The role and evolution of superoxide dismutases in algae

Superoxide is a natural byproduct of normal cellular functions and can obliterate key cellular components. Therefore it is important for a cell to efficiently detoxify superoxide. Superoxide dismutases catalyze the destruction of the superoxide radical (O2·-) into molecular oxygen and hydrogen peroxide. There are four known varieties of SODs, distinguished by their metal cofactor. These are the iron (Fe), manganese (Mn), copper-zinc (CuZn) and nickel, (Ni) forms. SODs play a particularly important role in photosynthetic organisms because they have two sources of radical oxygen: photosynthesis and respiration. This dissertation focuses on the physiology and molecular phylogeny of SODs in marine secondary red plastid-derived algae.; Photosynthetic eukaryotes have evolved from a singular primary endosymbiotic event where a heterotrophic cell engulfed and enslaved a cyanobacteria which eventually evolved into the photosynthetic plastid. These primary endosymbiotic eukaryotes diverged into the green and red plastid lineages. The secondary red plastid containing algae are the most successful in the modern ocean. Typically, all photosynthetic eukaryotes express nuclear-encoded FeSOD within the plastid. However, laboratory results suggest that the secondary red diatom, Thalassiosira pseudonana, possesses a MnSOD in the plastid, which can account for approximately 20% of the total Mn in a cell. This helps explain the unaccounted for Mn budget in diatoms and represents a unique use for MnSOD which may contribute to the success of diatoms in the modern ocean which is chronically limited for Fe.; The molecular phylogeny of the Fe- and MnSOD family was examined to further understand the phylogenetic position of secondary red algae with respect to the SODs. Because these enzymes are localized within organelles, the genes have likely derived from the original organelle endosymbionts. These data suggest that for all plastid-derived eukaryotes, FeSOD evolved from the primary endosymbiotic event and was transferred to the nucleus from the plastid because cyanobacteria and green sulfur bacteria are the last common ancestor for all of these organisms. This includes currently amitochondrial or aplastidic parasites that have subsequently lost these organelles. The MnSOD sequences show monophyly for primary and secondary red algae with a basal relationship to the primary green alga. Because green plants cluster separately with the cyanobacteria, this suggests that the MnSOD of primary green and primary and secondary red plastid eukaryotes evolved from the primary endosymbiotic host cell (i.e., last common ancestor for these groups) and the green plants derived MnSOD from the primary endosymbiont plastid. Thus, the Fe and MnSOD protein histories suggest alternative evolutionary pathways with FeSOD tracing the original endosymbiotic event and MnSOD describing multiple secondary events.