| Summary: | Anthropogenic activities release CO2 to the atmosphere, increasing the CO2 dissolved in the ocean. This process causes a change in seawater carbonate chemistry, lowering the pH and the carbonate saturation state (Ω) and affecting marine organisms that build calcium carbonate shells and skeletons. In the last decade, an increasing concern has grown regarding the effects of ocean acidification on calcifying organisms such as foraminifera and corals. The majority of these studies are laboratory experiments or short-term field observations which do not fully reflect the complexity of natural environments and long-term ecosystem scale responses. This dissertation aims to predict the response of foraminifera and corals under future higher concentrations of atmospheric CO2 by studying populations of benthic foraminifera and transplanted corals in natural low-pH, low-Ω springs, which constitute an in-situ ocean acidification laboratory.Chapter 2 explores the impact of Ω on benthic foraminifera abundance. Foraminiferal total abundance was lower at low-Ω springs than at control sites, although symbiont-bearing and non-calcareous foraminifera were less sensitive to ocean acidification conditions than calcareous symbiont-barren foraminifera. Chapters 3 and 4 explore the impact of Ω on coral survival, physiology and gene expression. Nubbins of Porites astreoides, Siderastrea siderea and Porites porites originating from low-Ω springs, and from ambient-Ω lagoon and reef settings were collected and transplanted to a low-Ω spring and to an ambient-Ω control site for two years. The physiological analyses suggest that slow-growing corals with high concentrations of Symbiodiniaceae, chlorophyll a and protein may be more resilient to ocean acidification conditions, although with reduced skeletal density. The transcriptome analysis of P. astreoides after one year of transplantation showed that genes involved in lipid metabolism, bicarbonate transportation and skeletal remodeling are differentially expressed at low-Ω spring transplantation site. These results suggest that corals transplanted to low-Ω spring have higher energy demands to counter the larger difference between the external seawater Ω and the internal Ω at the calcification site. The results of this dissertation indicate that impacts of ocean acidification on benthic foraminifera and corals are species-specific, although a general negative impact on calcification and increased energy requirements are observed under low Ω conditions.
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