Abstract
Extracellular enzymatic activities initiate microbially-driven heterotrophic carbon cycling in subsurface sediments. While measurement of hydrolytic activities in sediments is fundamental to our understanding of carbon cycling, these measurements are often technically difficult due to sorption of organic substrates to the sediment matrix. Most methods that measure hydrolysis of organic substrates in sediments rely on recovery of a fluorophore or fluorescently-labeled target substrate from a sediment incubation. The tendency for substrates to sorb to sediments results in lower recovery of an added substrate, and can result in data that are unusable or difficult to interpret. We developed a treatment using competitive desorption of a fluorescently-labeled, high molecular weight organic substrate that improves recovery of the labeled substrate from sediment subsamples. Competitive desorption treatment improved recovery of the fluorescent substrate by a median of 66%, expanded the range of sediments for which activity measurements could be made, and was effective in sediments from a broad range of geochemical contexts. More reliable measurements of hydrolytic activities in sediments will yield usable and more easily interpretable data from a wider range of sedimentary environments, enabling better understanding of microbially-catalyzed carbon cycling in subsurface environments.
Highlights
Heterotrophic microbial communities play an important role in organic carbon cycling in subsurface sediments
The extraction treatment presented here was developed to reduce the effects of adsorption on substrate recovery when measuring extracellular enzymatic activity in sediments using a fluorescently-labeled high molecular weight substrate, and to broaden the range of sediments in which enzyme activities can be measured using these substrates
The desorption treatment was especially effective at desorbing the high molecular weight portion of the added substrate (Figure 3B), resulting in higher proportions of highto low- molecular weight substrate, an effect that is evident for laminarin in core P13 from Guaymas Basin (Figures 2B,C)
Summary
Heterotrophic microbial communities play an important role in organic carbon cycling in subsurface sediments. Extracellular enzymatic activity is typically measured by addition of a fluorescently labeled substrate to an environmental sample, and hydrolysis is detected either as an increase in Hydrolysis Measurements with Competitive Desorption fluorescence as a fluorophore is cleaved (Hoppe, 1983) or as a change in molecular weight distribution as a fluorescent substrate is hydrolyzed into lower molecular weight products (Arnosti, 1996, 2003). In both cases, adequate recovery of the amended label or labeled substrate is necessary for interpretable results
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