I agree. The key symbiosis between coral and microalgae depends on fundamental thermodynamic equilibria of the carbonate chemistry of seawater - which are highly sensitive to temperature and atmospheric CO2, in very predictable ways. When living in coral becomes unprofitable for the algae, they leave. My instinct, from some experience with this system, is that introducing new species won’t do better than nature, nobody can beat thermodynamics. We have to reduce the CO2.