Coral Bleaching: Genotypic and Photosynthetic Diversity in Symbiotic Algae
It is well publicized that coral reefs world-wide are undergoing severe declines, a consequence of increasingly frequent episodes of ocean warming during the past few decades. Our laboratory has focused in recent years on understanding the genotypic and physiological diversity of the symbiotic algae in corals and their related cnidarian species in reefs around the globe.
Most conspicuously, loss of coral pigmentation reflecting the death or retarded growth of colonies, is due to reduced densities or nearly complete elimination of symbiotic algae (dinoflagellates; Symbiodinium sp.) associated with their hosts. Toward a greater comprehension of the phenomena that are increasingly decimating the ocean's "rain forests", we are pursuing when and why corals and their symbionts respond differentially to global climate changes. Our collaborative projects with the laboratory of Bill Fitt of the UGA Institute of Ecology, therefore are directed to finding the causes for coral bleaching in studies of the responses of dinoflagellates to increased temperature (exacerbated by global warming) and light stress (exacerbated by ozone layer depletion and clear, shallow water environments).
From PCR-based denaturing gradient gel electrophoresis (DGGE) and DNA sequencing analyses, we now have a rather comprehensive grasp that there is considerable symbiont genotypic variation that largely corresponds with host-specificities, and often temperature-stress susceptibility vs. resistance, in the Caribbean, the western Atlantic coast, Hawaii and much of the Pacific.
We also have documented that certain symbiotic dinoflagellate species exhibit greater sensitivity to stress conditions while distribution of most symbiont genotypes also correlates with high light and elevated temperature tolerance. This conjecture is substantiated long-term monitoring various reef sites, mostly in the Caribbean, using non-invasive measurements of photosynthetic rates but also seasonal sampling of tissue pigmentation, photosynthetic capacity and tissue accretion. Importantly, there are occasional shifts in predominant symbiont subtypes, but this varies among coral hosts.
In parallel with the coral reef studies, we utilize an extensive collection of dinoflagellates in the laboratory that have been cultured from various coral and other cnidarian sources; these are essential for detailed studies of gene expression and photosynthetic responses to controlled environmental changes. Through laboratory studies, we can reconstruct stress conditions to further dissect causative factors involved in bleaching. So far, we now know loss of dinoflagellate viability at elevated temperature closely correlates with rapid declines of photosynthetic activity. The deterioration stems from inefficient repair mechanisms, including impaired synthesis of photosynthetic components due to insufficient mRNA levels or impaired protein synthesis.
Finally, we are extremely excited to have developed new photosynthesis measurement methods that have revealed novel photosynthetic protection mechanisms that are prominent in among certain, stress-resistant, species of Symbiodinium.
Dr. Schmidt's long-term research interest have been to understand processes involved in the formation and maintenance of photosynthetic complexes of plants and algae. His work pioneered understanding how nuclear-encoded proteins are transported into chloroplasts and integrated into photosynthetic membranes. Other research has concerned how regulation of formation of multisubunit complexes is achieved, finding that it involves intricate processes that includes gene expression and subsequent control mechanisms (such as post-translational mRNA stabilization, RNA precursor maturation, translational regulation and post-translational protein stabilization and protein phosphorylation mechanisms). His laboratory also invented methods for reconstruction of the light-harvesting antenna complexes for photosynthesis from their purified apoproteins and purified chlorophylls and carotenoids. With molecular mutagenesis techniques, the latter work enabled mapping the proteins’ pigment binding sites, identification of the important roles in light absorption efficiency due to protein-protein interactions, and elucidation of light energy transfer/photoprotection pathways.
Laboratory Personnel and Their Major Projects:
LaJeunesse, T.C., W.K.W Loh, R.van Woesik, O. Hoegh- Guldberg, G.W. Schmidt, and W.K. Fitt. 2003. Low symbiont diversity in southern Great Barrier Reef corals relative to those of the Caribbean. Limnology and Oceanography 48: 2046-2054.
LaJeunesse, T.C., D.J. Thornhill, E.F. Cox, F.G. Stanton, W.K. Fitt, and G.W. Schmidt. 2004. High diversity and host specificity observed among symbiotic dinoflagellates in reef coral communities from Hawaii. Coral Reefs 23:596 - 603.
LaJeunesse, T.C., R. Bhagooli, M. Hidaka, L. de Vantier, T. Done, G.W. Schmidt, W.K. Fitt, and O. Hoegh-Guldberg. 2004. Differences in relative dominance between closely related Symbiodinium spp. In coral reef host communities across environmental, latitudinal, and biogeographic gradients. Mar. Ecol.-Prog. Ser. 284:147-161.
Thornhill, D.J., LaJeunesse, T.C., Kemp, D.W., Fitt, W.K. and G.W.Schmidt. 2006. Multi-year, seasonal genotypic surveys of coral-algal symbioses reveal prevalent stability or post-bleaching reversion. Mar. Biol. 148: 711-722.
