Symbiotic dinoflagellate algae residing inside coral tissues supply the host with the majority of their energy requirements through the translocation of photosynthetically fixed carbon. The algae, in turn, rely on the host for the supply of inorganic carbon. Carbon must be concentrated as CO2 in order for photosynthesis to proceed, and here we show that the coral host plays an active role in this process. The host-derived symbiosome membrane surrounding the algae abundantly expresses vacuolar H(+)-ATPase (VHA), which acidifies the symbiosome space down to pH ∼ 4. Inhibition of VHA results in a significant decrease in average H(+) activity in the symbiosome of up to 75% and a significant reduction in O2 production rate, a measure of photosynthetic activity. These results suggest that host VHA is part of a previously unidentified carbon concentrating mechanism for algal photosynthesis and provide mechanistic evidence that coral host cells can actively modulate the physiology of their symbionts.

Coral calcification is intricately linked to the chemical composition of the fluid in the extracellular calcifying medium (ECM), which is situated between the calcifying cells and the skeleton. Here we demonstrate that the acid-base sensing enzyme soluble adenylyl cyclase (sAC) is expressed in calcifying cells of the coral Stylophora pistillata. Furthermore, pharmacological inhibition of sAC in coral microcolonies resulted in acidification of the ECM as estimated by the pH-sensitive ratiometric indicator SNARF, and decreased calcification rates, as estimated by calcein labeling of crystal growth. These results indicate that sAC activity modulates some of the molecular machinery involved in producing the coral skeleton, which could include ion-transporting proteins and vesicular transport. To our knowledge this is the first study to directly demonstrate biological regulation of the alkaline pH of the coral ECM and its correlation with calcification.

Calcium carbonate (CaCO₃) is abundant on Earth, is a major component of marine biominerals and thus of sedimentary and metamorphic rocks and it plays a major role in the global carbon cycle by storing atmospheric CO₂ into solid biominerals. Six crystalline polymorphs of CaCO₃ are known-3 anhydrous: calcite, aragonite, vaterite, and 3 hydrated: ikaite (CaCO₃·6H₂O), monohydrocalcite (CaCO₃·1H₂O, MHC), and calcium carbonate hemihydrate (CaCO₃·½H₂O, CCHH). CCHH was recently discovered and characterized, but exclusively as a synthetic material, not as a naturally occurring mineral. Here, analyzing 200 million spectra with Myriad Mapping (MM) of nanoscale mineral phases, we find CCHH and MHC, along with amorphous precursors, on freshly deposited coral skeleton and nacre surfaces, but not on sea urchin spines. Thus, biomineralization pathways are more complex and diverse than previously understood, opening new questions on isotopes and climate. Crystalline precursors are more accessible than amorphous ones to other spectroscopies and diffraction, in natural and bio-inspired materials.