Contrasting patterns of peptidase activities in seawater and sediments: An example from Arctic fjords of Svalbard

The structural specificities and activities of microbial extracellular enzymes help determine the nature and quantity of substrates available for microbial uptake, and therefore the rate and location of organic matter transformation in the ocean. Previous work has demonstrated major differences in the spectrum of enzymes hydrolyzing polysaccharides in the water column and sediments, particularly in high-latitude locations. To determine whether pelagic–benthic contrasts in enzyme activities extend to other classes of organic matter, the hydrolysis of five peptide substrates was investigated in two Arctic fjords of Svalbard. Substrates were intended to measure leucine aminopeptidase activity (exo-acting; terminal-unit cleaving), plus chymotrypsin and trypsin activities (endo-acting; mid-chain cleaving). All substrates were readily hydrolyzed in surface sediments. In contrast, only two to four of the peptide substrates were hydrolyzed in the water column of both fjords. Chymotrypsin activities were undetectable or were far lower than trypsin and leucine aminopeptidase activities in the water column, but were comparable to the activities of other enzymes in sediments. The pelagic–benthic contrast in the spectrum of peptide substrates hydrolyzed in these fjords parallels patterns of hydrolysis previously observed for polysaccharides. Some organic matter may pass untouched through the water column due to a lack of the specific enzymes required for hydrolysis; these substrates could then selectively fuel benthic metabolism. Pelagic–benthic contrasts in enzymatic capabilities may be widespread in the ocean, and thus affect the processing of a significant fraction of marine organic matter. •Activities of 5 peptide-hydrolyzing enzymes were measured in 2 Arctic fjords.•Few peptides were hydrolyzed in the water column; all were hydrolyzed in sediments.•Organic carbon evades degradation in the water column due to lack of specific enzymes.•This organic carbon preferentially fuels microbial communities in sediments.