Thornhill, D.J., M.W. Daniel, T.C LaJeunesse, B.U. Bruns, G.W. Schmidt and W.K. Fitt. 2006. Natural infections of aposymbiotic Cassiopea xamachana scyphistomae from environmental pools of Symbiodinium. J. Exp. Mar. Biol. Ecol. 338: 50-56
Thornhill, D.J., T.C. LaJeunesse, D.W. Kemp, W.K. Fitt and G.W. Schmidt. 2006. Highly stable symbioses among Western Atlantic brooding corals. Coral Reefs 25: 515-519
Kemp, D. W., W. K. Fitt and G. W. Schmidt. 2008. A microsampling method for genotyping coral symbionts. Coral Reefs 27: 289-293.
LaJeunesse, T. C., H. R. Bonilla, M.E. Warner, M. Wills, G.W. Schmidt, and W.K. Fitt. 2008. Specificity and stability in high latitude eastern Pacific coral-algal symbioses. Limnology and Oceanography 53: 719-727.
Thornhill, D. J., D.W. Kemp, B.U. Bruns, W.K. Fitt and G.W. Schmidt. 2008. Correspondence between cold tolerance and temperate biogeography in western Atlantic Symbiodinium (Dinophyta). J. Phycology. 44: 1126 – 1135.
Long, T. A., Y, Okegawa, T. Shikanai, G.W. Schmidt and S.F. Covert. 2008. Conserved role of PROTON GRADIENT REGULATION 5 in the regulation of PSI cyclic electron transport. Planta. http://dx.doi.org/10.1007/s00425-008-0789-y
McCabe-Reynolds, J.C., Bruns B.U., Fitt, W.K. and G.W. Schmidt. 2008. Enhanced photoprotection pathways in symbiotic dinoflagellates of shallow-water corals and other cnidarians. Proc. Natl. Acad. Sci. USA 105: 13674-8.
McCabe-Reynolds, J.C., Bruns B.U., Fitt, W.K. and G.W. Schmidt. Prevalence of non-linear photosynthetic electron transport in symbiotic dinoflagellates of shallow-water cnidarians in hospice. Comparative and Integrative Biology (invited paper, in review)
Additional peer-reviewed publications from the G.W. Schmidt laboratory and grant support.
LaJeunesse, T.C. 2002. Diversity and community structure of symbiotic dinoflagellates from Caribbean coral reefs. Mar. Biol. 141: 387-400.
Warner, M.E., G.C. Chilcoat, F.K. McFarland and W.K. Fitt. 2002. Seasonal fluctuations in the photosynthetic capacity of photosystem II in symbiotic dinoflagellates in the Caribbean reef-building coral Montastraea. Mar. Biol. 141: 31-38.
Pochon, X., T.C. LaJeunesse and J. Pawlowski. 2004. Biogeographic partitioning and host specialization among foraminiferan dinoflagellate symbionts (Symbiodinium; Dinophyta). Mar. Biol. 146: 17-27.
LaJeunesse, T.C. 2005. "Species" radiations of symbiotic dinoflagellates in the Atlantic and Indo-Pacific since the Miocene-Pliocene transition. Mol Biol Evol 22: 570-581.
LaJeunesse, T.C., G. Lambert, R.A. Andersen, M.A. Coffroth, and D.W. Galbraith. 2005. Symbiodinium (Pyrrhophyta) genome sizes (DNA content) are smallest among dinoflagellates. J. Phycol. 41: 880-886.
LaJeunesse, T.C., S. Lee, S. Bush, and J.F. Bruno, 2005. Persistence of non-Caribbean algal symbionts in Indo-Pacific mushroom corals released to Jamaica 35 years ago. Coral Reefs 24: 157-159.
Kemp, D.W., C.B. Cook, T.C. LaJeunesse, and W.R. Brooks, 2006. A comparison of the thermal bleaching responses of the zoanthid Palythoa caribaeorum from three geographically different regions in south Florida. J. Exp. Mar. Biol. Ecol. 335: 266-276.
Todd, B.D., D.J. Thornhill, and W.K. Fitt. 2006. Patterns of inorganic phosphate uptake in Cassiopea xamachana: a bioindicator species. Marine Pollution Bull. 52, 515-521.
Barneah, O., I. Brickner, M. Hooge, V.M. Weis, T.C. Lajeunesse, and Y. Benayahu. 2007. Three party symbiosis: acoelomorph worms, corals and unicellular algal symbionts in Eilat (Red Sea). Mar. Biol. 151: 1215-1223.
Hunter, R.L., T.C. LaJeunesse, and S.R. Santos. 2007. Structure and evolution of the rDNA internal transcribed spacer (ITS) region 2 in the symbiotic dinoflagellates (Symbiodinium, Dinophyta). J. Phycol. 43: 120-128