Freshwater fish drink very little, while most marine fish ingest copious amounts of seawater to maintain salt and water balance. The European flounder is euryhaline and illustrates that no intestinal calcium carbonate is formed in freshwater (Figure A) and only a brief period in seawater results in the formation of x-ray opaque calcium carbonate precipitates in the intestinal lumen (Figure B). The calcium carbonate precipitates are formed regardless of feeding and are excreted to the surrounding seawater, where it impacts the pH balance.
Image credit: Rod W. Wilson, Exeter University
Until now, scientists believed that the oceans' calcium carbonate, which dissolves in deep waters making seawater more alkaline, came from marine plankton. The recent findings published in Science explain how up to 15 percent of these carbonates are, in fact, excreted by fish that continuously drink calcium-rich seawater. The ocean becomes more alkaline at much shallower depths than prior knowledge of carbonate chemistry would suggest which has puzzled oceanographers for decades. The new findings of fish-produced calcium carbonate provides an explanation: fish produce more soluble forms of calcium carbonate, which probably dissolve more rapidly, before they sink into the deep ocean.
Corresponding authors Drs. Frank Millero and Martin Grosell at the University of Miami's Rosenstiel School of Marine and Atmospheric Science and Dr. Rod Wilson of the University of Exeter note that given current concerns about the acidification of our seas through global CO2 emissions, it is more important than ever that we understand how the pH balance of the sea is maintained. Although we know that fish carbonates differ considerably in their chemical make-up, the team has really only just scratched the surface regarding their chemical nature and ultimate fate in the ocean. Scientists clearly need to investigate this further to understand what this means for the future health of the world's oceans.
Millero, Grosell and Wilson, who was the recipient of the University of Miami's prestigious 2005 Rosenstiel Award, along with Rosenstiel School Marine Biology and Fisheries graduate student Josi Taylor collaborated with other British and Canadian scientists to reach the conclusion published in the current issue of Science.
The researchers suggest that fish carbonates dissolve much faster than those produced by plankton, and at depths of less than 1,000 m. Less soluble carbonates, produced by plankton, are more likely to sink further and become locked up in sediments and rocks for tens or hundreds of millions of years before being released. Fish carbonates, on the other hand, are likely to form part of the 'fast' carbonate system by more rapidly dissolving into seawater.
"As a marine chemist who has been studying the global carbon cycle and its impacts on the pH of the water and marine ecosystems for 40+ years, these results offer an important piece of the equation," said Millero, professor of Marine and Atmospheric Chemistry at the Rosenstiel School. "By working with scientists in several disciplines we were able to come at this from different perspectives and combine data sets that hadn't been previously used together, to solve this problem. We can now employ the knowledge gained from this study to examine how ocean acidification due to the adsorption of CO2 from the burning of fossil fuels affects the ocean carbon system."
The combination of future increases in sea temperature and rising CO2 will cause fish to produce even more calcium carbonate, which is in sharp contrast to the response by most other calcium carbonate producing organisms. Fish's metabolic rates are known to increase in warmer waters, and this study explains how this will also accelerate the rate of carbonate excretion. In addition, our existing knowledge of fish biology shows that blood CO2 levels rise as CO2 increases in seawater and that this in turn will further stimulate fish calcium carbonate production.
"Depletion of fish stocks due to overfishing will obviously influence global calcium carbonate production attributable to fish, but the prediction of the impact of overexploitation is complex. Smaller fish which often result from exploitation produce more calcium carbonate for the same unit of biomass than bigger fish, a simple consequence of higher mass-specific metabolic rates in the smaller animals. In addition, the chemical nature of the calcium carbonate produced by fish, which determines solubility, almost certainly will depend on temperature, fish species, ambient pH and CO2 levels among other factors. The influence of such factors on this newly recognized and significant contribution to oceanic carbon cycling offers an exciting challenge for further study" said Grosell, associate professor of Marine Biology and Fisheries at the Rosenstiel School.
