Regulation of internal pH by the coldwater coral Desmophyllum dianthus

In the Patagonian fjords of Chile, large aggregations of the coldwater coral Desmophyllum dianthus build the structural and functional basis for a highly diverse benthic ecosystem. Interestingly, D. dianthus growths in both, high-pH (aragonite-supersaturated) and low-pH (aragonite-undersaturated) waters in near-surface and deep waters, respectively. This indicates a high adaptability of these corals to regulate and control calcification. Measurements of the skeletal boron isotopic composition (d¹¹B) in D. dianthus indicate an upregulation of the internal calcifying pH (pH_cf) in response to external pH (pH_sw) in culturing experiments simulating ocean acidification. A physiological underpinning of pH_cf upregulation in corals under different pH_sw is, however, so far lacking. Direct measurements at the site of calcification in corals are limited to a few studies on tropical corals. Comparable studies for coldwater corals are wanting. We used microsensors for pH, calcium and oxygen to assess pH_cf in D. dianthus in relation to calcium dynamics and respiration along the coral polyp under different pH_sw. We found pH_cf to be linked to pH_sw but no upregulation of pH_cf compared to pH_sw as well as a strong spatial heterogeneity in pH_cf. This suggests a highly complex pH regulation inconsistent with the hitherto upregulation models and suggests that rather the internal carbon pool and not pH is upregulated to enable calcification in D. dianthus.