Ocean sampling yields environmental sources of coral symbionts

The symbiosis between special algal species and reef corals is the foundation of a highly productive and biologically complex ecosystem, but our understanding of how this symbiosis is established by new corals has been limited by the fact that the symbiotic algae are difficult to find and study in the ocean.

But now a group of researchers has successfully identified algae of the genus known to represent coral symbionts, and has gone on to show that the isolated algae are indeed capable of establishing symbioses with new corals. The findings, which potentially bolster future efforts to protect and rehabilitate coral reefs, are reported by a group including Mary Alice Coffroth of the University at Buffalo and appear in the December 5th issue of Current Biology.

In response to environmental stresses, coral reefs around the world are in a decline due in large part to coral bleaching—loss of the symbiotic photosynthetic algae that live within corals and provide much of their energy. These symbiotic algae are essential to their host’s survival, but many corals must acquire their symbionts anew with the emergence of each generation. However, it has remained unclear how newly settled coral polyps acquire their symbionts in the ocean.

Organisms that resemble coral symbionts—dinoflagellates that are similar to those of the Symbiodinium genus that grow within corals—have been isolated from both sand and the water column; however, neither the locations of these populations nor their ability to establish symbioses is known. For both our understanding of reef ecosystems and their conservation, it is critical to recognize where these symbionts reside in the ocean environment.

In the new work, the researchers succeeded in identifying Symbiodinium in the water column as well as on ocean-bottom substrates. Most importantly, the researchers also demonstrated that a subset of Symbiodinium found in the water and on benthic substrates (that is, on algae and sediments) can infect new coral polyps. These isolates are therefore capable of establishing symbioses with corals and thus point to environmental sources of symbionts that may prove important in the recovery of reef-building corals after bleaching events.