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Comment by skew-aberration | Hacker News Reader

Coral gets its color from algae called zooxanthellae, which it depends on for photosynthesis.

Bleaching is actually due to the coral expelling those algae. Bleached corals are not dead, and remarkably they can not only let the algae in again but can also select a different algae species to cultivate (changing colour in the process)!.

This is thought to be a deliberate survival strategy to optimize the species of algae under different conditions (Adaptive Bleaching Hypothesis).

Bleaching is caused by the same stressors (like high temperature) which kill coral, but bleaching itself is not death of the coral.

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> Bleached corals are not dead

My open questions would be:

1. What's the consequence of corals expelling algae?

2. Why does heat make corals expel algae?

1. The coral slowly starves to death. Like a tree without leaves 2. It's complicated but the algae essentially start damaging the coral in these conditions. Their photosynthesis mechanism runs into overdrive, generating too many electrons. With no way to consume this much power it dumps the energy into making reactive oxygen species which it ejects. These damage the coral (and the algae). The same thing occurs in plants - a fern growing on the forest floor can't handle full sunlight (photodynamic stress). Plants are less temperature sensitive though because they have active cooling (transpiration). But temperature and quantity of light both affect the photosynthesis machinery.

That makes me wonder - I’m sure it’s not a novel thought - if there is a more suitable species of algae for these conditions we can inject into bleached corals?

Impressive that corals have such a survival mechanism. But hopeless if no other species of algae can enter another symbiotic relationship with them?

> That makes me wonder - I’m sure it’s not a novel thought - if there is a more suitable species of algae for these conditions we can inject into bleached corals?

TLDR Yes but it's complicated. There are more suitable algae species but AFAIK coral under heat stress will still often expel the algae even if the algae isn't the one stressing the coral. So you have to essentially breed a more tolerant coral and then provide the algae.

Now luckily there do exist more heat tolerant coral and algae species living alongside each other. Unfortunately those species are located in the Persian Gulf/inside the Strait of Hormuz so they are regularly subject to added stressors from Oil/NG extraction, military conflicts, and waste dumping. And of course their location makes research on them/collaboration between research institutions complicated.

And it's not like you can just transplant these coral somewhere else and they'll live. They are adapted to their environment and that environment plays a part in their heat tolerance as well. But research on these coral ecosystems will likely shed a light on how to help other coral species to adapt and evolve.

> TLDR Yes but it's complicated. There are more suitable algae species but AFAIK coral under heat stress will still often expel the algae even if the algae isn't the one stressing the coral.

Maybe the pharmaceutical industry can target a new market here...

It's not just heat, it's ocean acidification. As the concentration of CO2 in the atmosphere increases, CO2 in the water increases as well. When CO2 dissolves into water, it forms carbonic acid. This is the same stuff that makes fizzy drinks like soda taste sour.

So more CO2 in air = more CO2 in the water = lower PH in the ocean. I don't know the details but the more acidic water messes with the chemical processes that corals and other aquatic life depend on.

The connection is that shells, such as the hard part of corals, are made from calcium carbonate. When you put calcium carbonate in acid, it dissolves. This takes some of the acid out of the water.

In the long run, there are huge deposits of calcium carbonate on the ocean floor. Even if we burn all of the fossil fuels, the ocean eventually comes back to a neutral ph.

The problem is the short run. We're turning CO2 into carbonic acid at a rate that far exceeds what surface waters can handle. This is creating conditions where shellfish of all kinds are working overtime form proper shells. If they can at all. It even affects many kinds of plankton. This stress is causing large dieoffs. When we wipe out things at the base of the food chain, it has ripple effects through the rest of the ecosystem.

> Bleaching is actually due to the coral expelling those algae

Due to the coral or due to the algae? Put another way, could we engineer UV resistance into the algae?

It's not UV that's the problem but heat. We could possibly cultivate algae that are more heat resistant?

This is actually in process, though my understanding is that it doesn't seem to keep up with the pace of ocean temperature rise.

https://www.npr.org/2024/04/17/1245085914/coral-reefs-bleach...

I'm not an expert, but yes we can and we do.

And so does the coral!

If a species of algae cannot handle high temperatures without generating chemicals like hydrogen peroxide which trigger the bleaching, then that species is ejected sooner and more frequently during warmer months, creating selective pressure for species that can.

To survive these conditions without getting ejected, the algae need to essentially run their photosynthesis process more slowly. They produce less chlorophyll per cell and create sacrificial 'photoprotective carotenoid pigments' which act like sunscreen and dissipate that potential sunlight energy as useless heat (coral has a few tricks too fluorescent proteins).

So the coral is actually in a delicate balancing act, selecting the most aggressive/productive algae which won't also try to kill it during summer. Corals will learn to be more conservative in their selection and algae will get more conservative themselves. We want this process to happen quickly enough so that we don't lose coral coverage and diversity in the coming decades / centuries, but we can be confident that on longer timescales the coral will adapt, assuming we don't get runaway warming.

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