Temperature induced decoupling of enzymatic hydrolysis and carbon remineralization in long-term incubations of Arctic and temperate sediments

Abstract

Extracellular enzymatic hydrolysis of high-molecular weight organic matter is the initial step in sedimentary organic car- bon degradation and is often regarded as the rate-limiting step. Temperature effects on enzyme activities may therefore exert an indirect control on carbon mineralization. We explored the temperature sensitivity of enzymatic hydrolysis and its connec- tion to subsequent steps in anoxic organic carbon degradation in long-term incubations of sediments from the Arctic and the North Sea. These sediments were incubated under anaerobic conditions for months at temperatures of 0, 10, and /C176C. The short-term temperature response of the active microbial community was tested in temperature gradient block incuba- tions. The temperature optimum of extracellular enzymatic hydrolysis, as measured with a polysaccharide (chondroitin sul- fate), differed between Arctic and temperate habitats by about 8–13 /C176C in fresh sediments and in sediments incubated for months. In both Arctic and temperate sediments, the temperature response of chondroitin sulfate hydrolysis was initially similar to that of sulfate reduction. After months, however, hydrolysis outpaced sulfate reduction rates, as demonstrated by increased concentrations of dissolved organic carbon (DOC) and total dissolved carbohydrates. This effect was stronger at higher incubation temperatures, particularly in the Arctic sediments. In all experiments, concentrations of volatile fatty acids (VFA) were low, indicating tight coupling between VFA production and consumption. Together, these data indicate that long-term incubation at elevated temperatures led to increased decoupling of hydrolytic DOC production relative to fermen- tation. Temperature increases in marine sedimentary environments may thus…