| dc.description.abstract | The marine environment is hyperosmotic to the tissues of marine teleost fish, which results in passive water loss and ion gain throughout their lifetime. In order to avoid dehydration, marine teleosts drink seawater. Ingested seawater travels through the digestive tract, where it is manipulated in order to absorb water and selected ions. Calcium carbonate (CaCO3) is precipitated as part of this process, and aids water absorption by removing Ca2+ (calcium) and HCO3- (bicarbonate) ions from solution. This precipitate (hereby referred to as carbonate), which also contains some MgCO¬3, is then excreted to the environment as a waste product, where it can dissolve or become part of the sediment. The precipitation and dissolution of carbonate forms the marine inorganic carbon cycle, which we have only recently understood that fish form a significant part. In chapter one of this thesis, I have reviewed the literature to explore the many factors that affect the production of gut carbonates by fish, including absolute quantity, composition and how they affect the fate of carbonate after excretion. In chapter two I have experimentally investigated the effect of low temperature, from 3 to 14 °C, on the quantity and composition of carbonate excreted by lumpfish, Cyclopterus lumpus. Between 3 and 7 °C, carbonate excretion increased with a Q10 of 3.50, which is comparable to other species, and had no effect on the incorporation of MgCO3. This confirms that carbonate excretion rate responds to temperatures <10 °C in a similar way to the higher temperatures used in other studies. The consistent carbonate composition across tested temperatures suggests that any changes in dissolution rate of carbonates is likely to be due to temperature-associated changes in ΩCaCO3. In chapter three I investigated the effect of diets supplemented with calcium salts (CaCO3, Ca3(PO4)2, and CaCl2), two of which had elevated dietary pH buffering capacity, on the digestive physiology and carbonate production in freshwater-acclimated rainbow trout, Oncorhynchus mykiss. We found that consumption of any diet by these freshwater fish resulted in the alkalinisation of intestinal fluid with high levels of HCO3- ions resulting in the production and excretion of precipitated calcium carbonate minerals, previously thought to be exclusive to marine teleosts, which was particularly high in fish fed the CaCl2 or CaCO3 diets. This is the first known report of intestinal carbonate excretion by a freshwater fish. These results have consequences for modelling of global carbonate production by fish and potentially the production of aquaculture feeds. | en_GB |
