How do light-dependent corals cope with the almost complete darkness?

Authors:

Pérez-Rosales G, Rouzé H, Pichon M, Bongaerts P, Bregere N,  Carlot J,  Parravicini V, Hédouin L, Under The Pole Consortium. Coral reefs (2024) 

Article summary:

Most scleractinian corals depend on a symbiotic relationship with microalgae, which provides them with most of their energy needs. In symbiotic scleractinian corals, these algae themselves depend on light for photosynthesis. The presence of these corals at depth is therefore largely limited by the absence of light. However, exploration of the mesophotic zone (30-150 meters deep) has revealed their presence in these areas where light is extremely reduced, sometimes less than 5% of surface light! 

To better understand how certain symbiotic scleractinian corals are able to live at these depths, the authors studied specimens belonging to two species: Pocillopora cf. verrucosa and Pachyseris “speciosa” spp., collected between 6 and 60 meters, and 20 and 90 meters respectively. The samples were collected by UNDER THE POLE divers at four different sites in French Polynesia. For each coral species, the researchers compared several parameters at different depths:  

• the symbiotic algae species present, its density and its chlorophyll concentration,  

• the structure of the coral skeleton, 

• their main type of feeding (nutrient supply via algae or direct food capture by polyps). 

   

The authors discovered that in both coral species, the density of symbiotic algae and their chlorophyll concentration increase with depth, which compensates for the decrease in light and maintains a certain rate of photosynthesis. In addition, for both species, individuals living at greater depths exhibited micro-morphological differences in their skeletons that allowed them to increase the amount of light captured by the algae. Tissue analysis does not show that deep-water corals feed more directly than those on the surface, which could be necessary if the nutrient supply from symbiotic algae decreases. Finally, the species of symbiotic algae in corals did not differ between corals of the same species at different depths, but they did differ between different sites. 

These results show a high capacity for adaptation and acclimatization to the mesophotic zone in the two species of symbiotic scleractinian corals studied. However, further studies are needed to better understand the results, which have been summarized here in broad terms but showed differences between the sites studied.