CO₂-driven ocean acidification (OA) decreases seawater pH and carbonate ion concentrations, which can impact the calcification and physiology of marine calcifiers. These organisms form calcium carbonate skeletons and shells from a specialized calcification fluid that is, to varying degrees, isolated from surrounding seawater. The carbonate structures serve as archives, preserving the chemical signature of the calcification fluid, which can be analyzed using geochemical proxies. In the following thesis, I examine how different taxa respond to future ocean changes by exposing them to predicted future acidification scenarios. Additionally, I aim to understand if an organism’s resilience to the impacts of ocean acidification is linked to their ability to regulate their calcification fluid chemistry using geochemical proxies.
In Chapter 1, I investigate the geochemistry of three reservoirs important for biomineralization – seawater, the extrapallial calcification fluid (EPF), and the shell – of two commercially important bivalve species: Crassostrea virginica and Arctica islandica to understand if the boron isotope proxy is probing calcification fluid pH. Additionally, I examined the effects of three ocean acidification conditions (ambient: 500 ppm, moderate: 900 ppm, and high: 2800 ppm CO2) on the calcification and chemistry of the calcification fluid of the same three reservoirs for C. virginica. Comparisons of seawater and extrapallial fluid geochemistry indicated that the EPF has a distinct composition that differs from seawater. Additionally, our OA experiments show that EPF chemistry is significantly affected by ocean acidification, demonstrating that the biological pathways regulating or storing these ions are impacted by ocean acidification. I also found that shell δ11B does not faithfully record seawater pH, but rather was correlated with EPF pH, despite an offset from in situ microelectrode pH measurements. However, the δ11B-calculated pH values were consistently higher than microelectrode pH measurements, indicating that the shell δ11B may reflect pH at a more localized site of calcification, rather than pH of the bulk EPF.
In Chapter 2, I investigate the effects of four different seawater pH levels (8.03, 7.93, 7.83, and 7.63) on seven complexes of temperate coralline algae collected from New Zealand. I examined the photophysiology, calcification, and geochemical proxies to probe the internal carbonate chemistry of seven different species of coralline algae under simulated end-of-century ocean acidification scenarios. Under ambient conditions we found clear physiological differences between branching and encrusting species. We found that OA treatments only had a significant effect on calcification of three of the seven species, Corallina berteroi, Corallina spp., and Jania “bottlebrush.” Additionally, OA only affected the calcification fluid pH (pHCF) of two species, decreasing pHCF for both Corallina beteroi and Jania “feather.” Nonetheless, for all species pHCF was constantly upregulated compared to seawater pH, indicating a strong control over calcifying fluid chemistry. My results underscore the high resilience of coralline algae calcification under the different end-of-century ocean acidification scenarios. This tolerance to OA is related to the species’ ability to maintain a stable carbonate chemistry to support calcification as seawater pH declined.
Lastly, in Chapter 3 I build upon my work in Chapter 2 and examine the effects of multiple stressors (OA, warming, and irradiance) on four species of coralline algae. I found that multiple stressors impacted calcification of each species differently, with two species exhibiting additive effects of each stressor, one synergistic, and one antagonistic. Nevertheless exposure to multiple stressors still resulted in the greatest decrease in calcification. Notably, pHCF remained stable across all treatments, highlighting the exceptional control over pHCF and resilience of coralline algae to both individual and combined environmental stressors. We found that the regulation of pHCF may have come at an energetic cost and could explain decreased calcification due to increased energy expenditure or lack of energy under both OA and warming/irradiance, respectively. Additionally, carbon concentrating strategies could explain why some species may have fared better than others, due to being able to make use of CO2 diffusion under OA conditions rather than other more costly concentrating mechanisms. Lastly, we found that warming/irradiance led to a decrease in photophysiology for geniculate coralline algae, which could partly explain the decrease in calcification for those species. The stability of calcification of these foundational species is particularly susceptible to the combination of multiple stressors that are projected to impact marine ecosystems in the future, which is an important consideration in the context of ecosystem management where local stressors may further contribute to the degradation of foundational reef species.
Overall, this dissertation advances our understanding of how future environmental change affects bivalves and coralline algae. It also emphasizes the critical role that the regulation of a species’ calcification fluid pH regulation has in conferring resilience to future ocean conditions.
Caraveo B. J. A., 2025. Using boron isotopes to examine calcification fluid pH changes in marine calcifiers under environmental change. PhD thesis, University of California, Los Angeles. 146 p. Thesis.
