How a Healthy Coral Can Help a Bleached One Recover

Researchers at Stanford have published a lab study that shows bleached coral fragments can regain their symbiotic algae after brief physical contact with healthy fragments from the same genet. In controlled tanks, pairs of Pocillopora damicornis fragments were arranged so that five pairs touched and five pairs were separated by a few millimetres; touching pairs recovered while separated ones died.

High-powered microscopy showed that tissue fusion created a bridge that enabled rapid transfer of Symbiodiniaceae algae. Within days, algae began to move and by one week the symbiont population was doubling about every four days; consequently, a fragment that had one week of contact could continue to recover alone and reach full pigmentation in about seven weeks. Although polyp uptake of symbionts also occurred in mixed-genet pairings, it was too slow to support full recovery. Therefore, the contact-mediated route appears restricted to genetically matching donors and recipients, which is an important constraint for field use.

However, the experiments were conducted in laboratory tanks and did not test reef-scale conditions such as waves, predators or ongoing heat stress. Further research will be required to learn whether contact-based methods can be scaled or whether cultured symbionts could be used to help diverse coral genets.