Geochemistry is a science in which we collect samples of natural things – rocks, shells, feathers, bits of wood, teeth – and then measure some aspect of their chemical makeup to learn something about the world. Geochemistry can be used to learn about the organisms whose parts are being analyzed; for instance to figure out what they ate (the old adage “you are what you eat” is true here—many times a distinct chemical signature comes with eating certain food items). It can also be used to learn something about the environment in which an organism lived or a rock formed.
|Requisite beautiful-coral-reef shot. Kiritimati Island|
Corals are, by geochemistry-practitioner standards, awesome. For one thing, corals make their skeletons out of chemicals in seawater—as the water chemistry changes, so does the chemistry of the skeleton. Second, corals grow larger over time by adding new layers onto their skeletons, while the old skeleton often just sits there as a semi-permanent record of conditions at the time that bit of skeleton formed. This is like keeping track of daily weather on slips of paper added to a pile—you can then dig back through the pile to see how things have changed over time.
Third, corals also have a built-in time-stamp on these records: the density of the skeleton fluctuates with the seasons, leaving bands that can be seen by x-ray or CT scan in samples (see my earlier post). For coral samples collected live, these bands can be counted back in time; corals that are dead can be dated using another aspect of geochemistry: the amount of a particular radioactive element that decays at a known rate can be measured to back-calculate how long ago that coral was alive.
The fourth excellent/horrible thing about corals is this: while they sometimes act as passive recorders of water chemistry, both the density and the chemistry of the skeleton can also be affected by other things – notably how happy the coral is (corals that are heat-stressed “bleach” by expelling their colorful symbiotic algae, which screws with skeletal growth and chemical incorporation). Other aspects of coral biology such as spawning or food intake also can change the chemical signature.
If all of the different influences on the coral skeletal chemistry can be disentangled, there is fantastic potential for long reconstructions of both the environmental conditions in which the coral grew and the coral’s reactions to those conditions (over the last few100s of years, or even longer if dead corals are also used).
But that’s the hard part: disentangling. For one reason, we keep thinking that we know what controls each chemical signature (and this is the “royal we,” including me and other scientists), and then we figure out that it’s more complicated: we thought the concentration of strontium was a direct, unbiased measure of water temperature; now it seems that this is also very slightly affected by the skeletal growth rate. We also thought that the ratio of two different isotopes of oxygen in the skeleton was only controlled by water temperature and salinity (isotopes are different forms of the same element that behave the same chemically but have very slightly different weights), but then we figured out that calcification rate also matters.
|Not totally happy coral|
And here’s where our work comes in: faced with weird oxygen isotope data that couldn’t physically be explained by any combination of water temperature, salinity, or calcification, we knew there must another as-yet-unidentified impact at play. What we saw was a big jump in the baseline of the data after a major coral bleaching event.
Now, a quick tangent: coral bleaching is an extremely worrying phenomenon. Corals get most of their nutrition from the symbiotic algae they house in their tissues; when bleached, they can starve to death or become more susceptible to disease. With global water temperatures increasing, coral bleaching is becoming more frequent. The big question in the survival of coral reefs as we know them is therefore: can corals adapt?
One way corals might be able to adapt to warmer waters is by kicking out “weak” forms of symbiotic algae and trading them for genetic strains that are more resistant to heat stress. This is called the “adaptivebleaching hypothesis”—the idea being that by acquiring more heat-tolerant symbionts, corals can survive the onslaught of climate change (at least for a while).
Our data might be a reflection of this very phenomenon. If the symbionts the corals housed before the bleaching event were physiologically different than those after, maybe the skeletal chemistry would be different.
There has been a lot of activity lately from scientists trying to understand the nitty-gritty workings of coral calcification. We were able to synthesize this work and put forward a potential mechanism that could cause our observations (based mostly on a change in pH at the calcification site).
We also tried to test the idea directly by collecting lots of little nubbins (possibly the best technical term ever) of live coral. We identified their symbionts using DNA methods (and here “we” means my colleague Melissa Garren), and then the corresponding oxygen isotope values. What we found was a hint of a relationship—so we didn’t disprove our hypothesis.
|Our "nubbins" were essentially mini-core samples from large corals|
This area of research still needs more work, but it is exciting; if this signature is real, it could be used to retrospectively test for adaptive bleaching in other corals during other bleaching events. This is important to predict the outcome of bleaching events and manage coral reefs as we face increased heat stress. If corals can adaptively change symbionts, can we help them do this? Can we more effectively manage outplanting and reseeding efforts to restock damaged reefs?
I hope that this paper stimulates new ideas and more projects to help answer these questions.