Methanogens in municipal solid waste landfill leachates

Archaeal microbial communities present in municipal solid waste landfill leachates were characterized using a 16S rDNA approach. Phylogenetic affiliations of 241 partial length 16S rDNA sequences were determined. Sequences belonging to the Methanosarcinales order were dominant in the clone library and 60% of the clones belonged to the strictly acetoclastic methanogenic genus Methanosaeta. Sequences affiliated to the metabolically versatile methanogenic genus Methanosarcina represented 22 % of the retrieved sequences. Members of the hydrogenotrophic order Methanomicrobiales were also recovered in limited numbers, especially sequences affiliated to the genera Methanoculleus, Methanofollis and Methanospirillum. 8 euryarchaeal and 13 crenarchaeal sequences only were distantly related to any hitherto cultivated microorganisms, showing that archaeal diversity within the investigated samples was limited. Labscale incubations were performed with leachates mixed with several methanogenic precursors (acetate, hydrogen, formate, methanol, methylamine). Microbial populations were followed using group specific 16S rRNA targeted fluorescent oligonucleotidic probes. During the incubations with acetate, acetoclastic methanogenesis was rapidly induced and lead to the dominance of Archaea hybridizing with probe MS1414 which indicates their affiliation to the Methanosarcinaceae family. Hydrogen and formate addition induced an important acetate synthesis resulting from the onset of homoacetogenic metabolism. In these incubations, Methanosarcinaceae (hybridizing with probe MS1414) and Methanomicrobiales (hybridizing with probe EURY496) were dominant. Homoacetogenesis was also recorded for incubations performed with methanol and methylamines. In the methanol experiment, acetoclastic methanogenesis thereafter took place and archaea hybridizing with probe MS821 (specific for Methanosarcina spp.) forming strong cohesive clusters were observed to be the dominant population.