Abstract

Extracellular enzymes (EE) initiate heterotrophic remineralization by hydrolyzing high-molecularweight organic matter to substrates that are sufficiently small (approximately 600 Da) to be transported across cell membranes. An accurate understanding of EE associated remineralization processes in sedimentary deposits requires measuring patterns of extracellular enzyme activity (EEA) with minimal disturbance of natural sediment structure. In this study, two-dimensional patterns of extracellular enzyme (leucine aminopeptidase) activity in shallow-water, marine sediments from Great Peconic Bay, Long Island, New York were examined seasonally at sub-millimeter resolution by using a newly developed EE planar fluorosensor. Comparisons of spatially averaged, vertical enzyme activity profiles measured using this imaging sensor system and traditional sediment homogenization techniques verified the overall consistency of the methodology. The depth-averaged EEA (approximately 10 cm) varied seasonally with highest levels in the late spring through summer (0.2 μmol substrate g-wet-wt–1 hr–1) and lowest in the late fall and early winter (0.1 μmol substrate g-wet-wt–1 hr–1). EEA distributions, however, showed extensive small-scale horizontal heterogeneity as well as vertical variations. Both the input of reactive substrates (planktonic organic matter) and temperature differences accounted for major changes in EEA seasonally. In general, horizontal heterogeneity in EEA was greatest during warm seasons (summer, fall) as a result of increased macrofaunal activity. On the other hand, vertical variations are less significant during warm periods compared with cold periods as the sediment is more intensely reworked. Hot spots of elevated microbial activity from sub-millimeter to millimeter scales are observed in some seasons and are specifically associated with substrate inputs from phytoplankton blooms and particle reworking by infauna. The deposition of phytodetritus from an early spring bloom greatly enhanced surface sediment EEA, and at this time high EEA closely coincided with regions of elevated metabolite production as measured by NH+4 and ΣCO2 concentrations. Direct correlations between averaged EEA distributions and nutrient production rates were observed throughout the year but no correlations between EEA and pore water nutrient concentrations were present. Spatially resolved EEA directly tracks reactive particle distributions and is generally independent of solute transport mechanisms, such as bioirrigation, and redox conditions.

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