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Methanogenic archaea and CO 2 ‐dependent methanogenesis on washed rice roots
Author(s) -
LehmannRichter,
Großkopf,
Werner Liesack,
Frenzel,
Conrad
Publication year - 1999
Publication title -
environmental microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.954
H-Index - 188
eISSN - 1462-2920
pISSN - 1462-2912
DOI - 10.1046/j.1462-2920.1999.00019.x
Subject(s) - methanogenesis , anoxic waters , dry weight , biology , oryza sativa , methanol , phosphate , botany , zoology , food science , methane , nuclear chemistry , chemistry , biochemistry , ecology , organic chemistry , gene
Washed excised roots of rice ( Oryza sativa ) immediately started to produce CH 4 when they were incubated in phosphate buffer under anoxic conditions (N 2 atmosphere), with initial rates varying between 2 and 70 nmol h −1 g −1 dry weight of root material (mean ± SE: 20.3 ± 5.9 nmol h −1 g −1 dry weight; n = 18). Production of CH 4 continued for at least 500 h, with rates usually decreasing slowly. CH 4 production was not significantly affected by methyl fluoride, an inhibitor of acetoclastic methanogenesis. Less than 0.5% of added [2‐ 14 C]‐acetate was converted to 14 CH 4 , and conversion of 14 CO 2 to 14 CH 4 indicated that CH 4 was almost exclusively produced from CO 2 . Occasionally, however, especially when the roots were incubated without additional buffer, CH 4 production started to accelerate after about 200 h reaching rates of > 100 nmol h −1 g −1 dry weight. Methyl fluoride inhibited methanogenesis by more than 20% only in these cases, and the conversion of 14 CO 2 to 14 CH 4 decreased. These results indicate that CO 2 ‐dependent rather than acetoclastic methanogenesis was primarily responsible for CH 4 production in anoxically incubated rice roots. Determination of most probable numbers of methanogens on washed roots showed highest numbers (10 6 g −1 dry roots) on H 2 and ethanol, i.e. substrates that support CH 4 production from CO 2 . Numbers on acetate (10 5 g −1 dry roots) and methanol (10 4 g −1 dry roots) were lower. Methanogenic consortia enriched on H 2 and ethanol were characterized phylogenetically by comparative sequence analysis of archaeal small‐subunit (SSU) ribosomal RNA‐encoding genes (rDNA). These sequences showed a high similarity to SSU rDNA clones that had been obtained previously by direct extraction of total DNA from washed rice roots. The SSU rDNA sequences recovered from the H 2 /CO 2 ‐using consortium either belonged to a novel lineage of methanogens that grouped within the phylogenetic radiation of the Methanosarcinales and Methanomicrobiales or were affiliated with Methanobacterium bryantii . SSU rDNA sequences retrieved from the ethanol‐using consortium either grouped within the genus Methanosarcina or belonged to another novel lineage within the phylogenetic radiation of the Methanosarcinales and Methanomicrobiales . Cultured organisms belonging to either of the two novel lineages have not been reported yet.