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Published on October 20, 2006; 10.1104/pp.106.088963

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Received September 1, 2006
Accepted October 16, 2006

Localization and Role of Manganese Superoxide Dismutase in a Marine Diatom

Felisa Wolfe-Simon *, Valentin Starovoytov , John R. Reinfelder , Oscar Schofield , and Paul G. Falkowski

Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901
Department of Cell Biology and Neuroscience, Rutgers University, Piscataway, NJ 08854
Department of Environmental Science, Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901
Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901; Department of Geological Science, Rutgers University, Wright Geological Laboratory, 610 Taylor Road, Piscataway, NJ 08854

* Corresponding author; email: fwolfe{at}asu.edu.

Superoxide dismutase (SOD) catalyzes the transformation of superoxide to molecular oxygen and hydrogen peroxide. Of the four known SOD isoforms, distinguished by their metal cofactor (Fe, Mn, Cu\Zn, Ni), MnSOD is the dominant form in the diatom Thalassiosira pseudonana. We cloned the MnSOD gene, sodA, using the expression vector pBAD, overexpressed the product in Escherichia coli and purified the mature protein (TpMnSOD). This recombinant enzyme was used to generate a polyclonal antibody in rabbit that recognizes MnSOD in T. pseudonana. Based on quantitative immunoblots, we calculate that in vivo concentrations of TpMnSOD are approximately 0.9 amols per cell using the recombinant protein as a standard. Immunogold staining indicates that TpMnSOD is localized in the chloroplasts, which is in contrast to most other eukaryotic algae (including chlorophytes and embryophytes) where MnSOD is localized exclusively in mitochondria. Based on the photosynthetic Mn complex in photosystem II, cellular Mn budgets cannot account for 50-80% of measured Mn within diatom cells. Our results reveal that chloroplastic MnSOD accounts for 10-20% of cellular Mn, depending on incident light intensity and cellular growth rate. Indeed, our analysis indicates that TpMnSOD accounts for 1.4 (±0.2)% of the total protein in the cell. The TpMnSOD has a rapid turnover rate with an apparent half-life of 6-8 h when grown under continuous light. TpMnSOD concentrations increase relative to chlorophyll, with an increase in incident light intensity in order to minimize photosynthetic oxidative stress. The employment of a Mn-based SOD, linked to photosynthetic stress in T. pseudonana, may contribute to the continued success of diatoms in the low-Fe regions of the modern ocean.




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