A project undertaken at Monash University and led by Dr. Adam Kessler
The bright, white sands that you see on or near tropical reefs are made of (almost) pure calcium carbonate. This is the mineral that makes up the shells and skeletons of many marine favourites: corals, shellfish and even microscopic plankton. These white, carbonate sands are made from tiny fragments of these creatures.
Carbonate sands cover most of the sea floor around the Great Barrier Reef, and many other environments around Australia. These environments form important marine habitats, and support fisheries and tourism and make up the bulk structure of coral reefs.
Figure 1. a carbonate sand beach at Heron Island, QLD, Australia
Under acidic conditions, calcium carbonate dissolves. As the levels of carbon dioxide (CO2) in our atmosphere are rising, some of that CO2 reacts with the surface of the ocean, forming carbonic acid. You may have seen reports that some species of coral, shellfish and plankton are struggling to make their shells, because of this increased ocean acidity.
Like the corals and shells that form them, carbonate sands may dissolve as the acidity of the ocean rises. Currently, the ocean is not acidic enough for this to happen, and yet we are seeing evidence over reefs in Australia, Hawaii and the Caribbean that this is already beginning to happen. Therefore, something must be happening in the sediment that is causing this dissolution.
This project aimed to find out what microbial and chemical processes in the sediment lead to dissolution of reef sands.
In this project, we showed three important new findings:
(1) Fermentation is important in reef sediments. If you look in a textbook, there is a very well-established pattern of carbon mineralisation reactions that are usually thought to occur in sediments. We found that several of these reactions occur less than expected, and instead many of the microbes use fermentation as one of their main energy sources. This reaction is similar to the fermentation reactions that you might know performed by yeast in bread or beer.
(2) The sands on the reef are dissolving, but this dissolution is occurring only in microscopic cracks and crevices on the surface of individual grains of sand. Bacteria live in these cracks and the CO2 that they produce is just enough to make the water acidic enough that the grains start to dissolve. Even though these cracks are tiny, there are many cracks per grain (hundreds, or maybe thousands), and billions of grains on a reef, so the overall rate of dissolution is actually very fast!
(3) These two findings interact. The unexpected high rates of fermentation have two effects on dissolution. First, they create slightly more acidic conditions than the more classical, expected reactions, and therefore probably increase dissolution rates. Second, they create a chemical environment that can mask some of the signal that we use to measure dissolution, therefore potentially making our measurements of dissolution rates underestimates.
