AME 65:159-168 (2011)  -  DOI: https://doi.org/10.3354/ame01541

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 NH₄⁺ 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 NH₄⁺ production and the effects of a polyphenolic enzyme inhibitor, respectively. Casein additions increased the NH₄⁺ 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 NH₄⁺ concentration in samples with tannic acid indicated that inhibiting aminopeptidase activity reduced NH₄⁺ production rates. The concentration of the regenerated NH₄⁺ 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 NH₄⁺ in sediments and provide insights about mechanisms of enzyme hydrolysis and NH₄⁺ fluxes in estuarine sediments.

KEY WORDS: Leucine-aminopeptidase · Extracellular enzymes · Ammonium regeneration · Marine sediments