No abstract

Acetate is an important intermediate in the decomposition of organic matter in anoxic freshwater sediments. Here, we identified distinct microorganisms active in its oxidation and transformation to methane in the anoxic methanogenic layers of Lake Kinneret (Israel) profundal sediment by rRNA-based stable isotope probing (RNA-SIP). After 18 days of incubation with amended [U-(13)C]acetate we found that archaeal 16S rRNA was (13)C-labelled to a far greater extent than bacterial rRNA. We identified acetoclastic methanogens related to Methanosaeta concilii as being most active in the degradation and assimilation of acetate. Oxidation of the acetate-methyl group played only a minor role, but nevertheless 'heavy'(13)C-labelled bacterial rRNA templates were identified. 'Heavy' bacteria were mainly affiliated with the Betaproteobacteria (mostly Rhodocyclales and Nitrosomonadales), the Nitrospira phylum (related to 'Magnetobacterium bavaricum' and Thermodesulfovibrio yellowstonii), and also with the candidate phylum 'Endomicrobia'. However, the mode of energy gain that allowed for the assimilation of (13)C-acetate by these bacterial groups remains unknown. It may have involved syntrophic oxidation of acetate, reduction of chlorinated compounds, reduction of humic substances, fermentation of organic compounds, or even predation of (13)C-labelled Methanosaeta spp. In summary, this SIP experiment shows that acetate carbon was predominantly consumed by acetoclastic methanogens in profundal Lake Kinneret sediment, while it was also utilized by a small and heterogeneous community of bacteria.

An algal bloom of Peridinium gatunenese generally precedes the annual maximum of methane release from the profundal sediment of Lake Kinneret. Therefore, we investigated the response of the sediment methanogenic microbial community to simulated algal deposition. Addition of algal biomass on top of sediment cores resulted in increased CH 4 production rates and concentrations of the fermentation products acetate and propionate in the upper 4-cm layers with maximum values at 1-cm depth. Addition of algae to sediment slurries also resulted in increased CH 4 production rates and a transient increase of H 2 , propionate, and acetate concentrations within the first 10 d after addition. The composition of the active microbial community was determined by analysis of terminal restriction fragment polymorphism (T-RFLP) targeting ribosomal RNA and cloning and sequencing of reverse-transcribed 16S rRNA. Analysis of the sediment in the presence and absence of algae indicated that among the Bacteria, members of Deltaproteobacteria and Clostridiales responded by synthesis of ribosomes after 1 d of incubation and those of Bacteroidetes after 6 d. Among the Archaea, ribosomal RNA of the Methanosaetaceae (i.e., acetate-utilizing methanogens) slightly increased after 6 d. Algal deposition apparently stimulated ribosomal synthesis in these sediment microorganisms, thus resulting in increased activity. We conclude that these microorganisms were involved in degradation of the algal biomass resulting in transient release of acetate and other fermentation products and increased production of CH 4 .

The impacts of temperature on the activity and on the size as well as on the community composition of denitrifiers in an agricultural soil were studied in a controlled laboratory experiment. Soil slurries were incubated at different temperatures (4, 15, 20, 25, and 37 degrees C) under nonlimiting substrate conditions for 3 weeks. The abundance of the nitrate-reducer community in general was determined using the most probable number (MPN) technique; denitrifier activity and community composition were assessed by measuring potential denitrifier enzyme activity and by terminal restriction fragment length polymorphisms as well as by phylogenetic analysis of nitrite reductase gene amplicons (nirK and nirS). Increasing incubation temperatures resulted in gradually enhanced denitrification activity, but also in higher abundance of nitrate reducers and in different denitrifier community compositions. Genetic and physiological characterization of isolates purified from the highest dilution of the MPN series emphasized community differences. Overall, temperature apparently not only affected process rates but also resulted in the enrichment of denitrifiers and shifts in the community composition.