Abstract
Dimethylsulfide (DMS) is a climatically important trace gas produced by many marine phytoplankton species through the enzymatic cleavage of dimethylsulfonio-propionate (DMSP) by DMSP lyase. Although some algal species have high DMSP lyase activity, many species lack this activity even though they produce DMS. This observation raised an important question that formed the basis of this thesis: What is the source of DMS in marine algae that do not exhibit DMSP lyase activity? Two non-enzymatic pathways for DMS production in marine algae were identified. In the first pathway, DMS was produced from the enzymatic reduction of cellular dimethylsulfoxide (DMSO) in eight algal species that were tested, including Rhodomonas lens, which does not contain DMSP. An S-methionine sulfoxide reductase (MsrA) enzyme was proposed to be responsible for this activity in marine algae. In algae lacking DMSP lyase activity, DMSO reduction is expected to be one of the main sources of cellular DMS.
The second non-enzymatic pathway that was identified for the production of cellular DMS was through the oxidation of DMSP by reactive oxygen species, which are produced in all marine algae. Radical attack results not only in the direct production of DMS but also DMSO. Second-order rate constants for the formation of DMS and DMSO from the reaction of DMSP with the OH radical ranged from 0.5 - 1.2×108 M-1 s-1 and 1.1 - 1.3×108 M-1 s-1, respectively. In addition to examining production pathways, cellular DMS concentrations (DMSc) were constrained in several axenic marine algal cultures using published estimates of DMSc and literature models of membrane permeability. Based on this analysis, cellular DMS concentrations were estimated to range between 1 and 40 nM in the three algal species tested, which are much lower than the mM concentrations reported in the literature.
These results challenge our current understanding of the physiology of DMS in marine algae. The sources and losses of DMS in the cell do not provide for a sufficiently high concentration for DMS to act as an antioxidant, as has been previously suggested. Likewise, an alternative hypothesis suggesting that DMS acts as a prey deterrent is not supported by our constraints on DMSc concentrations. Additional understanding of the functional control of the various processes leading to the production and loss of DMS will ultimately lead to an understanding of the physiological role of DMS in marine algae.