Coral Reef News

The bleach boy

Here be monsters, says Roberto Iglesias-Prieto in this morning’s Plenary session. No, he hasn’t been at sea for too long, he’s talking about the symbiotic nature of corals. Most people think that corals are just rocks, and most of the rest think they’re either a plant or an animal. Of course, we know better, don’t we?

Corals depend on photosynthesis, he reminds us. The “most forgotten protein on the coral reef” is acpPC, which contains among other things, chlorophylls a and b. Poor acpPC. But at least Roberto hasn’t forgotten about it.

Light harvesting by corals is a tricky process. All light reaching the photosynthesizing symbionts is mediated through the host, either through the action of proteins affecting the nature of the light, or through the skeletal structure itself. That’s got to make things tough, right?

Nah. Here’s the good bit. It turns out that symbionts are better at photosynthesizing inside the coral than outside it. The reason lies in the light scattering properties of the coral surface. When you blam a coral with a laser, you get this:

Cool. Corals are very very good at harvesting light. So…?

Ah, photobleaching. A slide about the amount of CO2 us nasty humans have kicked up into the sky (and sea). And it’s not just how much, but how fast…

We’ve created a CO2 spike comparable with that during an interglacial, only on a timescale orders of magnitude faster. That of course means warming. And Roberto tells us that corals interpret thermal stress as light stress: as the temperature rises the maximum rate of photosynthesis is reduced. Even though the amount of light absorbed by the coral is the same, the rate of photosynthesis is lowered. Only one thing to say about that. Groooooo.

Corals are very good at harvesting light, as we’ve said. That’s great while the conditions are right, but photosynthesis is a bruising process. Normally the mechanism is able to recover during the full 24 hour period, so that by the time things kick off the next day all is well once again.

Too much light (or heat) is not a good thing though, and the photosynthetic process goes into overdrive.

That means it can’t recover fast enough, and a baaaad feedback cycle is kicked into action. Thermal stress leads to light stress. If the coral can’t recover from that, it suffers oxidative stress. If it can’t recover from that, the cell dies.

Next up, Roberto scares us with his “nightmare without monsters” — a figure showing how thermal and CO2 concentration changes can wipe corals out. But then something even more sobering: a graph pointing out that although we are currently faced with a range of choices about how we can react to the threat, taking even the most radical course of action might not help. It’s all to do with how long marine biologists live, apparently.

Sobering stuff, as ever.