Collaborative Research: Production and dynamics of DMSP and related compounds in response to oxidative stress in marine phytoplankton

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Oxidative stress is a pervasive problem for oxygen-evolving plants, and is likely to be especially important for marine phytoplankton growing in nutrient-impoverished waters with high fluxes of visible and ultraviolet (UV) solar radiation. Very little is known about how marine algae cope with oxidative stress. In this study, a new hypothesis is presented that dimethylsulfoniopropionate (DMSP) is a unique and very important antioxidant in the ocean that serves as a primary defense in combatting and alleviating oxidative stress. Its role as an antioxidant is likely because DMSP is the dominant cellular sulfur compound in, and a major organic constituent of many marine algae worldwide. Furthermore, preliminary results indicate that DMSP and its enzymatic lysis product, DMS are highly effective scavengers of toxic reactive oxygen species (e.g., hydroxyl radicals) in cells, removing these species faster, in some cases, than the well documented antioxidants, ascorbic acid and glutathione. DMSP and DMS oxidation yield dimethylsulfoxide (DMSO), which is also an effective oxidant scavenger as is acrylate, the other product of DMSP lysis. Together, these related compounds should serve as a multifunctional, highly flexible antioxidant system in DMSP-containing algae. To investigate this DMSP antioxidant system, the following related hypotheses are being tested by this research team: 1) Intracellular DMSP concentrations in phytoplankton will increase in response to chronic oxidative stress, and will decrease in response to acute oxidative stress. 2) Turnover of cellular DMSP is induced by oxidative stress, resulting in increased production of putative antioxidants. 3) Phytoplankton with high DMSP content and high DMSP lyase activity will be more resistant to oxidative stress than phytoplankton with low DMSP or low lyase activity. To test these hypotheses, axenic cultures of ecologically-important phytoplankton species and natural seawater populations are being exposed to various forms of oxidative stress, including high photon fluxes of visible light (400-700 nm) and/or UV radiation (290-400 nm), nutrient limitation (Fe and N), and addition of paraquat or copper. During these exposures, changes in cellular DMSP and related compounds will be determined along with DMSP lyase activity. Complementary measurements of other antioxidant defense systems (e.g., ascorbate, reduced and oxidized glutathione, ascorbate peroxidase, superoxide dismutase) will provide information on how the DMSP system varies in relation to these well-established antioxidant defenses. This project will elucidate the mechanisms that control DMSP levels in the ocean and its rate of conversion to DMS, which is globally-significant because of the important role of DMS in atmospheric chemistry and, possibly, climate. If the DMSP antioxidant hypothesis is correct, then an important, hitherto unknown, cellular function of DMSP will have been identified. Understanding the cellular physiology of DMSP and its relationship to other poorly-understood algal antioxidant systems will increase our understanding of the factors that control the distribution of ecologically-important phytoplankton in the sea. This study will also result in the interdisciplinary training of advanced undergraduate and graduate students in algal physiological ecology and oceanography. The PIs and students will disseminate results of this study through scholarly and public presentations, scientific journals, popular articles and freely-accessible web pages.