SGER: DMSP-Lyase Activity and CO2 Limitation in Marine Phytoplankton

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Bicarbonate (HCO3) dominates the dissolved inorganic carbon pool in the surface waters of the ocean. While some algal species can utilize HCO3 directly, the major source of inorganic carbon driving phytoplankton photosynthesis in the marine environment is CO2. Recent work indicates that the availability Of CO2 is a potential limit on the growth of large marine diatoms. Diffusion of CO2 across the cell membrane occurs directly in some phytoplankton species while others utilize a transporter for HCO?3 uptake, with the HCO3 converted to CO2 by carbonic anhydrase (CA). Extracellular CA activity is found in some species while others have intracellular CA activity. Algal species such as diatoms are"CO2-users" and do not take up HC03 ions directly, requiring high ambient levels of CO2. Conversely, other species such as haptophytes can proliferate under conditions of low CO2. This difference in carbon acquisition strategy suggests a mechanism whereby the composition of phytoplankton species within a community may be controlled by carbon limitation. In general, diatoms are also poor producers of dimethylsulfoniopropionate (DMSP) and contain little or no DMSP?lyase. On the other hand, haptophytes are important producers of DMSP and have high DMSP lyase activity. The cellular function of DMSP and the lyase enzyme in algae is controversial. Breakdown of DMSP via the DMSP?lyase enzyme liberates dimethylsulfide (DMS), acrylic acid and protons. Dr. DiTullio hypothesizes that the release of protons via the lyase?mediated reaction allows those species with the DMSP?lyase enzyme to effectively utilize HCO3 under conditions of low CO2 by forcing the equilibria of the carbonate/bicarbonate system toward CO2. This allows these species to alleviate carbon limitation and dominate over non?DMSP?producing species in low CO2 environments. This laboratory?based project will test the hypothesis that DMSP production in haptophytes is positively correlated to carbon limitation.