Response of Coralline Algae to ocean acidification: the Influence of pH and Inorganic Carbon Speciation at the Site of Calcification

Chair: Janice Lough

Cornwall, C.E.(1,2), Comeau, S.(1,2), McCulloch, M.T.(1,2)

1 The University of Western Australia, School of Earth and Environment and UWA’s Ocean Institute , Crawley, Western Australia 6009, Australia
2 ARC Centre of Excellence in Coral Reef Studies, The University of Western Australia, Crawley, Western Australia 6009, Australia

Background
Coralline algae (red algae) are ecologically important, via forming/binding reefs and providing a settlement substrate for many ecologically and commercially valuable species (e.g. abalone, corals, etc). Coralline algae are susceptible to ocean acidification (OA) through declines net calcification rates, but exhibit considerable variation in their responses. It is possible that some of this variability in responses can be explained by physiological differences between species. Integral in the response of coral net calcification to ocean acidification is the species-specific capacity to elevate the pH of the calcifying fluid (pHcf) at the site of calcification; it is unknown whether the same principle applies to coralline algae. Here we ask: 1) do coralline algae elevate pHcf at the site of calcification in the cell wall; 2) how are changes in net calcification rates of coralline algae under OA related to seawater pHT and calcifying fluid pHcf.

Methods
We examined the responses of net calcification and pHcf to OA (pHT 7.80 and 7.60, control 8.07) during a 10 week experiment. Species were chosen that have large apparent differences in their pHcf determined using their skeletal carbonate δ11B values.

Findings
We find the majority of coralline algal carbonates have high δ11B values (25 to 34 ‰), implying high pHcf values (>8.5) if only the tetrahedral B(OH)41- is incorporated. However, coralline algae precipitate calcite, rather than aragonite like shallow water corals, meaning high δ11B trigonal B(OH)3 may also be incorporated. This raises the question of how B(OH)3 uptake occurs, and whether is it related to the species of carbon utilised during calcification.

Conclusions
We find that the δ11B methodology used to determine pHcf for coralline algae needs to be carefully assessed due to the possible incorporation of B(OH)3. Another question highlighted by this study is whether coralline algae utilise HCO3 or CO32- for calcification.