd into DMS was only 10%. DMSPd turnover was significantly correlated with bacterial production (as measured by H-3-thymidine incorporation) and we estimate that DMSPd turnover contributed a mean of 3.4% of the carbon and similar to 100% of the sulfur required for bacterial growth in Gulf of Mexico surface waters. In addition to its role as a precursor of DMS, DMSP deserves attention as an important substrate for bacterioplankton in the euphotic zone.