Impact of Vitamin B12 and Nitrate on Transcript and Metabolite Abundances in Marine Diatoms.

Phytoplankton play countless roles in the support and regulation of marine ecosystems, as well as in global biogeochemical cycling processes. They are also, to varying extents, reliant on other physical and biological processes to supply their nutrient demands, such as the production of vitamin B₁₂ by bacteria and archaea or the regeneration and upwelling of nitrate. One such process in the global biogeochemical sulfur cycle is the pathway that begins with the production of dimethylsulfoniopropionate (DMSP) by marine phytoplankton and leads to the atmospheric formation of sulfate-based cloud condensation nuclei, which contribute to the Earth’s albedo. Nutrient limitation is thought to play a major role in the amount of DMSP produced by phytoplankton. Vitamin B₁₂ and nitrate are of particular interest due to their involvement as a co-factor and nitrogen source, respectively, in the synthesis of methionine, the precursor for DMSP. Laboratory-based nutrient limitation experiments have been performed on cultures of the diatom Phaeodactylum tricornutum. In addition to the B₁₂-dependent methionine synthase (MetH) P. tricornutum has a unique B₁₂-independent methionine synthase gene (MetE). Based on classic techniques, B₁₂ limitation had little impact on cell growth, whereas nitrate limitation had a significant effect on both culture health and DMSP concentration. Yet, targeted transcriptomic analysis (using Nanostring nCounter technology) and metabolomics analysis (using Nuclear Magnetic Resonance (NMR)) revealed complex changes in transcript abundance towards upregulated gene expression associated with the MetE gene in B₁₂ limited cultures, and shifts away from nitrogen-based metabolites towards DMSP in nitrate-limited cultures. These experiments help verify the role of B₁₂ in DMSP production and link the underlying metabolic pathways that drive the cellular portion of the sulfur cycle to ecosystem and global scale processes.