The Diversity and Bleaching Responses of Zooxanthellae in Kenyan Corals.

Abstract

Zooxanthellae of the genus Symbiodinium. the dinoflagellate endosymbioms. of many benthic cnidarians. are phylogenetically diverse. Molecular analyses of ribosomal RNA genes indicate multiple Symbiodimum species in 7 known phylotypes, A-G. The diversity of Symbiodirlium in corals from Kenya and sea anemones from the Mediterranean Sea was investigated by molecular methods. Symbiudinium in Kenya comprise phy[otype A, C and D zooxanthellae that occur pan-tropically. The majority of Mediterranean Symbiodinium comprise a distinct group of 'temperate A' zooxanthellae that may be regionally endemic. The zooxanthellal chloroplast psbA gene, encoding the D1 protein of photosystem II, was sequenced. The psbA and nuclear 248 rRNA gene trees were congruent. Resilience, i.e. the capacity for zooxanthelJae 10 recover after bleaching, to bleaching induced by elevated temperature and darkness was investigated in Porites cylindrica. Resilience was assessed by changes in zooxanthellal densities on termination of stressor. Resilience was influenced by the nature and duration of stressor. Zooxanthellae in corals subjected to relatively long durations of darkness were more resilient than those in corals treated for shorter durations. The opposite trend was evident for zooxanthellae in corals exposed to elevated temperature. The basis for these contrasting results may lie in different endodennal processes during treattnent with the two stressors. The recovery profile of corals that bleached on the reef was similar to those experimentally hleached using elevated temperature. No detectable changes in the molecular identity of zooxanthellae occurred on recovery. Porites cylindrica recently recovered from experimenlally induced bleaching and bleaching induced by natural stressors were subjected to a repetition of bleaching stressors to explore their capacity for acclimation, i.e. the development of resistance to bleaching stressors under laboratory conditions. Bleaching responses were not significantly affected by prior experience of bleading stressor. The relevance of these experiments on coral resilience and acclimation to field bleaching events is discussed.