Brief Contact Transfers Healthy Algae to Bleached Corals

Coral bleaching occurs when heat stress from warm ocean waters causes corals to expel their symbiotic dinoflagellate algae, and is a major cause of global coral reef decline. Now, Daniel Collins and colleagues have demonstrated that brief physical contact between healthy and bleached coral can restore algae to the bleached coral, leading to their full recovery. Just one week – and sometimes as little as three days – is enough to trigger this transformation. The algal recovery time is an order of magnitude faster than most recovery times in the wild, which in some cases can be too slow to prevent reef death. Collins et al. show that the algal transfer happens via tissue fusion between the corals, and as a result, the procedure is only successful for corals from the same genetic background (or genet). The researchers found that symbiont algae transfer via the uptake of polyps occurs upon contact in both same-genet and mixed-genet pairings, "revealing an additional, slower pathway for symbiont transfer that is insufficient on its own to support colony recovery," they write. They note that the next step will be to see whether these contact-based recovery strategies can be applied in the field at the scale of natural coral reefs.