Impacts of CO2 on acid-base balance, rectal base excretion and intestinal carbonate formation in marine fish

University Of Miami, Coral Gables FL

Investigators

Abstract

Combustion of fossil fuels continues to increase oceanic carbon dioxide (CO2) resulting in ocean acidification. In addition, periodic upwelling of CO2-rich water in certain zones results in significant acidification of surface waters, making examination of elevated CO2 levels relevant today. Although marine fish readily compensate for CO2 levels observed in upwelling regions and levels predicted for the next 200 years, relatively little is known about how these abundant vertebrates maintain acid-base balance (constant blood pH). Preliminary findings by Grosell and his team demonstrate that elevated external CO2 levels as low as 750 ppm CO2 induce an acid-base balance disturbance and at least partial compensation in marine fish. Grosell and his team are examining the mechanism by which marine fish maintain acid-base balance in the presence of elevated external CO2. A consequence of exposure to elevated CO2 in the surrounding water is an increase in blood CO2 that is balanced by a rapid increase in blood bicarbonate (HCO3-), buffering the acidosis and defending normal pH levels. Early experiments demonstrate that the increase in blood HCO3- is achieved by increased uptake of HCO3- from the water, presumably across the gills. Grosell and his team aim to identify the molecular nature of the transport proteins involved in this process. Blood CO2 and HCO3- form substrate for excretion of HCO3- by the intestine also, a process that is necessary for fish to maintain salt and water balance. Earlier studies by the PI and co-workers have demonstrated a relationship between HCO3- levels on the blood side of the intestinal tissue and the HCO3- excretion rates by the tissue. Thus, elevation of blood HCO3- during CO2 exposure will likely result in increased HCO3- excretion by the intestinal tissue and loss of blood HCO3-. The most recent findings by Grosell and his team confirmed this expectation and planned studies will examine if intestinal HCO3- transport systems adjust during CO2 exposure to facilitate retention of blood HCO3- levels. The HCO3- in the intestine of marine fish combines with the calcium (Ca2+) from ingested seawater to form calcium carbonate (CaCO3) in the intestinal lumen, which is subsequently excreted to the environment. The realization that marine fish, in this way, contribute substantially to the formation of CaCO3 in oceans is relatively novel and has lead Grosell and his team to test if elevated oceanic CO2 will result in increased CaCO3 production by marine fish. For the first time, the solubility of fish-produced CaCO3 will be determined as part of this project. These research activities will involve training of undergraduate students, PhD students, and a postdoctoral researcher, as well as a number of outreach activities in a community with highly diverse demography.