Press release of the University of Exeter:
Fish guts explain marine carbon cycle mystery
Research published today reveals the major influence of fish on maintaining the delicate pH balance of our oceans, vital for the health of coral reefs and other marine life.
The discovery, made by a team of scientists from the UK, US and Canada, could help solve a mystery that has puzzled marine chemists for decades. Published in Science, the study provides new insights into the marine carbon cycle, which is undergoing rapid change as a result of global CO2 emissions.
Until now, scientists have believed that the oceans' calcium carbonate, which dissolves to make seawater alkaline, came from the external 'skeletons' of microscopic marine plankton. This study estimates that three to 15 per cent of marine calcium carbonate is in fact produced by fish in their intestines and then excreted. This is a conservative estimate and the team believes it has the potential to be three times higher.
Fish are therefore responsible for contributing a major but previously unrecognised portion of the inorganic carbon that maintains the ocean's acidity balance. The researchers predict that future increases in sea temperature and rising CO2 will cause fish to produce even more calcium carbonate.
To reach these results, the team created two independent computer models which for the first time estimated the total mass of fish in the ocean. They found there are between 812 and 2050 million tonnes (between 812 billion and 2050 billion kilos) of bony fish in the ocean. They then used lab research to establish that these fish produce around 110 million tonnes (110 billion kilos) of calcium carbonate per year.
Calcium carbonate is a white, chalky material that helps control the delicate acidity balance, or pH, of sea water. pH balance is vital for the health of marine ecosystems, including coral reefs, and important in controlling how easily the ocean will absorb and buffer future increases in atmospheric CO2.
This calcium carbonate is being produced by bony fish, a group that includes 90% of marine fish species but not sharks or rays. These fish continuously drink seawater to avoid dehydration. This exposes them to an excess of ingested calcium, which they precipitate into calcium carbonate crystals in the gut. The fish then simply excrete these unwanted chalky solids, sometimes called 'gut rocks', in a process that is separate from digestion and production of faeces.
The study reveals that carbonates excreted by fish are chemically quite different from those produced by plankton. This helps explain a phenomenon that has perplexed oceanographers: the sea becomes more alkaline at much shallower depths than expected. The carbonates produced by microscopic plankton should not be responsible for this alkalinity change, because they sink to much deeper depths intact, often becoming locked up in sediments and rocks for millions of years. In contrast, fish excrete more soluble forms of calcium carbonate that are likely to completely dissolve at much shallower depths (e.g. 500 to 1,000 metres).
Lead author Dr Rod Wilson of the University of Exeter (UK) said: "Our most conservative estimates suggest three to 15 per cent of the oceans' carbonates come from fish, but this range could be up to three times higher. We also know that fish carbonates differ considerably from those produced by plankton. Together, these findings may help answer a long-standing puzzle facing marine chemists, but they also reveal limitations to our current understanding of the marine carbon cycle."
And what about the future? The researchers predict that the combination of increases in sea temperature and rising CO2 expected over this century will cause fish to produce even more calcium carbonate. This is for two reasons. Firstly, higher temperatures stimulate overall metabolism in fish, which drives all their biological processes to run faster. Secondly, increasing CO2 in their blood directly stimulates carbonate production by the gut specifically.
Dr Rod Wilson continues: "We have really only just scratched the surface of knowing the chemistry and fate of fish carbonates. Given current concerns about the acidification of our seas through global CO2 emissions, it is more important than ever that we understand how the pH balance of the sea is normally maintained. Because of the impact of global climate change, fish are likely to have an even bigger influence on the chemistry of our oceans in future. So, it is vitally important that we build on this research to help fully understand these processes and how this will affect some of our most precious marine ecosystems."
This study was carried out by the University of Exeter (UK), University of Miami (USA), University of Ottawa (Canada), University of British Columbia (Canada), Centre for Environment, Fisheries and Aquaculture Science (UK) and University of East Anglia (UK).
Dr Rod Wilson's research was supported by the Biotechnology and Biological Sciences Research Council (BBSRC).