CLIMATE CHANGE AND OCEAN ACIDIFICATION EFFECTS ON TROPICAL REEF MACROALGAE

Tropical marine macroalgae perform an essential role in coral reef function and health, however, their persistence in a rapidly changing ocean remains uncertain. The rise in sea surface temperatures and decrease in pH (ocean acidification = OA) are predicted to have damaging effects on marine calcifiers. Calcifying macroalgae have varied, often negative, responses to these conditions, however our lack of understanding about the mechanisms involved with calcification prevent us from interpreting these results fully. Thus, I conducted a series of experiments on five calcifying species, utilizing microsensors, radioisotopes, and mesocosms, in an attempt to define biotic and abiotic mechanisms involved in calcification and dissolution under OA. Microsensor work demonstrated that all species elevate the thalli surface pH 2-3X higher under OA, which promoted calcification. The use of a photosynthetic inhibitor revealed species-specific light-triggered thalli pH control that stimulated calcification, indicating strong biotic control over calcification. When exposed to OA conditions, stronger organismal control over calcification was shown to maintain calcification in the light. A major gap in our understanding of calcification under OA is whether it affects organismal capacity to form new calcium carbonate, or if dissolution occurs, reducing calcification rates. Using radioisotopes, I found that the ability to form new calcium carbonate under OA in the light was not affected in any species. This suggested that species with reduced net calcification were actually experiencing dissolution. This study also highlighted that all species were experiencing dissolution in the dark under OA. Finally, in a short-term growth experiment, I examined the combined effects of OA and increased temperature and found complex responses in species that are negatively affected by OA. This included a crustose coralline that appears to have an additive negative effect where temperature enhances the effect of OA but also a species that exhibited a negative effect which was evidently offset with increased temperature. Here, I define distinct abiotic (light, temperature, dissolution) and biotic (proton pump & photosynthesis), that are essential for understanding macroalgae persistence on future coral reefs.