Effects of Seawater Acidification on Hydrolytic Enzyme Activities

We have investigated the effects of seawater acidification on the activities of leucine aminopeptidase (LAPase), beta-glucosidase (BGase), phosphatase (P-ase), alpha-glucosidase (AGase), and lipase (L-ase), which are important promoters of degradation of marine organic matter, including proteins, carbohydrates, organic phosphorus compounds, and lipids. Seawater samples were collected from a eutrophic coastal area, from Tokyo Bay, and from oligotrophic pelagic waters outside the Kuroshio Current. Enzyme activities were measured using fluorogenic substrates added to the seawater samples, which were acidified from pH 8.2 to 5.6 by a chemical buffer. Spontaneous hydrolysis of the substrates was shown to be negligible in heat-inactivated control samples, thus validating our results. LAPase was the most sensitive to acidification; enzyme activity rapidly decreased from pH 8.2 to 7.8, corresponding to a realistic scenario of ocean acidification. L-ase activity also decreased with acidification. Activities of P-ase and BGase were relatively constant across the pH levels examined, suggesting that their activity is not appreciably influenced by acidification. The effect of acidification on P-ase activities differed between the coastal and semipelagic samples, and this was likely due to freshwater influence at the nearshore station. Because of the low activity of AGase in the sample, the effect of acidification on this enzyme could not be examined. The effects of acidification on enzyme activity appear to vary depending on enzyme type and location, but we conclude that acidification will cause changes in the cycling of organic matter in marine ecosystems, in particular to proteinous and lipid substances.