Thermophilic methanogens in rice field soil

The soil temperature in flooded Italian rice fields is generally lower than 30 degreesC. However, two temperature optima at approximate to 41 degreesC and 50 degreesC were found when soil slurries were anoxically incubated at a temperature range of 10-80 degreesC. The second temperature optimum indicates the presence of thermophilic methanogens in the rice field soil. Experiments with C-14-labelled bicarbonate showed that the thermophilic CH4 was exclusively produced from H-2/CO2. Terminal restriction fragment length polymorphism (T-RFLP) of archaeal SSU rRNA gene fragments revealed dramatic change in the archaeal community structure at temperatures > 37 degreesC, with the euryarchaeotal rice cluster I becoming the dominant group (about 80%). A clone library of archaeal SSU rRNA gene fragments generated at 49 degreesC 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.