Traditionally, oceanic calcification is credited to tiny marine plankton,mostly coccolithophores and foraminifera. These plankton combine seawater calcium with bicarbonate to form calcium carbonate, carbon dioxide and water, via the inorganic marine carbon cycle. When plankton die, calcium carbonate is released from their `skeletons' into the ocean where some of it sinks to the ocean floor to form limestone sediments while some of it dissolves under the high pressures and low temperatures in the deep ocean to produce calcium and bicarbonate, which increases the water's alkalinity. However, there is another source of calcium carbonate that has remained beneath the radar: bony fish guts!

Marine fish need to continually drink calcium-rich seawater in order to maintain the correct internal water and ion balance while bathed in their aquatic environment. Previous studies have shown that marine fish release bicarbonate into their intestines, which combines with calcium from the imbibed seawater to form calcium carbonate. This process allows fish to absorb water across the gut and also prevents them from developing kidney stones. The calcium carbonate precipitate (or `gut rocks') is then released into the surrounding water. Rod Wilson from the University of Exeter and colleagues from Canada, England and the United States sought to determine how much marine fish gut rocks contribute to global levels of calcium carbonate.

The authors used two independent models to estimate the total amount of fish in the planet's oceans. Taking into consideration the effect of both body size and temperature, they assessed global fish calcium carbonate production. The team estimates that a remarkable 3–15% of the calcium carbonate in the ocean comes from fish. Wilson and colleagues used a conservative approach for their calculations and suggest that the contribution of fish may actually be up to 3 times higher. These findings show that fish make a substantial, but previously unrecognized, contribution to the marine inorganic carbon cycle.

Next, the team sought to compare the influence of fish gut rocks on the environment with the effects of carbonates from more conventional sources. While calcium carbonate dissolves deep in the oceans to make the water more alkaline, studies have shown that carbonates also dissolve at much shallower depths and lower pressures than predicted to increase the total alkalinity of coastal waters; a phenomenon that has puzzled oceanographers for decades. Fish gut rocks have high levels of magnesium, which allows them to dissolve at shallower depths (<1000 m) and lower pressures. Collating this information,Wilson and colleagues suggest that the dissolution of fish gut rocks at shallow depths could help to explain the perplexing observation.

This study shows that fish may play an unexpected role in the marine inorganic carbon cycle. Magnesium-rich gut rocks expelled from fish are a major contributor to oceanic carbonates and may help explain the elevated alkalinity of shallow seawater. Moreover, as the oceans become more acidified due to increased atmospheric carbon dioxide levels, the authors suggest that fish are likely to make an even larger contribution to the inorganic carbon cycle. Higher temperatures and elevated levels of carbon dioxide in fish blood will likely lead to them producing even more gut rocks, at a time when plankton are expected to begin producing less calcium carbonate. Though many questions remain, it seems that researchers are finally getting to the guts of the carbon cycle.

Wilson, R. W., Millero, F. J., Taylor, J. R., Walsh, P. J.,Christensen, V., Jennings, S. and Grosell, M. (
). Contribution of fish to the marine inorganic carbon cycle.