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  • Writer's pictureAllan Dyen-Shapiro

Climate change and corals: the link the newspapers didn't give you

The combination of global climate change, an unusually strong El Niño event, and other factors affecting water temperature was recently reported to have caused massive bleaching of up to 1/3 of the world's corals. The newspapers got it right that this is big news. In Malaysia and the Philippines, two of the world's most populous countries, collapse of the coral reefs would lead to widespread famine and economic collapse. As coral reefs provide shelter or spawning grounds to ~1/4 of the world's fish and provide 10% of the world's photosynthesis (while taking up less than 1% of the world's surface area), the global implications of such a catastrophe would be dire.

The newspapers also got it right that a particular form of endosymbiosis, one organism living inside another, is key. Coral is a simple animal that attaches to a surface, becomes sessile, and engulfs a Symbiodinium, a photosynthetic dinoflagellate alga. As coral lives in nutrient-poor waters, algal photosynthesis provides the carbon nutrition that allows it to live. Heat stress leads the coral to expel its endosymbiont, resulting in death via starvation. The result is white, "bleached," rock.

As I read these stories, my mind as a former cell biologist/biochemist says, "How does this work?" And it's not just an academic concern: if one understands the symbiosis/expulsion process, replacement of the endosymbiont with varieties that tolerate heat might become possible. Indeed, it may be essential to saving these ecosystems.

The short story is that the answer isn't known. There you go: those of you who happened on this link in a chance Google search, the folks who routinely post TL;DR (too long, didn't read) on Facebook posts, you're done, you can stop thinking, you can open a Budweiser and turn on "Toddlers in Tiaras." You folks probably won't be interested in my fiction anyway; a community engaged with the sort of issues I address in my fiction probably won't be your thing.

For the rest of you, I now digress and describe an approach very common in plant and animal biology in the pre-molecular biology (or at least pre-genomics) era: physiology. Let's say you are interested in a certain process, say how crop plants respond to drought or how human cells respond to infection with a virus. You stress (withhold water/infect, respectively). Then you look at very early time points and see what happens. You time the kinetics of when these things happen. You interfere with the early steps using any tool you have and see if it effects the later steps. For example, let's say within thirty seconds of virus infection, phosphate is added to 1000 different proteins. Two hours later, a genes encoding anti-virus proteins are turned on. If in the presence of an inhibitor of phosphate addition the genes still get turned on, the phosphate addition was irrelevant to the response you were studying. By progressively eliminating responses as irrelevant, what's left is what's likely important.

There are numerous problems with this approach. First, the biology must be captured in the system you study in the lab. Do mammalian cells growing in a dish really capture all biology exhibited by cells growing in your liver? Often not. Second, the effect must be specific. The example I used, inhibiting phosphate addition, is like doing archeology with a sledgehammer rather than a fine-bristled brush: it affects lots and lots of things other than what the scientist thinks they are affecting. Third, signaling networks can be complicated. If a manipulation takes out a process that turns off a process that turns on another process, with this protocol, it is indistinguishable from the manipulation directly turning on the last process. In probably the biggest thing I got wrong in the 1994 - 2004 phase of my career, that's exactly the mistake in thinking I made, so this isn't just abstraction.

In well established systems, physiology was generally a very early approach to a problem, used mostly to define what things might be important, not to conclude what actually is important, at least without any certainty. In plants, one turns to genetics to establish relevance. Rather than huge perturbations to a system, one can very selectively perturb the system by interfering with often only one gene in an intact, real system. With humans, you generally don't have that luxury, but an important problem will get one thousand-fold more funding than an equally important problem in plant biology. (At least in the West. In China, where the central scientific problem is feeding 20% of the world's population on 7% of the world's arable land, the emphasis is opposite. They are happy to let Americans and Europeans cure cancer. Or not cure cancer and make certain Pharma companies rich, but that's the subject for another post.) Maybe any single experiment in human biology is as flawed as it was in the 1970s, but once the same question has been investigated in two dozen different ways, you tend to believe consistent results. And in mice, you do have genetics to work with, so anything we share with mice can be investigated in that system (albeit with much more difficulty than in plants).

Back to coral: it is impossible to grow most coral in a physiologically accurate fashion in a lab. The best studies harvest live coral from a reef and put it in a mesocosm (big containers of water harvested from the same place with all conditions kept as natural as possible). So, no genetics, no molecular biology, at least not as ways of studying the intact system.

My best literature searching techniques yielded the following answers (caveat, I never worked on coral, although I did do some marine botany in my 2005-2007 career, so I know something about it):

1. More reactive oxygen is produced in heat stressed corals than in non-heat stressed corals. Reactive oxygen is various forms of oxygen with extra electrons or with altered quantum spin state of electrons. It tends to dump electrons on other things and damage them. One of the things it damages is the photosynthetic apparatus. Artificial oxidative stress can cause symbiont expulsion.

2. High light levels can cause mis-regulation of photosynthesis with electrons shunted off onto oxygen, making superoxide (a form of reactive oxygen). Other types of reactive oxygen are made as secondary products. If scavengers of oxygen radicals are added, damage to the photosynthetic apparatus and symbiont expulsion can be prevented.

3. However, an alternative idea says that the reactive oxygen originates from the host animal, not from the algae. Heat stress has been shown to damage mitochondria in the coral. The less efficient electron transport leads to formation of reactive oxygen. These reactive oxygen species travel into the endosymbiont, and they can damage the photosynthetic apparatus there. This damage was unaffected by light level (even occurring in total darkness), and was unaffected by chemicals that inhibit photosynthesis, so it wasn't caused by anything related to photosynthesis.

4. All the other likely suspects, cellular actors that tend to do things in plants and animals, tend to change in interesting ways, but there's really no way to tie it all together.

Yeah, yeah, sure, stopping all fossil fuel use in the world is a great idea, but even if it all stopped today, so much damage has been done that these issues would still be important for heading off ecological catastrophe and economic collapse/famine in many regions. So here's the ideal experiment that can't happen yet: lab-grown coral in conditions that mimic the natural environment is allowed to establish symbioses with various lines of dinoflagellates and heat stress of varying degree is imposed. All relevant parameters of the symbiosis/expulsion process are quantified, and the system is perturbed to see what's most important using perturbants as sophisticated as those available in plant/animal systems. New lines of dinoflagellate cultures are developed that outperform the natural ones. These algae are introduced to reefs that are experiencing bleaching. As reefs can come back from single season bleaching events, this approach could save the reefs.

Now, convince the US Congress that this is sufficiently important to put as much money into this problem as they are putting into cancer research. (I would say weapons research, but that's too easy a target.) No? How about one tenth the money? One one thousandth?

Not happening any time soon. The scientifically illiterate population doesn't understand these issues, and thus isn't making noise. Even here in Florida, where much of the tourism and fishing depends on our aquatic ecosystems, and where our corals could be considered the most important in the world in that they are the only coral reefs anywhere with a high percentage of unique species found nowhere else, your average person is clueless. As is your average politician.

It falls on the few of us with a clue to spread the word. Please do so. Go ahead and comment on my Facebook page. Please. If this stuff isn't discussed, the planet is toast.

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