Both natural and anthropogenic hot environments contain appreciable levels of carbon monoxide (CO). Anaerobic microbial communities play an important role in CO conversion in such environments. CO is involved in a number of redox reactions. It is biotransformed by thermophilic methanogens, acetogens, hydrogenogens, sulfate reducers, and ferric iron reducers. Most thermophilic CO-oxidizing anaerobes have diverse metabolic capacities, but two hydrogenogenic species are obligate carboxydotrophs. Among known thermophilic carboxydotrophic anaerobes, hydrogenogens are most numerous, and based on available data they are most important in CO biotransformation in hot environments.

A moderately thermophilic, anaerobic, chemolithoheterotrophic, sulfate-reducing bacterium, strain CO-1-SRB T , was isolated from sludge from an anaerobic bioreactor treating paper mill wastewater. Cells were Gram-positive, motile, spore-forming rods. The temperature range for growth was 30-68 6C, with an optimum at 55 6C. The NaCl concentration range for growth was 0-17 g l "1; there was no change in growth rate until the NaCl concentration reached 8 g l . The pH range for growth was 6?0-8?0, with an optimum of 6?8-7?2. The bacterium could grow with 100 % CO in the gas phase. With sulfate, CO was converted to H 2 and CO 2 and part of the H 2 was used for sulfate reduction; without sulfate, CO was completely converted to H 2 and CO 2 . With sulfate, strain CO-1-SRB T utilized H 2 /CO 2 , pyruvate, glucose, fructose, maltose, lactate, serine, alanine, ethanol and glycerol. The strain fermented pyruvate, lactate, glucose and fructose. Yeast extract was necessary for growth. Sulfate, thiosulfate and sulfite were used as electron acceptors, whereas elemental sulfur and nitrate were not. A phylogenetic analysis of 16S rRNA gene sequences placed strain CO-1-SRB T in the genus Desulfotomaculum, closely resembling Desulfotomaculum nigrificans DSM 574 T and Desulfotomaculum sp. RHT-3 (99 and 100 % similarity, respectively). However, the latter strains were completely inhibited above 20 and 50 % CO in the gas phase, respectively, and were unable to ferment CO, lactate or glucose in the absence of sulfate. DNA-DNA hybridization of strain CO-1-SRB T with D. nigrificans and Desulfotomaculum sp. RHT-3 showed 53 and 60 % relatedness, respectively. On the basis of phylogenetic and physiological features, it is suggested that strain CO-1-SRB T represents a novel species within the genus Desulfotomaculum, for which the name Desulfotomaculum carboxydivorans is proposed. This is the first description of a sulfate-reducing micro-organism that is capable of growth under an atmosphere of pure CO with and without sulfate. The type strain is CO-1-SRB T (=DSM 14880 T =VKM B-2319 T ).

Recent advances in the field of microbial physiology demonstrate that carbon monoxide is a readily used substrate by a wide variety of anaerobic micro-organisms, and may be employed in novel biotechnological processes for production of bulk and fine chemicals or in biological treatment of waste streams. Synthesis gas produced from fossil fuels or biomass is rich in hydrogen and carbon monoxide. Conversion of carbon monoxide to hydrogen allows use of synthesis gas in existing hydrogen utilizing processes and is interesting in view of a transition from hydrogen production from fossil fuels to sustainable (CO2-neutral) biomass. The conversion of CO with H2O to CO2 and H2 is catalyzed by a rapidly increasing group of micro-organisms. Hydrogen is a preferred electron donor in biotechnological desulfurization ofwastewaters and flue gases. Additionally, CO is a good alternative electron donor considering the recent isolation of a CO oxidizing, sulfate reducing bacterium. Here we review CO utilization by various anaerobic micro-organisms and their possible role in biotechnological processes, with a focus on hydrogen production and bio-desulfurization.

Anaerobic treatment of a volatile fatty acid (VFA) mixture was investigated under psychrophilic (3 to 8°C) conditions in two laboratory-scale expanded granular sludge bed reactor stages in series. The reactor system was seeded with mesophilic methanogenic granular sludge and fed with a mixture of VFAs. Good removal of fatty acids was achieved in the two-stage system. Relative high levels of propionate were present in the effluent of the first stage, but propionate was efficiently removed in the second stage, where a low hydrogen partial pressure and a low acetate concentration were advantageous for propionate oxidation. The specific VFA-degrading activities of the sludge in each of the modules doubled during system operation for 150 days, indicating a good enrichment of methanogens and proton-reducing acetogenic bacteria at such low temperatures. The specific degradation rates of butyrate, propionate, and the VFA mixture amounted to 0.139, 0.110, and 0.214 g of chemical oxygen demand g of volatile suspended solids−1 day−1, respectively. The biomass which was obtained after 1.5 years still had a temperature optimum of between 30 and 40°C.