Sea urchins tolerate acid water

Sea urchin Echinoderm larvae showed no adverse effects when exposed to water that was relatively high in CO2

Sea urchins are likely to be able to adapt to increasingly acidic oceans resulting from climate change, according to new research.

When the animals, known as echinoderms, were exposed to water high in carbon dioxide early in their lives, there were no adverse effects.

Echinoderms are a diverse group that includes sea cucumbers and starfish.

Their natural resilience could represent a competitive advantage under some climate change scenarios.

The experiments, carried out by Nadia Suarez-Bosche, exposed larvae of the shallow-dwelling sea urchin Psammechinus miliaris to deep-sea water naturally rich in CO2.

After five days of incubation in the water samples, the scientists measured the physiological responses of the larvae and found that they were still growing and developing well even under the highest CO2 concentrations of up to 600 parts per million (ppm).

The current atmospheric CO2 concentration is around 390 ppm.

When CO2 dissolves in water, carbonic acid is formed. Previously, it was thought that the increasing acidity of seawater - caused by the oceans absorbing more CO2 from the atmosphere - would be damaging for these organisms.

It was thought the corrosive effect of the acid would harm their calcium carbonate skeletons.

The key to their ability to tolerate a wide range in water pH (the scale that determines how acid or alkali something is) comes from the variability of their natural habitat, even under present environmental conditions.

"Echinoderms are found all over the world's oceans, but particularly in coastal environments, where they are naturally exposed to huge fluctuations in pH," explained Dr Debora Iglesias-Rodriguez, leader of the research team and a co-author of the study.

Carbon 'Sink'

Echinoderms belong a group of organisms known as "calcifiers", which incorporate carbon from seawater directly into their skeletons in the form of Calcium Carbonate (CaCO3).

Whilst the scientists found no adverse effects on larval development or soft tissue production in the present study, they did observe a significant decrease in the amount of calcium carbonate that the organisms produced, resulting in smaller and thinner skeletons.

Given future acidification scenarios, "calcification could decrease but it would not prevent larvae from colonising the deep sea because they can tolerate these changes in the carbon chemistry of the water," explained Ms Suarez-Bosche.

However, this is likely to have implications for the global carbon budget, as calcification is an important removal process, or "sink", for carbon in the ocean.

Collaborative research suggests echinoderms currently contribute more than 5% to the total removal of inorganic carbon from the surface ocean to the deep sea, which happens when these organisms die and sink to the deep ocean - a process known as the "biological carbon pump".

Ocean acidification is widely considered one of the most pressing challenges in climate science but predicting the likely effects on the wide range of calcifying plants and animals in the ocean is complicated.

This has led to some contradictory findings among previous studies and difficulty in reaching a general consensus on the changes likely to occur.

The researchers believe their findings in sea urchins could potentially apply to other species of echinoderms, but more research is needed to find out.

Mimicking Nature

The study is the first to investigate the impact of elevated CO2 levels on echinoderms using seawater naturally high in CO2, without relying on experimental manipulation of seawater in the laboratory.

Seawater was collected during a research cruise to the Porcupine Abyssal Plain site in the North Atlantic in 2009.

Echinoderm spawn was immediately added to the sample, which was then sealed and left to incubate. Physiological measurements, pH and other aspects of the carbonate chemistry were taken at time zero and again at the end of the experiment.

"Unfortunately, we cannot do these experiments in-situ because of the extreme pressures involved and so this is not a perfect analogue for ocean acidification, but using this method mimics potential conditions as naturally as we can," explained Dr Iglesias-Rodriguez.

The scientists from the National Oceanography Centre presented their findings recently at the Challenger Society for Marine Science conference in Southampton, UK.

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