Effects of bleaching on oxygen dynamics and energy metabolism of two Caribbean coral species.

As mass coral bleaching events become more frequent, it is increasingly important to elucidate the factors underlying coral susceptibility and survival. We measured photosynthesis, respiration, and O2 concentration at the coral tissue surface, Symbiodiniaceae genotypes, and energy metabolic enzyme activities in Agaricia agaricites and Orbicella franksi throughout experimentally-induced thermal bleaching (+3 °C). A. agaricites colonies started to bleach two days into the thermal treatment and were fully bleached between Days 19-31. In contrast, O. franksi colonies only started to bleach on Day 12 and five colonies fully bleached between Days 24-38 while the remining three colonies took up 55 days. Both species experienced decreased photosynthesis and respiration rates as bleaching progressed. As a result, daytime O2 concentration at the coral surface shifted from hyperoxia in unbleached corals to normoxia in partially bleached corals, and to near hypoxia in fully bleached corals. Additionally, nighttime tissue surface O2 concentration shifted from hypoxia to normoxia, likely resulting from decreased symbiotic algae density, respiration, and photosynthates that fuel coral aerobic respiration. Genetic profiling of internal transcribed spacer 2 (ITS2) revealed differences in Symbiodiniaceae clade proportions between control and bleached colonies. Activity levels of energy metabolic enzymes did not significantly vary between control and bleached A. agaricites, but malate dehydrogenase and strombine dehydrogenase activities were significantly higher in bleached O. franksi colonies compared to controls. These differences were driven by the three O. franksi colonies that took the longest to bleach and contained >98 % Durusdinium sp. D1. The shifts in O2 dynamics within the microhabitat of bleached corals may have important implications for the metabolism of the coral holobiont while the changes in Symbiodiniaceae ITS2 profile and the upregulation of energy metabolic enzymes identify a potential factor contributing to bleaching dynamics.

Author Correction: Measuring light scattering and absorption in corals with Inverse Spectroscopic Optical Coherence Tomography (ISOCT): a new tool for non-invasive monitoring.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Measuring light scattering and absorption in corals with Inverse Spectroscopic Optical Coherence Tomography (ISOCT): a new tool for non-invasive monitoring.

The success of reef-building corals for >200 million years has been dependent on the mutualistic interaction between the coral host and its photosynthetic endosymbiont dinoflagellates (family Symbiodiniaceae) that supply the coral host with nutrients and energy for growth and calcification. While multiple light scattering in coral tissue and skeleton significantly enhance the light microenvironment for Symbiodiniaceae, the mechanisms of light propagation in tissue and skeleton remain largely unknown due to a lack of technologies to measure the intrinsic optical properties of both compartments in live corals. Here we introduce ISOCT (inverse spectroscopic optical coherence tomography), a non-invasive approach to measure optical properties and three-dimensional morphology of living corals at micron- and nano-length scales, respectively, which are involved in the control of light propagation. ISOCT enables measurements of optical properties in the visible range and thus allows for characterization of the density of light harvesting pigments in coral. We used ISOCT to characterize the optical scattering coefficient (µs) of the coral skeleton and chlorophyll a concentration of live coral tissue. ISOCT further characterized the overall micro- and nano-morphology of live tissue by measuring differences in the sub-micron spatial mass density distribution (D) that vary throughout the tissue and skeleton and give rise to light scattering, and this enabled estimates of the spatial directionality of light scattering, i.e., the anisotropy coefficient, g. Thus, ISOCT enables imaging of coral nanoscale structures and allows for quantifying light scattering and pigment absorption in live corals. ISOCT could thus be developed into an important tool for rapid, non-invasive monitoring of coral health, growth and photophysiology with unprecedented spatial resolution.