IMPACT OF VITAMIN B12 AND NITRATE ON DIMETHYLSULFONIOPROPIONATE (DMSP) CONCENTRATIONS IN <i>PHAEODACTYLUM TRICORNUTUM</i> AND <i>THALASSIOSIRA PSEUDONANA</i>

Abstract

Phytoplankton play countless roles in the support and regulation of marine ecosystems, as well as in global biogeochemical processes. One such process in the global sulfur cycle is the pathway, which is hypothesized to begin with the production of dimethylsulfoniopropionate (DMSP) by marine eukaryotic phytoplankton and result in the formation of sulfate-based cloud condensation nuclei that contribute to the Earth’s albedo. This concept is known as the CLAW hypothesis, but many of the intermediate steps, including DMSP production, are poorly understood. Nutrient limitation is thought to play a major role in the amount of DMSP produced by controlling the metabolic pathways. Vitamin B12 and nitrate are of particular interest due to their involvement in the synthesis of methionine, the precursor for DMSP, as a co-factor and nitrogen source, respectively. In culture experiments, B12 reduction had little impact on DMSP concentration of <i>Thalassiosira pseudonana</i> and <i>Phaeodactylum tricornutum</i>, while nitrogen reduction caused a significant increase in DMSP concentration. <i>T. pseudonana</i> also showed a further increase in DMSP concentration associated with the B12/nitrate reduction interaction. Nanostring gene expression analysis showed that DMSP was most highly correlated with the production and recycling of S-adenosylmethionine, not the B12 regulated methionine synthase. The increase in DMSP concentrations associated with reduced nitrogen supported the concept of a switch to sulfur-containing osmolytes to DMSP under low nitrogen conditions. These experiments showed that methionine recycling through SAM synthesis, as opposed to <i>de novo</i> methionine synthesis, were an important pathway in DMSP production. The results of this study help link the underlying metabolic pathways that help combat oxidative stress prompted by reduced nutrient conditions and those that drive the cellular portion of the sulfur cycle.