Long before dinosaurs thundered across the land, there was another group of animals that ruled an alien water-world we would barely recognize as our own. On this primordial earth, winds scoured a virtually lifeless landscape of rock and sand. But the ocean teemed with strange organisms amid the clouds of protozoa and bacteria. Fossils of frond-like filter feeders and jellyfish-like creatures have been found reliably dated to this period, about 600 million years ago. Many of these bizarre fossil species are likely early representatives of the
Cnidaria. These metazoan pioneers include jellyfish along with critters we know today as
Corals.
Most folks know that some corals, such as sea anemones, capture living prey and consume them internally. But many corals also harbor colonies of endosymbiotic microbes called Zooxanthella which allow the coral host to extract energy from sunlight. In short, these corals are both plant and animal. In many shallow water tropical or semitropical zones, large conglomerations of these plant/animal hybrid corals form large masses of carbonate exoskeletons and make up the physical foundation of coral reefs. Kill those microbial endosymbionts and the reef dies.
Left: A coral system in the Great Barrier Reef already showing early signs of bleaching can still serve as a refuge and nursery for entire ecosystems of marine creatures including a gorgeous blue starfish (L. laevigata). This reef can recover quickly. Right: Bleaching occurs when corals lose symbiotic algae and turn white. The center coral is still alive but those around it have been bleached. This reef will die. Photo courtesy of The University of Queensland
Corals are extremely sensitive to changes in temperature, salinity, and acidity. And as it turns out, CO2 has a well documented thermal (Global warming) effect and a lesser known chemical effect, both of which kill coral reefs:
Realclimate--Most of the carbon in seawater is in the form of HC(O3)-, while the concentrations of CO32- and dissolved CO2 are one and two orders of magnitude lower, respectively. The equilibrium reaction for CO2 chemistry in seawater that most cogently captures its behavior is CO2 + C(O3)2- + H2O == 2 HC(O3)- ... Le Chatlier's principal states that a perturbation, by say the addition of CO2, will cause the equilibrium to shift in such a way as to minimize the perturbation. The concentration of CO2 goes up, while the concentration of CO32- goes down. The concentration of HCO3- goes up a bit, but there is so much HC(O3)- that the relative change in HC(O3)- is smaller than the changes are for CO2 and C(O3)2-.
OK, here it is in layman's terms: Increasing quantities of atmospheric CO2 is a double whammy for coral reefs. Greenhouse Gases like CO2 raise the temperature of the air and eventually the ocean which directly knocks off the corals (And causes other problems). But on top of that, the CO2 asymmetrically overwhelms the chemical sinks leading to a series of reactions and counter reactions that increase acidity and this process will not be affected even if temperature levels ease back down. Higher acidities eat away the coral skeletons, just like limestone dissolves in vinegar, producing even more CO2.
Both the thermal and chemical shock produced by CO2 are harmful for the symbiotic algae and the coral in general, and thus both independently transform healthy reef systems into bleached-white graveyards marked by crumbling skeletons and piles of gray dust. This cycle of anthropogenic carbon to acid to coral bleaching is illustrated to the left by graphic artist Karen Wehrstein.
Why should we care about a bunch of disintegrating brainless marine animals? Well, aside from the aesthetic beauty, the reef systems of the world play a critical role in the global marine ecology similar in some ways to terrestrial rain forests. Destruction of the world's coral reefs would likely have the same kind of massive, immediate, and unpredictable impact on the marine biome as eliminating the old growth forests would on land.
The higher acidity also has consequences for other organisms; it wrecks the delicate food chain at every level and could possibly affect phytoplankton which is the basis of the entire food chain and which cleans our air of CO2 by converting it to oxygen. Life is opportunistic, which means in this case other, less desirable organisms such as red alga could bloom in the biological vacuum left behind, replete with toxins on the level of a genuine Weapon of Mass Destruction courtesy of Mother Nature. This could, for all we know, lead to another vicious feedback process; a tiny drop in photosynthesis means more CO2 in the atmosphere which in turn raises CO2 levels in the ocean and the process builds on itself. If so, the cycle might self amplify until a tipping point is reached at which point it becomes self sustaining. But it should be stressed that this latter example of a biochemical tipping point is highly speculative and greatly oversimplified.
Corals and Cnidarians in general are resilient; they've survived more ecological upheaval than any other group of metazoans known to science. They'll survive this industrial era as well. But the timescales on which they recover are distinctly out of sync with our own. Judging by prior extinction events, it can take these magnificent organisms hundreds of thousands to millions of years to recover or evolve into more adaptive forms and re-radiate across the planet. While geological time and evolutionary biology are clearly on the side of the Cnidaria, the same cannot be said of our civilization. Time and biology may be our salvation; or they could conspire to render human civilization a single flicker of metal and plastic stamped in one layer of strata, a thin seam of toxins and environmental catastrophe marking our brief debut and untimely exit from the stage of life.