As far as scientific interests are concerned, I’m not an “environmental” person at all. Somehow, ecology and conservation managed to remain profoundly boring to me despite the fact that my heart breaks every time I think about the havoc we’re wreaking in the biosphere. The same is true for physiology except there’s not even much of an emotional response to that, aside from a sullen disgust accompanying memories of endless lectures about fish kidneys.
After telling you how much I don’t care about ecology and physiology, it probably doesn’t come as a surprise that this post is kind of about both. You see, I study an animal with calcareous hard parts. It just so happens that these days, animals like that may be in trouble. Humans are pumping insane amounts of carbon dioxide in the air at a (geologically speaking) stupid rate, the oceans are swallowing it and slowly turning into a very salty fizzy drink. Calcium carbonate and acid don’t get along.
As it happens, the subject of calcifying critters versus ocean acidification also fits one of my recurring themes, i.e. that things are usually more complicated than you think. In this case, things are certainly much more complicated than I suggested in the previous paragraph. And thus, you shall be treated to a meandering about some of the complications I’ve come across 🙂
Many, many sea creatures have hard parts – shells or skeletons – made of calcium carbonate, the mineral that also makes up limestone and marble. The long list includes stony corals, calcareous sponges, crustaceans, nearly all molluscs, starfish, sea urchins and other echinoderms, a few groups of segmented worms, and even such unlikely suspects as the “soft bodied” acorn worms (Cameron and Bishop, 2012). And then we didn’t even count the algae and assorted single-celled beasties that the food chains of these animals stand on. The bottom line is that calcium carbonate is a damn big ecological deal.
(Below: a selection of marine carbonate users from Nick Hobgood‘s Wikimedia stash. Because Nick Hobgood is amazing.)
Calcium carbonate (chemical formula CaCO3), of course, is made of calcium and carbonate ions. How easy it is for animals to put into a skeleton depends on a variety of factors. In the most trivial sense, it matters how much of the ingredients you give them. Calcium is a standard component of seawater everywhere. Carbonate is also found normally in seawater. Its abundance is in a dynamic equilibrium involving carbon dioxide, carbonate, bicarbonate, carbonic acid and water. I find the chemistry difficult to get my head around, so here’s a figure from Feely et al. (2001) for our collective education:
(The numbers are the concentrations of the various chemical species under pre-industrial atmospheric CO2 levels and doubled CO2 levels, which could easily happen in the not too distant future.)
As far as I understand this system, the problem with CO2 is twofold. First, adding more of it decreases the pH of the water, and all forms of calcium carbonate are much more soluble at lower pH. Second, it decreases the concentration of carbonate ions, which makes them harder to obtain for shell-making purposes. (The guy who wrote this Encyclopedia of Earth article probably understands the whole thing rather better than me, though.)
Because I’m a pedant, I have to note that ocean “acidification” is a bit misleadingly named, because normal seawater is (and will be for the foreseeable future) far from being actually acidic. But the rules still apply – the lower the pH, the more difficult it is to make solid CaCO3.
This seems like a straightforward equation – keep pumping CO2 into the air, and shellfish will soon be shell-less (and thoroughly screwed). However, when you look at the actual reactions of real living creatures to acidification treatments (which Kroeker et al., 2010 summarised the then-existing literature for) – or indeed what happens to said creatures in the wild (as this “historical” study of reef corals by Cooper et al., 2012 did), a slightly more nuanced picture emerges.
First of all, of course organisms aren’t passive and helpless players in this game. Just like we can sweat or shiver to keep our body temperature right, calcifying things can regulate the environment in which their skeletons are manufactured. A study that looked at the chemical environment of the specialised space in which corals build their skeletons found that the pH of the fluid there is always higher than that of the surrounding seawater – corals do their darnedest to keep it where it’s good for them (Venn et al., 2013).
Having a specialised, “insulated” space for mineral deposition is a pretty common thing in the living world. The tiny algae called coccolithophores (who are single-handedly responsible for the White Cliffs of Dover) simply make their characteristic plates (literally “limescales,” hehe) inside their cells. Echinoderm larvae merge cells to create a “spicule factory”. Molluscs deposit their shells into the protected space between a tough organic layer and the soft tissue of the mantle. Et cetera.
On top of that, all organisms have proteins specialised to transport ions through cell membranes. Creatures who build hard parts can use such proteins to actively shuttle the materials they need even when they’re working against physics.
