Corals Resistant to Bleaching

One of the starkest reminders that the world is warming is the mass “bleaching” of corals. This happens because, when the water temperature climbs, the corals part company with symbiotic algae that live inside their cells and provide oxygen and remove waste. Their departure kills the coral. Why higher temperatures should cause these two partners to divorce isn’t known, but scientists have discovered that some corals are more immune to this effect than others. Those with fewer algae seem to be more tolerant, but the tradeoff might be slower growth and therefore less resilience to other threats. Speaking with Chris Smith, Brendan Cornwell has been studying this both in the lab and on reefs for real…

Brendan - We actually have to go and get in the water. So we want to know both what the temperature conditions are, so we deploy data loggers to each of these reefs to figure that out. So they basically, we zip tie them to the rock and then they sit there for the entire year, or a few months, constantly recording the temperature, and then we go get them. But then the other thing we want to know is how the corals react to a really standardised set of conditions. So we actually have to heat them up ourselves and bleach them, and ask how much they bleach when we give them a specific amount of heat stress, in order to identify more or less bleaching resistant populations.

Chris - So you've got real world data of what's going on, where you see the bleaching. But you've also got samples that you can subject to conditions that enable you to work out just how much they'll take before they bleach. So you can pinpoint where it's likely to be worse, where it's likely to be better.

Brendan - Exactly. And the sort of nice thing about it is that these are colonies of polyps, right? So the coral itself is made up of lots and lots of different polyps. They're all clonal. So they're all genetically identical and they're all making up the colony itself. And so when you take a little piece of the colony, you don't actually kill the entire thing. You just take what we call a nubbin, and that nubbin is what we're actually doing the experiment on. And so what that allows us to do is both figure out what that colony can and can't stand in terms of heat stress, but also it lets us go back to that colony over and over again, and ask if that changes.

Chris - Right. So if one looks at the corals that are coming out of these experiments as clearly more resilient, compared with corals that are more vulnerable to bleaching at temperature escalations. When they do this, is there any difference in terms of the performance of the corals, when they're growing, when they're not trying to bleach?

Brendan - That is one of the questions that we've started to ask ourselves, is essentially what do you gain by being bleaching resistant and why hasn't bleaching resistance just spread throughout the population, if it's so favourable when conditions warm? Part of what we found in this study, is that we think we see a trade off between how fast the coral can grow and how readily it bleaches. We think that has to do with the overall abundance of symbiont in the coral tissue itself. So how much of those algae, how many are actually in the tissue? We think that matters for both how severe the bleaching is, but also for how quickly they grow. So we think when they have more algae, what we're seeing is that their growth rate is slightly higher than if they have fewer algae, which logically makes some sense.

Chris - Indeed. So what one would anticipate then is that the ones that have fewer algae are going to grow more slowly, but they're going to be a bit more resistant.

Brendan - Exactly, yeah. They'll lose overall, as a proportion of the initial population that they were harbouring, they'll lose fewer of those.

Chris - Now, if the motivation for doing this is to study corals that are more bleaching resistant, and then perhaps even use those to repopulate decimated reefs elsewhere, is there not a danger then that we could be selecting subpopulations of coral that actually are going to be much slower growers? So our recovery times might be quite a lot longer?

Brendan - Yeah. I think that is definitely a possibility and it might not be something that we can necessarily overcome, but might just have to be something that we kind of know is coming. So yeah, it could be that we select for these bleaching resistant colonies that are more slow growing, which will mean that they won't be able to deal with the other stress that we might lay on them. It could be that they overall are just slower, moving into the future, because they have to adopt that strategy in order to not face such severe bleaching when temperatures warm up.

Chris - Are you seeing a sort of overgrowth of the resistant forms, sort of moving into the space vacated by what would've been their faster growing, but more vulnerable cousins, which have now obviously been killed off in some places by bleaching events?

Brendan - We are not. So we have not been at this particular location for long enough, I think to have seen that. But there are now indications in the literature that something like that might be happening. Essentially that we're imposing such strong selection on these populations, that we are seeing an evolutionary response. It's really difficult to know whether or not that is strictly due to inheriting that response from your parents, or if it's due to some sort of acclimation. But there are indications now from studies on the great barrier reef, that there is some sort of shift in terms of the overall bleaching resistance and resilience of the populations, which, that's definitely one reason why that might be the case.

Chris - And are the resistant forms sort of represented across coral populations, or are we looking at a future where we have a much more restricted pool of species, which are the resistant ones, but which in turn might not actually therefore support such a diverse reef, and all of the other attendant species that would normally make it their home?

Brendan - That's hard to know, but to me, it seems reasonable to think that some of the more bleaching prone species will likely be the first to be lost. So I do expect that the composition of these reefs, in terms of species that you can find there, will shift as temperatures warm. I think that's probably inevitable.