Theory 08/Marine Biology & Fisheries /Coral reef science/3 min
Bleaching Is a Symbiont Problem
Two reefs, same water, same heatwave. One turns white and one does not.
Coral bleaching thresholds are set by the composition of the symbiont community rather than by water temperature alone, so reefs with the same temperature history can bleach differently, and the threshold should be predicted from symbiont type rather than from a degree-heating metric.
01The evidence
- Bleaching susceptibility and mortality of corals were shown to be determined by fine-scale differences in symbiont type, which is the observation a composition account requires.Europe PMC
02The tension
Bleaching is forecast with degree-heating weeks: accumulate temperature above a local threshold and predict bleaching. The metric works on average and fails on individual reefs, and the failures are attributed to acclimatisation or to local conditions rather than to the community that actually determines tolerance.
03The theory
A coral is a symbiosis. The algal partner supplies photosynthate, and different symbiont types have different thermal optima. A colony dominated by a heat-sensitive type will bleach at a lower temperature than one dominated by a tolerant type.
If composition sets the threshold, then the reef-scale threshold is a weighted average of the colony thresholds, with the weighting set by the distribution of symbiont types. That makes the threshold a measurable community property rather than a physical constant.
The prediction is that bleaching should be predicted better by community composition than by accumulated heat, and that reefs which recovered from a previous event should bleach at a higher temperature because the sensitive types were removed. That is a testable hysteresis.
The management consequence is that restoration should focus on symbiont diversity rather than on coral cover, and that bleaching forecasts should include a composition term.
04What this is not
Degree-heating metrics treat the coral as a uniform organism with a fixed thermal limit. This treats it as a community with a distribution of limits, which explains why identical heat produces different outcomes.
05Predictions
- P1Bleaching should be predicted by symbiont composition better than by accumulated heat.
- P2Previously bleached reefs should show a higher threshold on repeat exposure.
- P3Transplanting tolerant symbionts should raise the threshold in a season.
06What would refute it
Bleaching tracking accumulated heat regardless of composition would restore the physical threshold.
07What it would change
It would change reef restoration and bleaching forecasting.
08Evidence base
- Bleaching susceptibility and mortality are determined by fine-scale differences in symbiont typehttps://europepmc.org/article/PMC/PMC2492480