Thermophilic anaerobes in Arctic marine sediments induced to mineralize complex organic matter at high temperature Casey Hubert, 1 * † Carol Arnosti, 2 Volker Brüchert, 1‡ Alexander Loy, 3 Verona Vandieken 1§ and Bo Barker Jørgensen 1,4 1 Department of Biogeochemistry, Max Planck Institute for Marine Microbiology, Bremen, Germany. 2 Department of Marine Sciences, University of North Carolina, Chapel Hill, NC, USA. 3 Department of Microbial Ecology, University of Vienna, Vienna, Austria. 4 Center for Geomicrobiology, Aarhus University, Aarhus, Denmark. Summary Marine sediments harbour diverse populations of dormant thermophilic bacterial spores that become active in sediment incubation experiments at much higher than in situ temperature. This response was investigated in the presence of natural complex organic matter in sediments of two Arctic fjords, as well as with the addition of freeze-dried Spirulina or individual high-molecular-weight polysaccharides. During 50°C incubation experiments, Arctic thermo- philes catalysed extensive mineralization of the organic matter via extracellular enzymatic hydrolysis, fermentation and sulfate reduction. This high temperature-induced food chain mirrors sediment microbial processes occurring at cold in situ tem- peratures (near 0°C), yet it is catalysed by a com- pletely different set of microorganisms. Using sulfate reduction rates (SRR) as a proxy for organic matter mineralization showed that differences in organic matter reactivity determined the extent of the thermo- philic response. Fjord sediments with higher in situ SRR also supported higher SRR at 50°C. Amendment with Spirulina significantly increased volatile fatty acids production and SRR relative to unamended sediment in 50°C incubations. Spirulina amendment also revealed temporally distinct sulfate reduction phases, consistent with 16S rRNA clone library detec- tion of multiple thermophilic Desulfotomaculum spp. enriched at 50°C. Incubations with four different fluo- rescently labelled polysaccharides at 4°C and 50°C showed that the thermophilic population in Arctic sediments produce a different suite of polymer- hydrolysing enzymes than those used in situ by the cold-adapted microbial community. Over time, dormant marine microorganisms like these are buried in marine sediments and might eventually encounter warmer conditions that favour their activation. Dis- tinct enzymatic capacities for organic polymer degra- dation could allow specific heterotrophic populations like these to play a role in sustaining microbial metabolism in the deep, warm, marine biosphere. Introduction Anaerobic microbial communities catalyse the degrada- tion of organic matter in marine sediments. Particles reaching the seafloor are comprised of a complex mixture of carbohydrate, protein, nucleic acid and lipid molecules that are generally too large for direct microbial uptake (Weiss et al., 1991; Hedges et al., 2001; Arnosti, 2004). Initial depolymerization steps in anaerobic degradation pathways involve microbial production of extracellular enzymes. Hydrolytic cleavage of large substrates results in smaller monomeric products for cell internal uptake and metabolism. The ability of microbial communities to access and degrade complex organic compounds in surface sediments determines the fraction of sediment organic matter that is mineralized and that which is per- manently buried (Burdige, 2007). Initial hydrolysis thus sets off a cascade of transformations in which products of hydrolysis may be rapidly degraded by fermentation and anaerobic respiration (Arnosti, 2004). These sediment processes are depicted schematically in Fig. 1. In anoxic marine surface sediments where sulfate is abundant, microbial sulfate reduction is generally the dominant ter- minal mineralization step (Jørgensen, 1982; 2006; Fig. 1). In many shelf sediments, high sulfate reduction rates (SRR) therefore indicate an active reaction cascade and Received 22 August, 2009; accepted 14 December, 2009. *For cor- respondence. E-mail casey.hubert@newcastle.ac.uk; Tel. (+44) 191 246 4864; Fax (+44) 191 222 6502. Present addresses: † School of Civil Engineering and Geosciences, Newcastle University, Newcastle Upon Tyne, UK; ‡ Department of Geology and Geochemistry, Stock- holm University, Stockholm, Sweden; § Institute of Biology, University of Southern Denmark, Odense, Denmark. Environmental Microbiology (2010) 12(4), 1089–1104 doi:10.1111/j.1462-2920.2010.02161.x © 2010 Society for Applied Microbiology and Blackwell Publishing Ltd