Impact of Vitamin B12 and Nitrate Availability on the Concentration of Particulate Dimethylsulfoniopropionate in Phytoplankton

The production of particulate dimethylsulfoniopropionate (DMSP_p) was examined in natural phytoplankton communities from the South Atlantic Bight near Savannah, Georgia, during an expedition in June 2015. Vitamin B₁₂ and nitrate were added to seawater samples from a coastal and an oceanic site, both of which contained low-biomass, cyanobacteria-dominated communities. Under nitrate-limited conditions, irrespective of changes in B₁₂ levels, DMSP_p concentrations increased. DMSP_p concentrations of these mixed phytoplankton communities did not appear to be limited by the availability of B₁₂. In a laboratory experiment, DMSP_p concentrations in the diatom Phaeodactylum tricornutum were measured after the removal of vitamin B₁₂ and nitrate from a synthetic seawater culture media. DMSP_p concentrations increased under nitrate-limited conditions, irrespective of changes in B₁₂ levels, and are argued to be the result of increased biosynthesis. DMSP_p concentrations in P. tricornutum were unaffected by B₁₂ limitation. It is hypothesized that P. tricornutum is using the B₁₂-independent methionine synthase MetE to synthesize DMSP_p rather than the B₁₂-dependent methionine synthase MetH.