Anoxic subsurface sediments contain communities of heterotrophic microorganisms that metabolize organic carbon at extraordinarily slow rates. In order to assess the mechanisms by which subsurface microorganisms access detrital sedimentary organic matter, we measured kinetics of a range of extracellular peptidases in anoxic sediments of the White Oak River estuary, NC. Nine distinct peptidase substrates were enzymatically hydrolyzed at all depths. Potential peptidase activities (Vmax) decreased with increasing sediment depth, although Vmaxexpressed on a per cell basis was approximately the same at all depths. Half-saturation constants (Km) decreased with depth, indicating peptidases that functioned more efficiently at low substrate concentrations. Potential activities of extracellular peptidases acting on molecules that are enriched in degraded organic matter (D-phenylalanine and L-ornithine) increased relative to enzymes that act on L-phenylalanine, further suggesting microbial community adaptation to access degraded organic matter. Nineteen classes of predicted, exported peptidases were identified in genomic data from the same site, of which genes for class C25 (gingipain-like) peptidases represented more than 40% at each depth. Methionine aminopeptidases, zinc carboxypeptidases, and class S24-like peptidases, which are involved in single-stranded DNA repair, were also abundant. These results suggest a subsurface heterotrophic microbial community that primarily accesses low-quality detrital organic matter via a diverse suite of well-adapted extracellular enzymes.
The identities and biochemical properties of extracellular enzymes present in natural environments are poorly constrained. We used a series of competitive inhibition experiments with samples from a freshwater environment (the Tennessee River at Knoxville, TN, USA) and a marine environment (Bogue Sound, NC, USA) to characterize the range of substrate specificities of naturally occurring enzymes that hydrolyze L-leucine 7-amido-4-methylcoumarin (Leu-AMC), L‑proline-AMC (Pro-AMC), and L-arginine-AMC (Arg-AMC)—putative substrates for leucyl-aminopeptidase, prolyl-aminopeptidase, and arginyl-aminopeptidase, respectively. Extracellular peptidases which hydrolyzed Arg-AMC and Leu-AMC demonstrated affinity for up to 8 other amino acids, whereas those hydrolyzing Pro-AMC in the Tennessee River, and Arg-AMC at Bogue Sound, were more specific to proline and arginine, respectively. Patterns of substrate affinity showed that Leu-AMC (at both sampling sites) and Arg-AMC (at Bogue Sound) were primarily hydrolyzed by enzymes other than leucyl-aminopeptidase and arginyl-aminopeptidase, respectively. The set of naturally occurring peptidases in both environments showed greater affinity towards a subset of amino acids. These amino acids were on average larger, yielded more free energy from oxidation to CO2, and tended to be depleted in aged organic matter. These relationships indicate that pathways of amino acid diagenesis are at least partially controlled by the substrate specificities of the peptidases involved in protein degradation.
