Coral copes with acidic ups and downs

Coral reefs can tolerate rising and falling ocean pH levels but are still in danger from increasing ocean acidity, say marine scientists. 

Carles Pelejero from Institut de Ci?ncies del Mar, Barcelona, Spain, and colleagues looked at a large coral from Flinders Reef in the western Coral Sea and, using boron isotope measurements, traced the ocean’s acidity over the past 300 years. 

Acidity values in the reef water rise and fall in 50 year cycles, the researchers claim.   They didn’t expect these cycles, expecting CO₂ levels from burning fossil fuels to dominate reef-water pH. ’We thought we would be able to see a tendency towards acidification for the last decades,’ Pelejero told Chemistry World. 

The coral has so far coped with these changes in acidity, but in future, as the oceans become more acidic the reefs will grow progressively weaker, said Pelejero.

’Natural cycles in reef-water pH could enhance or mitigate the vulnerability of corals to future ocean acidification.’ When the low-pH part of the cycle coincides with a drop in ocean pH from increased CO₂ absorption, ’waters in Flinders Reef will experience pH levels with no precedents,’ said Pelejero. As pH drops, the aragonite saturation state - which controls coral calcification - will be too low for corals to develop, he added. 

This evidence that coral can tolerate rising and falling pH is ’slightly good news’, said coral-reef expert Jason Hall-Spencer from Plymouth University, UK. The study’s limitation, according to Hall-Spencer, is the sample size of just one coral core. ’You don’t know what that says about the whole reef,’ he said. ’It would be good if the study were backed up with replication’ - from other parts of the same reef and also other reefs around the world. Palejero concedes this limitation and plans to work towards building a ’more global oceanic picture’. Katharine Sanderson