Physiology and biogeochemistry of bleached and recovering corals from Hawaii
Coral bleaching, the loss of endosymbiotic zooxanthellae and/or chlorophyll a (chl a), results in corals that appear white. Photosynthesis and photosynthetically fixed carbon translocated to the host decreases, so bleached corals may need to rely on energy reserves to recover. Impact on coral host physiology is not well understood. Porites compressa and Montipora capitata were experimentally bleached (treatment corals) with 30°C seawater for one month. Additional fragments were maintained at ambient temperatures (∼27°C; control corals). Fragments recovered on the reef for eight months. Buoyant weight, photosynthesis, respiration, quantum yield of photosystem II (Fv/F m), chl a, zooxanthellae, total lipid, lipid class, carbohydrate, and protein concentrations were analyzed at 0, 1.5, 4, and 8 months of recovery. After bleaching, treatment corals of both species were white, had lower chl a, Fv/Fm), energy reserves, and photosynthesis relative to control corals. Decreased total lipids of both species reflected decreased storage and structural lipids. Respiration and basal metabolic demand remained unchanged in treatment relative to control corals of both species. P. compressa replenished total lipid, carbohydrate, and protein reserves after chl a and photosynthesis recovered. M. capitata replenished total lipid, carbohydrate, and protein reserves while chl a and photosynthesis remained low. To verify these physiological changes, stable carbon and oxygen isotopes of the skeleton (δ13Cs; δ18O s) and nitrogen and carbon isotopes of host tissue (δ15 Nh; δ13Ch) and zooxanthellae (δ15Nz; δ13Cz) were analyzed. δ13Cs and δ18 Os of both species reflected decreased calcification and metabolic fractionation, resulting in values approaching equilibration with seawater, rather than changes in photosynthesis or seawater temperature. Thus, their use as a proxy for bleaching events is limited. δ15N h reflected the presence of zooxanthellae, while δ15N z reflected dissolved inorganic nitrogen incorporation for mitotic division and chl a recovery. δ13Ch and δ13Cz indicated that M. capitata switched from photoautotrophy to heterotrophy during recovery, while P. compressa was solely photoautotrophic. Overall, P. compressa relied on stored energy reserves to survive bleaching, while M. capitata also acquired a significant amount of fixed carbon from heterotrophy. Strategies employed by the coral host may be more important for recovery from bleaching than genetic differences in the zooxanthellae.