Dissolved organic matter dynamics in an Arctic catchment: Linking DOM chemistry, bioavailability, and microbial community composition

Dissolved organic matter (DOM) export from land to water is an important component of global and regional carbon budgets, and terrestrial DOM, in turn, influences metabolism of aquatic ecosystems. I investigated the underlying mechanisms controlling DOM processing along a terrestrial-aquatic gradient in an arctic tundra catchment through field surveys and manipulative experiments measuring DOM chemistry, microbial community composition (MCC), and microbial activity. Multivariate analyses revealed distinct DOM composition and MCC in tussock, birch-willow, and wet sedge tundra soils using phospholipid fatty acid analysis and denaturing gradient gel electrophoresis analysis of 16S rRNA. The latter method also was used to differentiate bacterial assemblages in surface waters. Fungal and total active microbial biomass was significantly greater in tussock (174 and 678 mug C g soil-1) than wet sedge soils (43 and 456 mug C g soil-1). In short-term incubations (3 hours), DOM chemistry accounted for 46% of the variance in bacterial production (BP), when DOM from each site was added to a mixture of microbes from all sites (BP ranged from 0.15 to 2.55 mug C mL -1d-1). Effects of MCC were weaker when each microbial community received a common mixture of DOM (BP ranged from 0.05 to 0.96 mug C mL-1d-1). In longer-term (days to weeks) mesocosm experiments, soil water DOM enhanced lake BP 321 to 672% relative to controls, and these effects were indirect through shifts in bacterial community composition. Such shifts also influenced the bioavailability of DOM photo-products; initially, photo-oxidized DOM reduced BP, likely due to the production of harmful byproducts, but increased contact time (days to weeks) between photo-products and bacteria shifted community composition and enhanced BP relative to dark controls.; Overall, my results suggest that spatial heterogeneity influences DOM dynamics across the landscape. Microbial communities appear to contain a full complement of functional groups with respect to C processing, and variation in substrate supply selects for inactive groups over relatively short time scales (days to weeks). The contact time between microbial communities and DOM may therefore be an important control on microbial activity and DOM processing along flowpaths; it is likely that such controls operate in temperate zone and tropical systems as well.