Abstract
It has been suggested that reduced biogenic sulfur compounds, in particular dimethylsulfide (DMS), could account for a significant flux of sulfur from the oceans to the atmosphere. In order to investigate this hypothesis, and to ascertain the biological source of DMS, we have determined its concentration in surface and deep sea water at a number of stations, and compared the results with indicators of biological activity, e.g., the concentration of chlorophyll a. We have observed a close correlation between the concentration of DMS in seawater and indicators of algal activity, both in their geographical distribution and in vertical profiles through the water column. The concentration of DMS in seawater shows a sharp decline near the base of the euphotic zone; only very low levels of DMS are present in the deep ocean. The concentrations in the surface ocean average 50 ng S(DMS)$\text{l}^{-1}$. From this concentration, an annual flux of 36 Tg S$\text{yr}^{-1}$from the oceans to the atmosphere can be estimated. Besides DMS, we have observed the presence of other dissolved organosulfur compounds in sea-water, among them methylmercaptan, carbon disulfide, and dimethyl disulfide. These compounds occur in significantly lower concentrations than dimethylsulfide. We have found DMS to be produced in pure, axenic cultures of marine planktonic algae. This supports the suggestion (derived from the marine distribution of DMS) that it is produced by marine primary producers. We have also observed the release of very large amounts of DMS from coral organisms, especially under conditions of physiological stress.