Extracellular Enzymatic Hydrolysis of High Molecular Weight Organic Carbon in Eastern Mediterranean Sapropelic and Non-Sapropelic Subsurface Sediments

Abstract:

Organic carbon availability is an important constraint on microbial activity in the subsurface. Since most sedimentary organic matter is likely high molecular weight and complex, bioavailability of organic carbon is closely tied to activities of extracellular enzymes that hydrolyze organic macromolecules into transportable sizes. In part due to methodological difficulties, few measurements of extracellular enzymatic activities have been made in marine sediments below ca. 20cm depth. We measured extracellular hydrolysis of specific polysaccharides in deep sediments from sapropel and non-sapropel sections of a single core from the Eastern Mediterranean. In order to counteract adsorption of the substrate onto sediment particles, we developed an extraction protocol utilizing competitive desorption and mild heating. This treatment improved substrate recovery from incubation subsamples 5- to 10-fold, and enabled us to detect enzymatic activity in deep subsurface sediments. The wide variation in TOC between proximal sediment layers in this core provided an excellent opportunity to investigate (i) the rate at which subsurface microbial communities can hydrolyze a diversity of organic substrates, and (ii) rates and ranges of enzymatic capabilities as a function of sediment depth, organic carbon load and microbial community composition. Our experiments were carried out in long-term incubations (3-6 weeks), in which substrates were readily hydrolyzed, but hydrolysis rates differed among substrates and among sediment sections. Activity was not correlated with depth, but was highest in sections with highest organic carbon content. Isolation of strains able to grow directly on the substrates of interest are underway, and provide a promising path forward to illuminate mechanisms driving potential hydrolytic activity in the subsurface.