Enzyme activities in the water column and in shallow permeable sediments from the northeastern Gulf of Mexico C. Arnosti * , K. Ziervogel, L. Ocampo, S. Ghobrial University of North Carolina-Chapel Hill, Department of Marine Sciences, CB #3300, Chapman Hall, Chapel Hill, NC 27599, USA article info Article history: Received 23 January 2009 Accepted 19 June 2009 Available online 27 June 2009 Keywords: enzymes polysaccharides carbohydrates permeable sediments water column carbon cycling abstract The activities of extracellular enzymes that initiate the microbial remineralization of high molecular weight organic matter were investigated in the water column and sandy surface sediments at two sites in the northeastern Gulf of Mexico. Six fluorescently labeled polysaccharides were hydrolyzed rapidly in the water column as well as in permeable sediments. This result contrasts with previous studies carried out in environments dominated by fine-grained muds, in which the spectrum of enzymes active in the water column is quite limited compared to that of the underlying sediments. Extracts of Spirulina, Isochrysis, and Thalassiosira were also used to measure hydrolysis rates in water from one of the sites. Rates of hydrolysis of the three plankton extracts were comparable to those of the purified polysaccharides. The broad spectrum and rapid rates of hydrolysis observed in the water column at both sites in the north- eastern Gulf of Mexico may be due to the permeable nature of the sediments. Fluid flux through the sediments is sufficiently high that the entire 1.5 m deep water column could filter though the sediments on timescales of a few days to two weeks. Movement of water through sediments may also transport dissolved enzymes from the sediment into the water column, enhancing the spectrum as well as the rate of water column enzymatic activities. Such interaction between the sediments and water column would permit water column microbial communities to access high molecular weight substrates that might otherwise remain unavailable as substrates. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Heterotrophic microbial communities play key roles in carbon cycling in marine systems, regenerating nutrients, respiring organic substrates to CO 2 , and repackaging dissolved organic matter into microbial biomass. These communities are active in the water column, either free in solution or attached to particles, and in sediments. Although the fate of most marine organic matter is remineralization to CO 2 , the specific means by which microbial communities in seawater and in sediments carry out these processes is less well understood. In particular, the extent to which microbial communities in seawater and in sediments from the same location resemble or complement each other in specific functional capabilities is largely unknown. Investigations of microbial activities and microbial communities generally focus either on the water column or on sediments; only a few studies of microbial communities or activities (e.g. Tholosan et al., 1999; Arnosti, 2000, 2008; Hewson et al., 2003) have investigated seawater and sediments simultaneously. The remineralization of organic matter by microbial communi- ties is initiated by the activities of extracellular enzymes. In order to access substrates with a molecular weight greater than ca. 600 Da (Weiss et al., 1990), some (unknown) fraction of a heterotrophic microbial community must produce extracellular enzymes to hydrolyze high molecular weight substrates to sizes sufficiently small to permit uptake into the bacterial cells. These enzymes are sensitive to the structural features of their substrates, such that accessing a particular macromolecule requires the activity of enzymes suited to hydrolyze specific chemical bonds. The few comparisons (Arnosti, 2000, 2008) made to date have found that sedimentary and seawater microbial communities from the same location differ fundamentally in their spectrum of enzyme activi- ties. They therefore also differ in their abilities to access high molecular weight substrates, an observation with profound impli- cations for rates and locations of carbon cycling. These previous investigations of specific enzyme activities, in common with most studies of sedimentary carbon cycling, have focused on organic-rich sediments. Other types of sedimentary environments, in particular less organic-rich coastal sand beds, have been comparatively understudied. Such sediments, however, dominate continental shelves (Hall, 2002). Recent studies of * Corresponding author. E-mail address: arnosti@email.unc.edu (C. Arnosti). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2009.06.018 Estuarine, Coastal and Shelf Science 84 (2009) 202–208