Increased hardiness of one common coral provides some hope among evidence of warming oceans
By Gerard Wynn
One common coral species survived higher temperatures through a process of acclimatisation over several months, showed a study published in the journal Science on Thursday, raising the possibility of a smaller climate impact than previously feared.
Corals are primitive animals which harbour plant-like algae which they also depend on to trap sunlight and feed them sugars.
As temperatures rise, corals eject the algae, causing coral bleaching and ultimately death if the algae do not quickly return.
The impact of climate change on corals is of great interest, firstly because of their role as the foundation for huge natural reefs teeming with fish life, and secondly because they appear vulnerable even to slight increases in ocean temperatures.
However, one common species can acclimatise to higher temperatures, Thursday’s study found.
“Our results show both short-term acclimatory and longer-term adaptive acquisition of climate resistance. Adding these adaptive abilities to ecosystem models is likely to substantially slow predictions of demise for coral reef ecosystems,” said the Stanford University authors.
The study left open the important question of what upper temperature limit there may be on such acclimatisation, and whether this ability applied to other coral species. They also allowed that ongoing ocean acidification as a result of manmade carbon emissions may reduce the ability of corals to acclimatise in this way.
A recent report by the UN panel, the Intergovernmental Panel on Climate Change (IPCC) ranked coral bleaching alongside melting Arctic sea ice as one of the first major climate impacts.
Corals acquire their vivid colours from the algae which they harbour, and the chlorophyll pigment they contain and use for photosynthesis.
The scientists analysed the performance of the common, so-called table top coral, which comprises a large percentage of hard coral cover on Pacific reefs.
“We chose Acropora hyacinthus for this study because it is a dominant reef-builder and especially sensitive to environmental stress, making its ability to acclimate or adapt extremely important to the future of coral reef ecosystems as climate change proceeds,” they said.
They compared the heat tolerance of the same coral species in shallower pools, where temperatures rose higher at low tide, with those inhabiting deeper water.
They showed that corals native to the shallower pools could better survive a ramp up in temperatures.
They then tested for acclimatisation by transplanting corals from shallower into deeper water and vice versa.
They found that the bleaching resistance of corals transplanted from deeper to shallower water improved, which they termed moderately variable (MV) and highly variable (HV) pools respectively.
“The experiments showed higher chlorophyll-a retention during heat stress in colonies transplanted to the HV Pool than in the same colony transplanted to the MV Pool.”
“Although the MV Pool corals acquired heat resistance when moved to the HV Pool, they did not achieve the resistance of native HV corals.”
They concluded that corals native to the warmer water had built up a superior heat resistance over generations, but that even individuals used to cooler waters could produce at least some heat tolerance in the short term.
They traced the acclimatisation to the greater use of heat resistant proteins.
In its report last month, the IPCC documented widespread damage to corals in recent decades, linked both with the trend in rising sea temperatures, and particular warming events such as the regular El Nino weather phenomenon.
Average sea surface temperatures of the Indian, Atlantic and Pacific Oceans rose by 0.65, 0.41 and 0.31C over the period 1950–2009, it found.
“The conclusion based on outputs from a wide range of emissions scenarios and models is that preserving more than 10 percent of coral reefs worldwide would require limiting warming to less than (an average of) 1.5 degrees compared to pre-industrial levels,” the report said.