Temperature induced bleaching in the mesophotic coral Alveopora ocellata

Mesophotic coral ecosystems (MCEs) are found between 30 and 150 m and are characterized by low-light conditions. Those underexplored reefs are considered less affected by anthropogenic disturbances, wave action, and climate change due to their depth and distance from shore.

In the past decades, mass coral bleaching events (associated with elevated temperatures) caused the loss of ~75% of corals globally. The grave status of shallow reefs led to the suggestion that mesophotic corals may possibly replenish shallow reefs in the case of severe loss or serve as a temporary refuge.

Mesophotic corals present a remarkable fluorescence polymorphism, attributed to fluorescent proteins. As fluorescence was previously suggested to serve as a potential indicator for coral health, we aimed to examine the effect of elevated temperatures on highly-fluorescent (HF) and low-fluorescent (LF) morphs of the mesophotic coral Alveopora ocellata. We found that A. ocellata did bleach after a short-term (14 days) thermal stress of +4°C above ambient sea temperature. Under higher temperature, corals gradually lost their algal symbionts (7.7X10⁶ to 14X10⁶ cells per cm²), cellular photosynthetic pigments (6.6X10^-7 to 3.4X10^-7 µg chlorophyll a) and presented reduced maximal quantum yield (F_v/F_m). Throughout the experiment, HF corals presented 44 ± 6 % lower zooxanthellae densities, 117 ± 33% higher cellular chlorophyll a concentration, and six-fold higher fluorescence. The differences found between the morphs may indicate that fluorescence is unfavorable under thermal stress. Furthermore, this demonstration of a bleaching process in a mesophotic coral, demonstrates the vulnerability of MCEs to the predicted global warming.