Thermophilic methanogens in rice field soil

The soil temperature in flooded Italian rice fields is generally lower than 30 degrees C. However, two temperature optima at approximately 41 degrees C and 50 degrees C were found when soil slurries were anoxically incubated at a temperature range of 10-80 degrees C. The second temperature optimum indicates the presence of thermophilic methanogens in the rice field soil. Experiments with 14C-labelled bicarbonate showed that the thermophilic CH4 was exclusively produced from H2/CO2. Terminal restriction fragment length polymorphism (T-RFLP) of archaeal SSU rRNA gene fragments revealed a dramatic change in the archaeal community structure at temperatures > 37 degrees C, with the euryarchaeotal rice cluster I becoming the dominant group (about 80%). A clone library of archaeal SSU rRNA gene fragments generated at 49 degrees C was also dominated (10 out of 11 clones) by rice cluster I. Our results demonstrate that Italian rice field soil contains thermophilic methanogenic activity that was most probably a result of members of the as yet uncultivated euryarchaeotal rice cluster I.