Black corals contain photosynthetic algae

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Recent research has discovered that zooxanthellae (the algae that provides essential nutrients to its host coral) is not just the reserve of shallow water corals.

The research, published in the journal Proceedings of the Royal Society B, shows that black corals also harbour the beneficial photosynthetic algae, also known as Symbiodinium, despite popular belief that there would be little requirement due to the depths that black corals are commonly found; depths that have very little in way of useful light penetration.
 
Scientists at the Hawaii Institute of Marine Biology(HIMB), collected 14 black coral species from around Hawaii at depths of between 10m and 396m.
 
The corals were then examined for the presence of algae using molecular and histological (tissue studies) techniques. The results were surprising, as 71% of the species contained algae.  
 
PhD student, Daniel Wagner at HIMB, who led the investigation, states: "Because black corals are predominantly found in deep and dark environments, most people assumed that they could not harbour these photosynthetic symbiotic algae. At this point we do not know how these algae are able to exist in extreme environments, and it certainly highlights how little we know about deep reefs." 
 
The study represents the deepest record of Symbiodinium to date, and is an important discovery in the field of coral biology, it also implies that some members of these algae have a broad environmental range, with some extremely diverse habitat preferences.
 
Black corals have cultural and economical significance in the State of Hawaii, where black coral is considered the official gemstone and is harvested commercially for use in the precious coral jewellery industry.
 
For further information see the paper: "Azooxanthellate? Most Hawaiian black corals contain Symbiodinium", Daniel Wagner, Xavier Pochon, Leslie Irwin, Robert J. Toonen and Ruth D. Gates doi: 10.1098/rspb.2010.1681