DEEPLIFE scientific publication in The ISME Journal (2021)

A new vision of coral reefs - Coral reefs are one of the most productive and diverse ecosystems on Earth. Hard coral, which builds reefs, relies mainly on a symbiotic relationship with microalgae (zooxanthellae) to power, among other things, the energetic needs of calcification. During the symbiosis, the coral provides limited inorganic nutrients, while algae share essential organic compounds (e.g., sugars) derived from their photosynthetic activity. Thus, the symbiosis between reefbuilding corals (photosynthetic scleractinian corals) and microalgae underpins the health and productivity of tropical coral reef ecosystems.

Although coral ecosystems extend into the depths, this symbiotic relationship has mainly been studied in shallow waters (<30 m) because of technical limitations imposed by traditional scientific scuba diving. We do not know how deeper corals, inhabiting large and vastly underexplored mesophotic coral ecosystems, modulate their symbiotic associations to grow in environments that receive less than 1% of surface light.

To better understand how reef-building corals can survive so far away from their presumed light optimum, the DEEPHOPE program brough together the scientific expertise of an international consortium and the diving exploration expertise of UNDER THE POLE. This program made it possible, for the first time, to study of scleractinian corals on a wide depth gradient (6 to 172 m) and geographic scale (5 archipelagos of French Polynesia) in order to push the limits of our knowledge of corals.

This study allowed the discovery of the deepest photosynthetic hard coral at 172 m depth. The analyzes of symbiotic communities show that the coral, identified as Leptoseris hawaiiensis, survives at such depth by hosting Symbiodiniaceae, predominantly of the genus Cladocopium, but also a single species of endolithic algae from the genus Ostreobium. These algae in the coral skeleton are adapted to photosynthesize in near-darkness using shorter wavelengths compared to other zooxanthellae and optimize light capture. Morphologically, L. hawaiiensis is characterized by a thin flat skeleton, which is optimal for light harvesting and reducing skeletal carbonate deposition. Thus, this coral species uses different strategies to acclimate and survive in extreme low light conditions.

The present results report a new depth record for photosynthetic scleractinian corals at 172 m and provide a new vision of coral reefs. These ecosystems have been extensively studied in shallow waters, which represents less than one-fifth of the total depth range of the tropical coral reefs. Depth is a dimension that can no longer be ignored in future studies to understand coral ecosystems. A better knowledge of coral ecosystem functioning is necessary for the implementation of effective management and preservation measures of these threatened ecosystems.

Summary from the article "Symbiotic associations of the deepest recorded photosynthetic scleractinian coral (172 m depth)." 

Read the full article via the link below. 

Authors

Rouzé H., Galand P.E., Medina M., Bongaerts P., Pichon M., Pérez-Rosales G., Torda G., Moya A., Under The Pole Consortium, Raina J.B., Hédouin L.