Effects of Increased Temperature and CO2 on Dimethylsulfoniopropionate (DMSP) Concentration in Symbiodinium microadriaticum and Associated Changes in Methionine Synthase Activity
McLenon, Amanda Lorraine
DiTullio, Giacomo R.
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Dinoflagellate algae of the genus Symbiodinium occur as endosymbionts in a variety of hosts including coral. Oxidative stress has been linked to a breakdown in this relationship, known as bleaching. The predicted increases in atmospheric pCO₂ will be accompanied by increased sea surface temperature and ocean acidification. The response of Symbiodinium spp. to environmental changes can dictate survival of their hosts and could contribute to the ecological success of coral reef ecosystems. Preliminary studies established high concentrations of the sulfur compound dimethylsulfoniopropionate (DMSP) in Symbiodinium spp. with increased temperature. This study examines the potential of DMSP as an antioxidant in the algae, Symbiodinium microadriaticum (CCMP1633) isolated from a cnidarian host. Specifically, the synergistic effects of increased temperature and CO₂ were investigated. An HPLC assay for the activity of the enzyme B12-dependent methionine synthase was modified for use with algal cultures and used to determine whether production of methionine, a precursor to DMSP, was a potential point of regulation in an oxidative stress response. Increased CO₂ did not have a significant effect on any parameters measured. Yet DMSP was found to increase in cultures exposed to elevated temperature (33ºC), which correlated with an increase in reactive oxygen species (ROS), a drop in photosynthetic efficiency of photosystem II (PSII), and evidence of the photoprotective mechanism known as xanthophyll cycling. Cells also increased significantly in biovolume, and SYTO-BC stain indicated increased DNA content, suggesting arrested cell division. Methionine synthase activity did not correlate to DMSP concentration. This may be due to turnover of DMSP to other antioxidant compounds, inhibition of methionine synthase by nitric oxide, or switching to the B12-independent form of methionine synthase. The findings of this study provide insight into the responses of algal symbionts to environmental changes, and shed light on the potential use of DMSP and other known photoprotective mechanisms under oxidative stress.