Meeting Abstract
The effects of increased proton concentration in seawater, during ocean acidification, is believed to result in the dissociation of calcium from carbonate and subsequent protonation of carbonate into bicarbonate ion, a form that is unable to complex with calcium to retain calcium carbonate shells. This model assumes a constant supply of calcium during the proton-dependent dissociation process. This research project was undertaken to assess whether calcification disruption may occur through a decrease in organismic calcium concentration in low pH water due to possible transport inhibition between H+ and Ca2+ at cellular sites of uptake. Partially purified gill plasma membrane vesicles (PMV) from American lobsters (Homarus americanus) were prepared using a tissue homogenization process in hypotonic buffer, followed by centrifugation purification. Vesicle protein content was determined using the Bradford protein assay. 45Ca uptake at 50 µM, 1, 5, and 50 mM, by PMV loaded with a mannitol-based buffer at pH 8, were measured using three external pH conditions (pH 8, 7 and 6) in the same mannitol-based buffer for time periods from 1 to 30 min. 45Ca influx rates by PMV at each of the 4 calcium concentrations were greatest at pH 8 and least at pH 6, suggesting that increased proton concentration inhibited the uptake of calcium. Hyperbolic influx curves, following Michaelis-Menten kinetics, were obtained by plotting 45Ca influx vs. calcium concentration at each pH value. Increased proton concentration reduced 45Ca influx Jmax, with less effect on Km, tentatively suggesting either a non-competitive or mixed effect of H+ on 45Ca2+ transport. Results of this preliminary study suggest that increased seawater proton concentration may limit the uptake of calcium and thereby contribute to reduce calcification processes.
