Biogeochemistry and physiology of bleached and recoverying Hawaiian and Caribbean corals

Coral reefs are declining globally due to a combination of direct and indirect human impacts. Much of this decline can be attributed to prolonged exposure to elevated sea surface temperatures which induces coral bleaching – a process whereby corals lose their endosymbionts and/or their endosymbiotic pigments resulting in corals that appear pale or white. Corals have extremely different responses to bleaching events: some corals bleach and die, others bleach and recover, and some do not visibly bleach at all. In the absence of abundant photosynthetically fixed C, corals may rely on one or more of the following strategies to sustain themselves and promote recovery: (1) catabolize stored energy reserves, including lipids, carbohydrates, and/or proteins, (2) reduce respiration rates, (3) decrease skeletal growth, (4) increase heterotrophy or (5) shuffle or change their endosymbiont type(s). Although mounding species of coral have been shown to survive bleaching events in greater abundance than branching species, the underlying mechanism(s) for mounding coral resilience is unknown. Furthermore, controlled bleaching and recovery experiments coupled with detailed carbon budgets that incorporate autotrophy and multiple heterotrophic sources (i.e. zooplankton and dissolved organic carbon) do not exist for Caribbean corals. Therefore, two controlled tank experiments, one in Hawaii and the other in Puerto Morelos, Mexico were conducted to understand the bleaching and recovery responses in the Hawaiian coral Porites lobata and the three Caribbean corals Montastraea faveolata, Porites astreoides, and Porites divaricata. Four major findings were observed: 1) Bleaching resilience in the mounding coral P. lobata is due to it harboring a thermally tolerant endosymbiont type combined with an ability to actively metabolize zooplankton acquired C and utilize DOC as a significant fixed C source, 2) Bleached P. astreoides were capable of meeting greater than 100% of metabolic demand by increasing feeding rates, 3) All Caribbean corals took up dissolved organic carbon as a source of fixed carbon when bleached, 4) M. faveolata and P. astreoides are more resilient to single bleaching than P. divaricata. This study represents the first comprehensive assessment of the underlying traits that confer bleaching resilience in a mounding Hawaiian coral. Furthermore, these studies represent a comprehensive physiological and biogeochemical analysis of bleached and recovering Caribbean corals and the first to detail their carbon budgets. The ability for bleached corals to maintain high photosynthetic capacity coupled with the ability to utilize exogenous C sources appears to be an underlying theme in resilience to bleaching. Based on these findings, species such as P. lobata, M. faveolata, and to a lesser extent P. astreoides are more likely to survive a single bleaching event than the branching P. divaricata or the previously studied branching coral P. compressa.