Chemolithotrophic acetogenic H2/CO2 utilization in Italian rice field soil

Acetate oxidation in Italian rice field at 50 degrees C is achieved by uncultured syntrophic acetate oxidizers. As these bacteria are closely related to acetogens, they may potentially also be able to synthesize acetate chemolithoautotrophically. Labeling studies using exogenous H-2 (80%) and (CO2)-C-13 (20%), indeed demonstrated production of acetate as almost exclusive primary product not only at 50 degrees C but also at 15 degrees C. Small amounts of formate, propionate and butyrate were also produced from (CO2)-C-13. At 50 degrees C, acetate was first produced but later on consumed with formation of CH4. Acetate was also produced in the absence of exogenous H-2 albeit to lower concentrations. The acetogenic bacteria and methanogenic archaea were targeted by stable isotope probing of ribosomal RNA (rRNA). Using quantitative PCR, C-13-labeled bacterial rRNA was detected after 20 days of incubation with (CO2)-C-13. In the heavy fractions at 15 degrees C, terminal restriction fragment length polymorphism, cloning and sequencing of 16S rRNA showed that Clostridium cluster I and uncultured Peptococcaceae assimilated (CO2)-C-13 in the presence and absence of exogenous H-2, respectively. A similar experiment showed that Thermoanaerobacteriaceae and Acidobacteriaceae were dominant in the C-13 treatment at 50 degrees C. Assimilation of (CO2)-C-13 into archaeal rRNA was detected at 15 degrees C and 50 degrees C, mostly into Methanocellales, Methanobacteriales and rice cluster III. Acetoclastic methanogenic archaea were not detected. The above results showed the potential for acetogenesis in the presence and absence of exogenous H-2 at both 15 degrees C and 50 degrees C. However, syntrophic acetate oxidizers seemed to be only active at 50 degrees C, while other bacterial groups were active at 15 degrees C. The ISME Journal (2011) 5, 1526-1539; doi:10.1038/ismej.2011.17; published online 3 March 2011