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ABSTRACT Anoxic subsurface sediments contain communities of heterotrophic microorganisms that metabolize organic carbon at extraordinarily low 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 ( V max ) decreased with increasing sediment depth, although V max expressed on a per-cell basis was approximately the same at all depths. Half-saturation constants ( K m ) 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. IMPORTANCE Burial of organic carbon in marine and estuarine sediments represents a long-term sink for atmospheric carbon dioxide. Globally, ∼40% of organic carbon burial occurs in anoxic estuaries and deltaic systems. However, the ultimate controls on the amount of organic matter that is buried in sediments, versus oxidized into CO 2 , are poorly constrained. In this study, we used a combination of enzyme assays and metagenomic analysis to identify how subsurface microbial communities catalyze the first step of proteinaceous organic carbon degradation. Our results show that microbial communities in deeper sediments are adapted to access molecules characteristic of degraded organic matter, suggesting that those heterotrophs are adapted to life in the subsurface. 

Local sources of particles and precursor gases have long been considered as the major control for the ground‐level particle number concentration in an urban environment. Here we show the existence of two distinct sources. The first source was detectable during morning and afternoon rush hours and was defined by high black carbon concentrations. Particle number concentration inversely correlated with the local planetary boundary layer height. The particle size distributions were characterized by a wide range of modal diameters and did not exhibit detectable modal growth. This source was attributed to vehicular emissions. The second source yielded particle number concentration comparable to those during the rush hours and was detected six times over the 3‐week measurement campaign. Small particles produced by this source were recorded during the midday after the diminishment of the rush‐hour traffic effects. The particles exhibited prolonged modal growth over 8 hr, which may indicate a regional scale nucleation event. The data suggest that these particles were likely formed above the nocturnal boundary layer after sunrise and were subsequently transported to the surface through convective mixing. Overall, the nocturnal and convective boundary layer evolution was found to be closely associated with the of small particle event and the most important factor affecting the ground‐level particle number concentration. Shallow nocturnal boundary layers trapped pollution near the ground leading to particle number concentrations over 10⁴cm⁻³.