The direct role of enzyme hydrolysis on ammonium regeneration rates in estuarine sediments

Abstract

Benthic ammonium (NH4 +) regeneration in coastal marine sediments has a fundamental role in nitrogen (N) cycling and N supply to primary producers. Nitrogen regeneration involves benthic microbial mineralization of organic-N, which, in turn, depends on protein hydrolysis. These processes were examined in Aransas Bay (Texas, USA) sediments by monitoring NH 4+ evolution as a function of enzyme activity in controlled sediment slurries. Casein and tannic acid were added to evaluate the direct role of aminopeptidase on NH 4+ production and the effects of a polyphenolic enzyme inhibitor, respectively. Casein additions increased the NH 4+ concentration from 19 ± 0.3 to 737 ± 150 μM in 120 h, a final concentration 4.3-fold higher than that of control samples and 2.9-fold higher than that of samples with casein and tannic added together. Lower NH 4+ concentration in samples with tannic acid indicated that inhibiting aminopeptidase activity reduced NH 4+ production rates. The concentration of the regenerated NH 4+ related directly to aminopeptidase activity in controls (r = 0.86, p < 0.01), casein-enriched (r = 0.89, p < 0.01), and casein plus polyphenol treatments (r = 0.71, p < 0.01) over the first 72 h. The results demonstrate the importance of aminopeptidase in regenerating NH 4+ in sediments and provide insights about mechanisms of enzyme hydrolysis and NH 4+ fluxes in estuarine sediments. © Inter-Research 2011.