For coral reefs it´s not just ocean acidification, but also local chemistry that counts

CO2 dissolves carbonate – so CO2 is bad. Sea cucumbers eat carbonate. But at least they do give something good back to the water.

That´s what we learn from new research led by Stanford´s Carnegie Institution for Science that was published yesterday in the Journal of Geophysical Research.

The reasoning of the researchers is simple. One could say that coral bleaching increases with temperature rise – or even predict coral reefs’ extinction from a certain pH value as CO2 levels grow higher and higher and ocean acidification continues. But what in the end matters most is CaCO3 buildup versus CaCO3 breakdown. And then you have to include various biological processes – also the ones that favour carbonate dissolution, they say, like those strange marine creatures we somehow failed to give a more intrinsic name, sea cucumbers.

Lab results by the Carnegie group indicate that sea cucumbers around Australia’s Great Barrier Reef could be responsible [because they are so abundant – about one sea cucumber per square meter] for about half of the nighttime CaCO3 breakdown, simply by grazing the coral sands of reef systems.

During the dissolution in their digestive tract the sea cucumbers actually ‘produce’ alkalinity, the researchers say:

“In aquarium incubations with Stichopus herrmanni and Holothuria leucospilota total alkalinity increased by 97 ± 13 and 47 ± 7 μmol kg⁻¹, respectively. This increase was due to CaCO₃ dissolution, 81 ± 13 and 34 ± 6 μmol kg⁻¹ and ammonia secretion, 16 ± 2 and 14 ± 2μmol kg⁻¹, respectively, for these species.”

From this they conclude sea cucumbers could – perhaps – help improve the resilience of shallow seas around reef system to ocean acidification – as long as these waters don’t mix too much:

“Thus, in a healthy reef, bioeroders dissolution of CaCO₃ sediment appears to be an important component of the natural CaCO₃ turnover and a substantial source of alkalinity as well. This additional alkalinity could partially buffer changes in seawater pH associated with increasing atmospheric CO₂ locally, thus reducing the impact of ocean acidification on coral growth.”

Sounds like these creatures could do some good work. Bottleneck could become their supply of fresh carbonate sand. But not to worry – other research has actually shown we can feed sea cucumbers from above.