Physiological roles of dimethylsulfoniopropionate (DMSP), DMSP lyase, dimethylsulfide (DMS) and dimethylsulfoxide (DMSO) in phytoplankton

Dimethylsulfide (DMS) is a biogeochemically important sulfur gas that is emitted from the ocean and can potentially affect global climate. The main source of marine DMS is from the enzymatic cleavage of dimethylsulfoniopropionate (DMSP), which is produced by phytoplankton. Both phytoplankton and bacteria produce enzymes that cleave DMSP into DMS and acrylic acid. DMS may be oxidized to dimethylsulfoxide (DMSO) in seawater, and the concentration of DMSO is normally much higher than that of DMS. Thus, DMSO production is also a very important factor affecting DMS cycling. The aim of this dissertation research was to examine factors that affect DMSP lyase activity (DLA) and the production of DMSP, DMS, and DMSO in phytoplankton cultures and natural seawater. UV radiation did not affect chlorophyll a (Chl a) normalized concentrations of DMS and particulate DMSO (DMSOp) in natural water and Amphidinium carterae, but reduced particulate DMSP (DMSPp):Chl a in A. carterae cultures. Thalassiosira pseudonana grown in nitrogen-limited media showed an increase of intracellular DMSP concentrations after the growth peak, while DMSP concentration did not increase in UV-exposed cultures after 60 hours of exposure. When the oligotrophic Sargasso Sea water was amended with 1 muM ammonium and 62.5 nM phosphate, Chl a showed a 13-fold increase, while Chl a-normalized DMS, DMSP, and DLA decreased substantially. The results of these experiments suggest that nutrient concentrations and biomass normalized DLA, DMS and DMSP concentrations appeared to be negatively correlated. DLA was measured at various locations in the North Atlantic Ocean, Southern Ocean and Ross Sea during the period 2002 and 2005. Depth profiles of DLA:Chl a in the Sargasso Sea and the Southern Ocean showed higher DLA:Chl a near the surface and lower ratios at depth while the Gulf of Maine and Ross Sea Polynya did not show any distinct patterns of DLA:Chl a with depth. The Ross Sea had the highest surface DLA per unit volume of seawater (7.80 nM min-1), however, surface DLA:Chl a was 5 to 20 times higher in the oligotrophic Sargasso Sea than any other oceanic water sampled. Evidence supports a role for nutrient concentrations and light intensity as important factors affecting DLA. These observations support a role for DMSP and its enzymatically cleaved products as antioxidants in phytoplankton cells. However, not all DLA variations can be explained by light and nutrients and it is possible that these sulfur compounds have functions other than as antioxidants.