Antioxidants, another use for DMSP and its associates

DMSP is known to act as an osmoprotectant, cryoprotectant and in some cases a grazing deterrent. Interestingly, DMSP and its derivatives may have another use, as antioxidants which protect against oxidative stressors such as high light levels. Sunda et al., (2002) investigated this by testing DMSP reactivity with the free radical OH^. and production under various oxidative stressors in Emiliania huxleyi and the diatom Thalassiosira pseudonana.

OH^. radicals were used to oxidise DMS to form DMSO, MSNA and MSA in a step-wise manner. DMSP also oxidised in a similar way. Reaction rates with OH^. and cellular concentrations were worked out for DMSP and MSNA and compared to information from the literature. It was discovered that DMSP reacts with OH^., but Acrylate and DMS are 20-60x more reactive. DMS also reacts with singlet oxygen and is lipid soluble, so could protect photosynthetic membranes. Therefore, cleavage of DMSP by DMSP lyase could increase protection. DMSO and MSNA also reacted, and with its low solubility DMSO could concentrate to high levels in the cell.

For the response of production in the face of oxidative stressors, CO₂ and Iron limitation experiments were firstly conducted in bottles with nutrient enriched seawater. CO₂ limitation cultures were grown to high cell volumes to reduce CO₂ by 40 times. For Iron limitation, cells were transferred from a medium containing iron to one without. Cultures were grown and harvested regularly for measurements of DMSP, DMS, total cell volume, chlorophyll a and APX activities (an enzyme associated with antioxidant defence) in T. pseudonana. Under both CO₂ and iron limitation, T. pseudonana showed a 20-60x increase in DMSP, increased DMS/cell-volume ratios, and elevated APX activities with reduced growth and chlorophyll a levels, indicating they were under oxidative stress. E. hux showed a similar trend. It would also have been interesting to directly quantify synthesis of DMSP lyase, to examine if it was upregulated.

For experiments using UV light, the control culture was grown under fluorescent light (no UV), whereas the UV treatments were grown in 30% sunlight and exposed to full UV, UV with (more harmful) UV-B filtered or with (almost) all UV-A and UV-B filtered. Total DMS, total DMSP, cell volume and cell chlorophyll a were measured. DMSP and DMS production was higher in sunlit treatments relative to controls, due to exposure to UV light. Interestingly, DMS and DMSP production peaked when UVB was filtered but UVA was unfiltered, rather than at full exposure. The authors put this down to production being underestimated, as increased production may be balanced by consumption of DMSP and DMS and also photolysis of DMS. 

DMS and DMSP production in response to UV highlights a negative feedback mechanism which directly benefits algae. Increased UV exposure will lead to higher DMS release, causing increased cloud formation above the bloom, so shielding the bloom from UV light and oxidative stress. Although, I think the lower DMS and DMSP levels found at the highest exposures should examined closely for the idea to be fully vindicated. It could also be interesting to look at more recent publications to see if Symbiodinium in coral show similar responses and whether this impacts the ability of coral to cope with oxidative stress and so bleaching. 

Sunda, W. K. D. J., Kieber, D. J., Kiene, R. P., & Huntsman, S. (2002). An antioxidant function for DMSP and DMS in marine algae. Nature, 418, 317-320.