Physiological differences between coral tips and bases during bleaching

Written for my Targeted Research Project while studying with Stanford's Overseas Studies Program in Australia through the Center for Marine Studies at the University of Queensland

We investigated the physiological differences between coral tips and bases during bleaching. Our research focused on three questions: (1) How do the tips and bases of coral branches physiologically differ during a bleaching event? (2) How do the tips and bases respond differentially to further heat stress? (3) How will the light pressure on photosynthesis differ between tips and bases, and before and after further heat stress? We found that initially the tips have less chlorophyll per zooxanthellae and less zooxanthellae than the bases. With further heat stress, both tips and bases lost zooxanthellae at equivalent rates. This suggests that tips and bases may have different starting points, either due to acclimation to light conditions or issues relating to coral growth. Our fluorescence measurements did not reveal a significant difference in light pressure either between tips and bases or before and after heat stress. We also found evidence of one colony (Colony 4) which responded to further heat stress by losing chlorophyll per zooxanthellae instead of the actual zooxanthellae cells. This suggests a different mechanism of bleaching, possibly brought about by genetic variation in the host and/or symbiont.

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