Fish carbonates - their nature and fate within the marine inorganic carbon cycle

Lead Research Organisation: University of Exeter
Department Name: Geography


This proposal is based on a fundamentally important and previously unexpected change to our understanding of the marine inorganic carbon cycle. It follows our recent revelation that calcium carbonates excreted by fish make a significant contribution but our current estimates vary over more than a 10-fold range (3 to 45 % of global marine carbonate). BACKGROUND: As humans burn more fossil fuels, atmospheric concentrations of the 'greenhouse gas' carbon dioxide (CO2) rise contributing to climate change. Atmospheric CO2 is in balance with CO2 dissolved in the oceans, in something referred to as the marine-atmospheric carbon cycle. Whatever happens to CO2 in the oceans, will ultimately have an effect on CO2 in the atmosphere, and hence can influence global climate. When CO2 dissolves in seawater, it forms bicarbonate ions. An important part of the marine-atmospheric carbon cycle is the reaction of this bicarbonate with seawater calcium to produce a solid precipitate of white calcium carbonate (the mineral found in limestone). These precipitates are very dense, and sink to the ocean bottom in a continuous 'rain' of white crystals. The rate at which they form and sink (or re-dissolve) is important in the marine carbon cycle. The majority of calcium carbonate is generated by marine life that promotes this reaction to make a hard protective 'shell'. The most famous organisms involved in this 'biogenic' calcification are corals. However, those thought to produce the most are actually microscopic phytoplankton called coccolithophores that live in the open ocean. Dense skeletons of these and other microscopic organisms are normally considered to be the only important source of marine carbonates. Scientists collect samples in deep ocean traps, to measure this carbonate 'rainfall' for use in computer models of the carbon cycle. We have recently discovered that marine fish also produce substantial amounts of precipitated calcium carbonate, but for a very different purpose. They produce it in their intestines, by drinking large volumes of seawater and actively promoting the reaction of seawater calcium with bicarbonate ions that are produced by their own metabolism. Fish then excrete the precipitated calcium carbonate into the surrounding seawater, where it probably mixes with all the better known (planktonic) sources. In fact, some tropical fish will excrete calcium carbonate equivalent to its own dry body weight every year! We have conservatively estimated that the contribution of fish may be up to 45% of the total global carbonate production. This novel discovery suggests that fish also contribute to the marine carbon cycle, but scientists who model this cycle have never previously taken this into account. Indeed, the unusual chemistry of fish carbonates (which are more soluble than carbonate from more traditional sources), may explain a phenomenon that has puzzled oceanographers for decades - the rapid dissolution of 'apparently insoluble' carbonates in the upper layers of the ocean. Our research is a multi-disciplinary project that for the first time aims to precisely model how much calcium carbonate is produced by marine fish under different environmental conditions and determine its fate within in our oceans. This will also help with predictions about how carbonate excretion by marine fish will be affected by future environmental changes, such as temperature and CO2. We predict that fish will become even more important in this regard in the future, whereas marine plankton will become less important. Thus a precise understanding of this fish contribution to the global marine carbon cycle is both a novel and environmentally important topic.

Related Projects

Project Reference Relationship Related To Start End Award Value
NE/H010092/1 31/03/2010 09/12/2011 £18,896
NE/H010092/2 Transfer NE/H010092/1 11/03/2012 06/02/2014 £13,287
Description The main aims of this project (led by PI Rod Wilson) were to provide an improved understanding of how a suite of marine environmental parameters influence rates of carbonate production in marine fish - and which can then be integrated to develop new models of oceanic marine carbonate production. The main findings of the experimental feeding and production rate elements of the project are reported by PI Wilson in this return. I oversaw the analysis and interpretation of the carbonate products from the fish and provided supervision on the SEM based analysis. This work has contributed a large set of new data on the crystal products that a wide suite of temperate water fish species produce. Many of these are, somewhat unexpectedly, very similar to those previously described from Caribbean locations and, in combination with other NERC funded work suggest considerable uniformity (at least within fish families) in terms of the carbonate products being produced by fish. The other novel finding from this area of the project was that many of these carbonates also apparently contain phosphate that appears to be incorporated into the crystal lattice (or certainly bound within the crystal aggregates). This has significance because of the potential impacts of crystal dissolution potential. A number of publications are pending from this work (the PDRA was on maternity leave and this has delayed outputs).
Exploitation Route the main applications result to developing revised models of ocennic inorganic carbonate production - work being conducted in collaboration with Cefas.
Sectors Environment

Description Leverhulme Trust grant
Amount £156,000 (GBP)
Funding ID RPG-2017-024 
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 12/2017 
End 11/2020