Coral calcification mechanisms across a natural environmental mosaic in Hawai'i

Coral calcification is key to coral reef growth and function but may be compromised under increasing global and local stressors. Corals modify the carbonate chemistry of their calcifying fluid to facilitate calcification, but little is known about how these mechanisms vary across the substantial differences in reef seawater conditions that can occur over as little as a few kilometers. Here, we used boron-based geochemical proxies (δ¹¹B, B/Ca) to investigate how three common Hawaiian coral species (Montipora capitata, Porites compressa, Porites lobata) regulate the carbonate chemistry of the calcifying fluid along a natural environmental mosaic of seawater carbonate chemistry and significant wave height. We found that calcification mechanisms were governed by complex species and site interactions: while all species generally differed from each other in their calcifying fluid chemistry, they also responded differently to site-specific environmental conditions. These results highlight that there are varying degrees of calcification mechanism plasticity in response to changing environmental conditions. Furthermore, species-specific patterns of pH upregulation inside the calcifying fluid were good predictors of calcification responses to ocean acidification and warming in at least two of the three species, with M. capitata being a clear winner under future ocean conditions. Our findings provide important insights into how corals calcify across a natural environmental mosaic and highlight the differential potential for an adaptive capacity in calcification mechanisms in the face of intensifying climate change.