As if there were not already enough reasons to save coral reefs, scientists studying the Red Sea have discovered another one: they can be major sinks for carbon dioxide.
Coral reefs grow from two major chemical reactions: photosynthesis and calcification. Reef photosynthesis stores carbon, just as forests do on land, but calcification takes hydrogen carbonate from the water and stores some of the carbon in coral bodies, while releasing the rest. Measurements indicate the net effect is usually for coral reefs to be sources of slightly more carbon dioxide than they capture, although in most cases not by enough to alter the global carbon budget much.
In three papers published last year a team studying some of the world’s most unusual coral reefs show there is an important exception to this.
The Red Sea is almost entirely surrounded by deserts, making it subject to regular dust storms. Professor Hamish McGowan of the University of Queensland and co-authors found that 24 hours after dust falls on them the reefs of the gulf of Aqaba/Eilat, an arm of the Red Sea, experience a pulse of photosynthesis. This can pull enormous quantities of carbon dioxide out of the water over several days. Since this CO2 eventually gets replenished from the atmosphere, the net effect is to draw millions of tonnes of carbon out of the air, balancing the emissions of a small country. Even smoothed out on an annual basis, the reefs drew twice as much carbon dioxide from the atmosphere as the surrounding waters.
“This process was previously thought to be impossible, but our research proves otherwise,” McGowan said in a statement.
McGowan told IFLScience another team studying the phenomenon confirmed the growth pulses are the result of nutrients in the dust supercharging both coral and their symbiotic photosynthesizing zooflagellates. This is not stopped by the fact the low nutrient density of the dust.
Most coral reefs do not have neighboring deserts, let alone surrounding ones, but McGowan is keen to explore whether those on Australia’s northwest coast experience similar dust-driven growth. Moreover, while the Caribbean is far from the Sahara, it is known to be fertilized by its dust. No one yet knows if the same goes for the reefs there. Until we know how widespread the phenomenon McGowan observed is, we can’t tell how important it is to the global climate budget.
Coral reefs are frequently devastated by nutrient-rich runoff from floods. McGowan told IFLScience the volume of material in these cases is much greater, an example of having far too much of a good thing. If anyone is thinking of fertilizing reefs with dust as a form of carbon sequestration it would need to be done with great care.
The Red Sea reefs are known to tolerate temperatures at which other corals would bleach and die. Hopes have been expressed that transplanting these heat-adapted corals to other parts of the world could be a last-ditch way of saving reef ecosystems.
McGowan is interested in the possibility that the dust helps buffer the corals against heatstroke. However, in the course of their work, McGowan and colleagues identified another factor that contributes to the area’s reef’s resilience, and can’t be replicated elsewhere. “These reefs are growing in a hyperarid environment,” McGowan told IFLScience. “Dry air blows off the surrounding deserts and causes 3 meters of evaporation [a year], that has to be replaced through the Straits of Tiran.” This cools the surface waters so that the temperatures the corals experience are not as high as had been thought. Most other coral reefs are in much more humid areas, where evaporation is far lower.
The study is open access in the Journal of Geophysical Research: Oceans with papers on other aspects of the research in Frontiers in Marine Science and the Journal of Geophysical Research: Atmospheres.