Mitochondrial electron transport activity and metabolism of experimentally bleached hermatypic corals

• Change to Metabolism and mitochondrial electron transport system activities are reduced for experimentally bleached corals.
• Reduction in biomass and/or oxidative damages lead to the reduced mitochondrial electron transport system activities.
• Mitochondrial electron transport system activities can be used as a tool for coral health assessment.

Bleached corals (Porites cylindrica and Galaxea fascicularis) were obtained through extended incubation (over 45 days) under light depletion and privation: low light and dark conditions, and heat stress (32 °C). The colonies in the different treatments became bleached and had reduced metabolic rates, photosynthesis, calcification and respiration; reduced biomass, zooxanthellae density, and chlorophyll a concentrations; and reduced mitochondrial electron transport system activity, which represent potential respiration rates. The most important reduction in mitochondrial electron transport activity was shown when the activities were normalized by the unit of surface and not by the unit of host protein. This result indicates that the reduction in activity could be mainly explained by the reduction of biomass and tissue thickness. However, increased Manganese Superoxide dismutase (Mn SOD) activity, a mitochondrial SOD, suggests that ROS production occurs in the mitochondria under heat stress with the consequence of potentially damaging the electron transport system. The reduced calcification rates observed are hypothesized to be the results of a decrease in the energy available for calcification due to the reduced photosynthetic rates, limiting the availability of substrates for respiration and therefore the energy production, and the decreased in the number of active mitochondrial electron transport system. Electron transport system activity associated with respiration is the basis of all metabolic processes and is not biased by incubation like traditional measurements of respiration in an aquarium. Therefore, ETSA could be used as an overall indicator of coral health, especially for host animal health.