Most days, I get up, go to work, and zap corals with lasers. No, I am not building some twisted underwater version of a fly swatter, nor would I ever want to harm a coral. Rather, I am studying the chemical composition of coral skeletons to better understand how they grow, and hopefully improve our understanding of their sensitivity to ocean acidification.
 
I am using a technique called Raman spectroscopy. It is fundamentally different to conventional approaches for analyzing the chemistry of coral skeletons. While many studies have investigated element and isotope ratios by putting dissolved skeletons into a mass spectrometer, Raman spectroscopy is based on the scattering of light from a laser directed onto the coral skeleton. This offers several advantages: (i) it is fast, taking only 1 second per measurement, (ii) it can be applied on small spatial scales, down to about 1/100 the width of a human hair, and (iii) it is non-destructive, as the skeleton is not damaged in any way.
 
Based on the way the laser light is reflected from the skeleton, I can infer the chemical composition of the fluid from which it precipitated. This provides us with key insights into the mechanisms by which corals build their skeletons, and whether the skeleton-building process is sensitive to the rapid changes in seawater chemistry driven by invasion of human CO2 emissions into the ocean.
 
This new avenue of research into coral growth has led to several recent publications. The first paper developing the technique was published in November 2017 in Biogeosciences. Two follow-up application papers were both published this month, in Proceedings of the Royal Society B and Frontiers in Marine Science special issue on coral calcification. Within just the past year, we have already gained key insights into coral calcification with Raman spectroscopy, and this is just the beginning! Stay tuned for several new publications coming in the near future.

Last week, corals all over the world "leaped" higher up on their skeletons. What was the special occasion? Just another full moon. My latest publication in the journal Coral Reefs shows that Porites corals build new sheets of skeleton, called “dissepiments”, each lunar month. Dissepiments are essentially the ladder rungs in the skeleton that the living coral polyps uses to climb. The living polyp rests on the upper-most dissepiment for one lunar month while building the rest of its skeleton, and then builds a new dissepiment during the full moon. Why the full moon? Many corals, including Porites, are perforate, meaning that the skeleton does not completely enclose each polyp. Rather, the polyps across the colony are connected, and thus their growth needs to be synchronized. It seems that the full moon provides a cue to synchronize the dissepiment-building process. One key outcome of this finding is that we can use dissepiments as monthly time markers in the skeleton. In the paper, we demonstrate using dissepiments that seasonal changes in skeletal extension rates are responsible for producing the annual density bands in Porites corals. Further, we can detect stressful events in the history of a colony based on anomalies in the spacing between consecutive dissepiments.

I am especially proud of this research because it required a lot of patience of foresight. Due to the nature of this study, it took my entire PhD to complete. That lunar rhythms exist in coral skeletal growth was not an entirely new idea and it had been hypothesized for decades that dissepiments form on a lunar basis. But this had not been tested in a formal way. Previous attempts to test the lunar rhythm hypothesis were somewhat ambiguous because they tracked dissepiments over just a couple months. We knew we needed a longer study. We designed a study in which we first stained living Porites colonies in Palau with a stain that is incorporated into the skeleton. And then we waited for the corals to build their dissepiments. We waited 6 months before we returned to collected the first skeletal samples. Then we waited another 15 months before we collected another set of samples. This allowed us to track the formation of dissepiments over multiple periods of time. All told, by the time I analyzed all the samples, 4 years had passed since I first started designing the study.

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