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Dimethylsulfoniopropionate (DMSP) plays important roles in oceanic carbon and sulfur cycling and may significantly impact climate. It is a biomolecule synthesized from the methionine (Met) pathway and proposed to serve various physiological functions to aid in environmental stress adaptation through its compatible solute, cryoprotectant, and antioxidant properties. Yet, the enzymes and mechanisms regulating DMSP production are poorly understood. This study utilized a proteomics approach to investigate protein changes associated with salinity-induced DMSP increases in the model sea-ice diatom Fragilariopsis cylindrus (CCMP 1102). We hypothesized proteins associated with the Met-DMSP biosynthesis pathway would increase in relative abundance when challenged with elevated salinity. To test this hypothesis axenic log-phase cultures initially grown at a salinity of 35 were gradually shifted to a final salinity of 70 over a 24-h period. Intracellular DMSP was measured and two-dimensional gel electrophoresis was used to identify protein changes at 48 h after the shift. Intracellular DMSP increased by approximately 85% in the hypersaline cultures. One-third of the proteins increased under high salinity were associated with amino acid pathways. Three protein isoforms of S-adenosylhomo-cysteine hydrolase, which synthesizes a Met precursor, increased 1.8- to 2.1-fold, two isoforms of S-adenosyl Met synthetase increased 1.9- to 2.5-fold, and S-adenosyl Met methyltransferase increased by 2.8-fold, suggesting active methyl cycle proteins are recruited in the synthesis of DMSP. Proteins from the four enzyme classes of the proposed algal Met transaminase DMSP pathway were among the elevated proteins, supporting our hypothesis and providing candidate genes for future characterization studies.

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.