As the oceans warm and the pH of the water drops, the future looks bleak for corals. Mass bleaching events occur when the conditions are hot and coral hosts lose the algal lodgers that provide their energy and nutrients. Tiny polyps also struggle to lay down their delicate calcified skeletons in more acidic waters. ‘The current rates of change in the environment are dangerously rapid in relation to the rate of coral genetic adaptation’, says Hollie Putnam from the University of Hawaii, USA, suggesting that corals may not be able to evolve fast enough to keep pace with the damaging effects of climate change. However, Putnam and her colleague Ruth Gates wondered whether there was a glimmer of hope for the apparently doomed animals. Could adult corals pass on advantages to their young that might safeguard their future through alternative mechanisms such as epigenetics? Putnam headed out, into the warm shallow waters of Hawaii's Kaneohe Bay, to collect samples of the cauliflower coral (Pocillopora damicornis) to find out whether exposing the adults to future climate conditions might improve their offspring's chances.
Carefully nurturing the corals in the lab, Putnam kept some in natural seawater (26.5°C and 417 μatm PCO2) while others were placed in water that mimicked predicted ocean conditions (28.9°C and 805 μatm PCO2) for 1.5 months. Then, knowing that the corals release their larvae around the time of the full moon, Putnam transferred the adults to collecting bins to capture the youngsters, ready to expose half of them to modern seawater conditions while the others were placed in the future conditions for 5 days before testing to see how they faired.
Measuring the photosynthetic activity of the coral symbionts, and the metabolic and calcification rates of the corals, Putnam was surprised to see that the warmer and more acidic conditions did not seem to affect the corals’ ability to grow or their metabolic rate. However, the adult's symbionts suffered: their net photosynthetic rate plummeted by almost 80% while the ratio of algal photosynthesis to the respiration rate of the intact coral also plummeted. But how would the offspring of adults that had already had a dose of the future cope in the tougher conditions?
The larvae of parents that had been incubated in the future ocean conditions were smaller and had a lower metabolic rate than the larvae of parents in normal seawater. However, when the future-conditions larvae were placed in the warmer and more acidic conditions, their size-adjusted metabolic rate increased.
‘It is often thought that the early life stages of marine organisms are the most sensitive’, says Putnam; however, she and Gates have shown that the larvae performed better when their parents had been preconditioned to the more stressful future conditions. ‘This is counter to what would be predicted’, she says. Putnam also suspects that the parents that had been prepared in the future climate conditions give their youngsters a head start either by providing them with additional energy reserves or by passing on benefits through epigenetic modifications to the DNA (that do not change the molecular code of a gene but instead alter when the gene is activated), which may offer protection to their offspring. In addition, Putnam is keen to continue studying the youngsters through to adulthood and beyond, to find out whether their own young will benefit from the parents’ experience: maybe the coral's future is less bleak than we thought.
