The salty marine environment is dehydrating and marine fish are left with seawater as the only source of hydration. In land-dwelling animals ingestion of seawater results in severe diarrhea which, in itself, is dehydrating. In contrast, marine fish are able to absorb fluids from ingested seawater and can successfully eliminate the associated salt gain. The present project aims to further characterize the intestinal processes that allow for these vertebrates to exist in abundance in oceanic environments. Specifically, uptake of chloride across the intestine, which is important for the absorption of water, is the focus of the present research. The PI's previous NSF funded research identified a novel pathway for chloride absorption, which in most fish accounts for at least 50% of the total uptake. This chloride uptake pathway involves the secretion of large quantities of base into the intestinal lumen, but excretion of this base could ultimately result in a challenge to the acid-base balance of the fish. This challenge is met in marine fish by excretion of a proportional amount of acid across their gills to maintain normal blood pH. Through the use of molecular techniques and studies of cells lining the intestinal lumen the goal of this research is to identify the specific proteins involved in this unique chloride uptake pathway. In addition, recently developed equipment, which allows for the determination of both oxygen consumption and ion transport by fish intestine, will be employed to test that hypothesis that absorption of fluid from ingested seawater has a high metabolic cost. Through the course of this research graduate and undergraduate researchers will be trained. In addition, the findings of this research will be incorporated into a university course about the environmental biology of fishes and into an environmental health curriculum targeted to high school teachers in the local public school system, which has a highly diverse demography.
PI: Martin Grosell The ocean is a dehydrating environment and marine fish are forced to drink seawater as their only source of rehydration. The absorption of water by the intestine is a significant physiological challenge and the present project aimed and gaining further understanding of the processes allowing for absorption of water from ingested seawater. The marine fish intestine secretes high levels of bicarbonate which reacts with calcium in the ingested seawater to produce calcium carbonate. The process of forming calcium carbonate leads to a reduction of the osmotic pressure in the intestinal fluids which facilitates diffusive water absorption by the intestine. The concentration of bicarbonate in the intestinal fluids appears to be tightly regulated, in part by secretion of acid into the intestinal fluids. Acid secretion acts to titrate some of the bicarbonate in the intestinal fluids which aids in further reduction of intestinal fluid osmotic pressure and thereby water absorption. The above grant resulted in identification of the specific transport proteins involved in bicarbonate secretion by the intestinal epithelium and resulted in 28 publications so far. In addition, the PI has contributed to several text books during the award period. As part of the activities under this grant, we quantified the excretion of precipitated calcium-carbonate from fish and combined these rates with information about the total mass of fish in our oceans to reach an estimate of global calcium-carbonate production by fish. This estimate amounted to 3-12% of the total global calcium-carbonate production which was previously attributed mainly to microorganisms.