Thus, most mineralising organisms have ways of dealing with environments not necessarily friendly to mineralisation. Some naturally have to handle a wide range of such environments, like shellfish who live in estuaries and experience changes in salinity (and calcium concentration, and pH) with every tide.
As you might expect, different organisms can react very differently to the same challenge, ocean acidification in this case (e.g. Ries et al., 2009). They could be badly affected, they could just not give a damn… or even up their game and increase their calcification rates, as a brittle star was observed to do (Wood et al., 2008). That sort of compensation appears costly in other areas, though – Wood et al’s brittle stars suffered muscle loss, for example. It’s also worth noting that even closely related species can be affected in pretty different ways. In a study of two oyster species of the same genus and similar natural habitats, the larvae of one just shrugged off a bit of acidification, whereas those of the other struggled to build their baby shells under identical conditions (Miller et al., 2009).
Overall, ocean acidification is going to be bad for most creatures who rely on calcified hard parts, but how bad and what kind of bad will vary greatly if the above examples are anything to go by.
However, carbon dioxide is not only known as an ingredient for sea soda. In fact, it’s probably better known as the baddie in the global warming story. The oceans aren’t just going to get lower on the pH scale. They are also getting warmer. And because scientists want to know everything, some of them went out and investigated what the combination of these two does to calcifying critters.
And there, in an ironic twist that allows us to feel a tiny bit of relief in the short term, they saw that a little warming (but not too much!) can actually mitigate the effects of acidification for some creatures. Make no mistake, baby sea urchins are still very screwed when you plunge them into “future” ocean water. But they are somewhat less screwed if you include warming in the package than if you just pump a load of CO2 into their tank (Byrne et al., 2010).
Calcifying animals in a real marine environment don’t exist in isolation – they prey on, are preyed on, help out and compete with members of other species. Will their different reactions to the changing chemistry of the ocean overhaul entire ecosystems? (I don’t read much ecological literature, so there are probably unseen-by-me studies of this out there…)
Lastly, there is a factor we always have to contend with when discussing environmental change, and that is adaptation. The experiments I’ve cited so far examine the reactions of individual organisms to changed circumstances, but the real change is occurring over a time scale of many generations for many of these creatures. As soon as you have such generation-spanning processes, evolution becomes a player. Will ocean calcifiers be able to adapt? Are there ways to predict which of them will?
There’s exactly one title in my reference manager that deals with that sort of thing, a breeding experiment/simulation study about a sea urchin and a mussel population by Sunday et al. (2011), but again, that paper points out how the ability to adapt will be highly dependent on the genetic variation present in a population. Genetic variation is not just species-dependent, it varies even within a species unless the entire species is one interconnected population (which does happen sometimes, see the recent news about giant squid). And there are many, many species in the ocean. Most of which we probably haven’t even seen, let alone studied.
Byrne M et al. (2011) Unshelled abalone and corrupted urchins: development of marine calcifiers in a changing ocean. Proceedings of the Royal Society B 278:2376-2383
Cameron CB & Bishop CD (2012) Biomineral ultrastructure, elemental constitution and genomic analysis of biomineralization-related proteins in hemichordates. Proceedings of the Royal Society B 279:3041-3048
Cooper TF et al. (2012) Growth of Western Australian corals in the Anthropocene. Science 335:593-596
Feely RA et al. (2001) Uptake and storage of carbon dioxide in the ocean: the global CO2 survey. Oceanography 14:18-32
Kroeker KJ et al. (2010) Meta-analysis reveals negative yet variable effects of ocean acidification on marine organisms. Ecology Letters 13:1419-1434
Miller AW et al. (2009) Shellfish face uncertain future in high CO2 world: influence of acidification on oyster larvae calcification and growth in estuaries. PLoS ONE 4:e5661
Ries JB et al. (2009) Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification. Geology 37:1131-1134
Sunday JM et al. (2011) Quantifying rates of evolutionary adaptation in response to ocean acidification. PLoS ONE 6:e22881
Venn AA et al. (2013). Impact of seawater acidification on pH at the tissue–skeleton interface and calcification in reef corals. PNAS 110:1634-1639
Wood HL et al. (2008) Ocean acidification may increase calcification rates, but at a cost. Proceedings of the Royal Society B 275:1767-1773