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Environmental context Cobalamin, or vitamin B12, is receiving increased attention as a critical trace nutrient in the growth and metabolic processes of oceanic phytoplankton and bacterial communities. We present evidence that indicates B12 has a more significant role in the biogeochemical cycling of the climatically important compounds dimethylsulfide and dimethylsulfoniopropionate than previously understood. Several possible mechanisms are examined that link cellular-level processes involving B12 to global-scale biogeochemical processes involving the oceanic cycling of dimethylsulfoniopropionate and dimethylsulfide. Abstract Evidence is presented showing that dissolved dimethylsulfoniopropionate (DMSPd) and dimethylsulfide (DMS) concentrations are influenced by the availability of vitamin B12 in two oceanographically distinct regions with different DMS production capacities, the central equatorial Pacific Ocean and the Ross Sea, Antarctica. In both locations, addition of B12 to incubation experiments resulted in decreases in DMS and, in some cases, DMSPd concentrations relative to unamended controls. In no case did increasing iron availability significantly (α=0.1) alter DMS concentrations relative to controls. The relative decreases in DMS between B12 addition and control experiments were significant (α=0.1) in five of seven experiments conducted at ambient iron levels. Overall, DMS concentrations were on average 33.4% (±15.1%; 1 standard deviation) lower, relative to unamended controls, by the end of incubation experiments when B12 was added. Declines in DMSPd were observed in three of five experiments. Similar trends were observed when B12 was added to iron-supplemented bottle incubation experiments (30.4±10.4% lower final DMS concentrations in +B12Fe treatments relative to +Fe treatments). Several possible molecular-level explanations exist for this link between B12 and DMS production, including potential B12 dependence of methyltransferase enzymes involved in both DMS and DMSP degradation. Although the enzymology of these reactions remains unclear, the relationships described here provide evidence for plausible mechanisms behind the microbial modulation of oceanic DMS.

Environmental context Dimethylsulfoniopropionate (DMSP), a small sulfur compound biosynthesised by algae, plays an important role in global climate, particularly in polar regions. We investigated salinity effects on DMSP levels, and provide the first experimental measurements of DMSP and associated physiological changes in a polar diatom across to a range of gradual salinity shifts representative of sea-ice conditions. Quantitative estimates of DMSP in polar diatoms following salinity changes will facilitate new mathematical models to predict seasonal responses and reactions to climate change. Abstract Although extreme environmental gradients within sea-ice have been proposed to stimulate dimethylsulfoniopropionate (DMSP) accumulation in diatoms, a taxa whose temperate counterparts show relatively low concentrations, this has yet to be experimentally validated across a range of salinities representative of sea-ice conditions. The present study examined changes in DMSP concentrations in the widespread polar diatom Fragilariopsis cylindrus in response to gradual salinity shifts representative of those encountered during sea-ice formation and melt. DMSP concentrations were elevated up to 127% in 70-salinity cultures. Low-salinity shifts decreased intracellular DMSP concentrations in a gradient-dependent manner that suggests DMSP recycling rather than release under milder hyposalinity shifts. Permeable membranes were detected in ~45% of 10-salinity cells; therefore, loss of membrane integrity may only partially explain DMSP release in the lowest-salinity group. Growth rates, photosynthetic efficiency of photosystem II and reactive oxygen species detection indicated only partial impairment by salinity stress in this organism. Thus, experimental evidence supports the role of DMSP as a compatible solute in the acclimation of a sea-ice diatom across large salinity gradients and measurements of associated physiological changes will improve interpretation of environmental measurements.