Biological Consumption of Dimethyl Sulfide in the Marine Euphotic Zone: Results of Radioisotope Experiments

Abstract

The flux of dimethyl sulfide (DMS) to the marine atmosphere, hypothesized to have an important climatic effect via modulation of cloud albedo, depends critically on the rapidity of biological cycling of DMS in the water column, which may be much more rapid than its turnover by air—sea exchange. We developed and tested a radioisotope method to measure microbial DMS consumption using 14C-labeled DMS. Results from whole-water incubations in the NE Pacific and Puget Sound, Washington show that CO2 and particulates (cell material) are the primary products and are typically produced in ratios of about 2: 1, suggesting methylotrophic use of DMS as a carbon source, although specific DMS consumers have not yet been isolated from ocean surface waters. Because of low specific activity and low in situ [DMS], a quasi-kinetic approach was required, and estimates of in situ DMS consumption ranged from <0.1 nM d—1 to 0.5 nM d—1, yielding biological turnover times of 3-70 d. In the NE Pacific, rates were consistently lower than those calculated from a chloroform-inhibition method, and the radioisotope method most likely provides a lower bound on the true consumption rates. However, even the lower estimates of biological DMS consumption rates are faster than typical loss to outgassing, indicating that most DMS produced in the upper water column is consumed within the water column.