High activity and low temperature optima of extracellular enzymes in Arctic sediments: implications for carbon cycling by heterotrophic microbial communities

The rate of the initial step in microbial remineralization of organic carbon, extracellular enzymatic hydrolysis, was investigated as a function of temperature in permanently cold sediments from 2 fjords on the west coast of Svalbard (Arctic Ocean). We used 4 structurally distinct polysaccharides (chondroitin sulfate, fucoidan, xylan and pullulan) to determine the temperature-activity responses of hydrolysis of a related class of compounds. All 4 enzyme activities showed similarly low temperature optima in the range of 15 to 18°C. These temperature optima are considerably lower than most previous reports of temperature optima for enzyme activities in marine sediments. At 0°C, close to the in situ temperature, these enzyme activities achieved 13 to 38% of their rates at optimum temperatures. In one experiment, sulfate reduction rates were measured in parallel with extracellular enzymatic hydrolysis in order to determine the relative temperature responses of the initial and terminal steps in microbial remineralization of carbon. The temperature optimum of sulfate reduction, 21°C, was considerably lower than previous reports of sulfate reduction in marine sediments, but is consistent with recent studies of psychrophilic sulfate reducers isolated from Svalbard sediments. A calculation of potential carbon flow into the microbial food chain demonstrated that the activity of just one type of polysaccharide-hydrolyzing enzyme could in theory supply 21 to 100% of the carbon consumed via sulfate reduction across the temperature range investigated here. These characteristics suggest that these extracellular enzymes are well adapted to permanently cold temperatures