We know increased temperatures can stress corals and cause bleaching – which is when they symbiotic algae that produce energy for the corals are lost, causing the coral to turn white, stop growing and often die. But, with climate change, corals are often subjected to extremes of both heat and cold.
And now, a study published in Scientific Reports has shown that both heat and cold can stress corals, but they do so in different ways.
Melissa Roth, Ralf Goericke and Dimitri Deheyn from Scripps Institution of Oceanography studied an important species of reef building coral called Acropora by growing fragments in individual tanks and then subjecting them to a change in temperature – either plus or minus 5 degrees C from an original temperature of 26 degrees C.
The team found that Acropora subjected to a decrease in temperature showed a greater decline in growth and the amount of photosynthetic symbionts in their cells in the first 5 days of the experiment than those that were heat treated. This could be because the enzymes required for photosynthesis work more slowly at lower temperatures and because cold shock can cause the coral cells containing the symbionts to be jettisoned into the water.
However, over the final 15 days of the 20 day experiment, the cold treated corals fared much better. They acclimated and began to recover their growth rate and stabilise their symbiont numbers. In contrast, heat treated corals did progressively worse with prolonged exposure to increased temperature. Photosynthesis decreased and eventually the corals bleached and stopped growing.
The authors suggest that although temporary extremes of cold could threaten corals, it is a prolonged increase in temperature that is more damaging, which is worrying given the increases in temperature seen in many areas of the ocean.