Cauliflower corals cannot migrate as temperatures rise Free

Humans have long worked with so-called sentinel species to warn of impending danger: from bats and birds accumulating insecticides in their bodies that warn of pollution, to canaries alerting colliers to carbon monoxide down mines. But possibly the largest sentinel organism on the planet is the Great Barrier Reef. In recent years, it has suffered repeated bleaching as sea temperatures rise. One hope has been that threatened corals may be able to seek refuge from increasing heat by moving further poleward. But for that to occur, corals would need to be able to adapt to new temperature environments and it wasn't clear whether they can. However, Peter Edmunds from California State University, Northridge, USA, knew that if he could measure the metabolic and photosynthetic rates of corals collected from different latitudes on the planet – to identify whether they had significantly different thermal tolerances – he could begin to discover whether the exquisite symbiotic organisms stand a chance as global temperatures increase.

Teaming up with colleagues at the University of Guam, USA (13.5°N), and the University of the Ryukus, Japan (26.6°N), Edmunds collected small samples of cauliflower corals (Pocillopora spp.) off the coast during the northern hemisphere autumn, transferring the corals to seawater tanks in his colleagues’ laboratories. Then, he selected individual portions of coral, warmed or cooled them in the dark – when the photosynthetic algae that reside within the coral's body could not photosynthesise and produce oxygen – and measured the coral's oxygen consumption to determine its metabolic rate at that temperature. Next, Edmunds turned on an LED lamp to trigger the algae to begin photosynthesising, and measured their oxygen production. Repeating the process at temperatures ranging from ∼21°C up to ∼35°C, Edmunds was able to plot how the increasing temperature impacted the corals’ ability to live and photosynthesise. Six months later, he travelled to the University of California's Gump Station, Moorea, in the South Pacific (17.5°S), to repeat the process during the southern hemisphere autumn to investigate the coral's ability to respond to rising temperatures south of the equator.

However, when Edmunds plotted the oxygen consumption rates of all of the corals, they were remarkably similar. Living at different latitudes had not altered their ability to withstand heat. And the same was true for the corals’ photosynthetic algal lodgers, which all produced oxygen at roughly the same rate across the ∼15°C temperature range. ‘This result is inconsistent with the notion of local adaptation of reef corals’, says Edmunds, warning, ‘it provides little hope that migrations of Pocillopora spp. in the Pacific could modulate their regional response to increasing temperature’.

But it's not all doom and gloom. After looking in more detail at the annual temperature ranges and seasonal light variations at the three locations, the team suspects that the most northern cauliflower corals in Okinawa might be able to accumulate larger food reserves over winter, as their respiration declines, which could help them to weather rising summer temperatures, offering some hope. And Edmunds discovered a possible explanation for the resilience of P. meandrina to bleaching during the 2019 heatwave in Moorea, which killed off 71% P. cf. effusa corals. The algae that reside within P. meandrina are able to increase their photosynthesis up to ∼31°C, giving the coral a ∼3°C safety margin above current average water temperatures, which could have been sufficient for protection in 2019.

So, it seems that migrating to cooler climes may not offer hope for the survival of cauliflower corals, but Edmunds is keen to conduct similar experiments across a wide range of locations to find out what the future holds for other species.