BioEd Online

Animals such as sea stars, sea urchins and sea lilies bury much more carbon than anticipated, according to the first study to estimate echinoderms' contribution to ocean carbon storage.

Studies of biological carbon in the oceans tend to focus on organisms that drift through the shallows, such as plankton, because they are known to store carbon in the form of calcium carbonate, which they transport to the sea floor when they die.

Mario Lebrato suspected that bottom-dwelling animals such as echinoderms also store large amounts of calcium carbonate, and wondered how large a role they might have in the global carbon cycle.

While still an undergraduate at the University of Southampton, UK, Lebrato, now a PhD student at the Leibniz Institute of Marine Science in Germany, set out to study the rates at which echinoderms absorb calcium carbonate and what happens to the carbon when they die. "The funding for this was initially derived from my pocket because nobody believed in the echinoderm [carbon] contribution," says Lebrato.

He and his colleagues collected echinoderm samples from both deep sea and shallower waters at multiple latitudes in the Atlantic Ocean. At every stop, they tried to collect a few echinoderms from each of the phylum's five main classes: sea stars (Asteroidea), sea urchins (Echinoidea), brittle starts (Ophiuroidea), sea cucumbers (Holothuroidea) and sea lilies (Crinoidea). Only fully grown adult animals were collected, and all were cleaned thoroughly before being freeze dried and disintegrated into powder for carbon analysis.

Overlooked organisms

The team calculated carbon measurements for the classes that they collected at all sample sites. They then used these data to inform estimates of carbon content for echinoderms collected at various latitudes from other oceans around the world.

The researchers later paired their carbon estimates with population density and mortality data for the different echinoderm classes worldwide to determine how much calcium carbonate full-sized animals store in their bodies and how quickly that carbon was buried by sediment after the animals die.

Lebrato and his colleagues report in the journal ESA Ecological Monographs that, worldwide, echinoderms capture around 0.1 gigatonnes of carbon per year¹. This is less than the global capture resulting from pelagic organisms — a figure that ranges from 0.4 to 1.8 gigatonnes depending on the sources considered — but still represents a sizeable carbon pump. By comparison, human activities lead to around 5.5 gigatonnes of carbon being pumped into the air every year.

"Echinoderms are found in all ecosystems at all depths worldwide and have bodies that can be composed of more than 80% calcium carbonate … so we were almost expecting a result like this," says Lebrato.

Eye opener

"I was definitely surprised by the magnitude of the values reported in this study, but their approach seems sound, so the reported numbers are probably fairly accurate," says palaeoceanographer Justin Ries of the University of North Carolina at Chapel Hill.

"These numbers are eye opening, but I suspect there's more to come," says Craig Smith, a biological oceanographer at the University of Hawaii at Manoa. The numbers that this study reveals are probably an underestimate because vast regions of the Equatorial Pacific have high echinoderm biomass but are not well studied, he says.

"Because this project had to extrapolate over large areas, there are no doubt hot spots that were glossed over and not included in the global average," says Smith. These hot spots, he explains, need to be targeted for future study.

One key issue that the research raises is the potential effect that ocean acidification resulting from increasing atmospheric carbon dioxide might have on echinoderms and their carbon-storage abilities. "If the echinoderms end up being disproportionately susceptible to ocean acidification then it's conceivable that the dissolving of echinoderm-derived sediments will be one of the earliest effects of ocean acidification on the global carbon cycle," says Ries. "In fact, maybe it already is."