Schmidt, O; Wüst, PK; Hellmuth, S; Borst, K; Horn, MA; Drake, HL: Novel [NiFe]- and [FeFe]-Hydrogenase Gene Transcripts Indicative of Active Facultative Aerobes and Obligate Anaerobes in Earthworm Gut Contents, Applied and Environmental Microbiology, 77, 5842-5850 (2011)
The concomitant occurrence of molecular hydrogen (H2) and organic acids along the alimentary canal of the earthworm are indicative of ongoing fermentation during gut passage. Fermentative H2 production is catalyzed by [FeFe]-hydrogenases and Group 4 [NiFe]-hydrogenases in obligate anaerobes (e.g., Clostridiales) and facultative aerobes (e.g., Enterobacteriaceae), respectively, functional groups that might respond differently to contrasting redox conditions. Thus, the objectives of this study were to assess the redox potentials of the alimentary canal of Lumbricus terrestris and analyze hydrogenase transcript diversities of H2 producers in glucose-supplemented gut content microcosms. Although redox potentials in the core of the alimentary canal were variable on an individual worm basis, average redox potentials were similar. Lowest redox potentials occurred in the foregut and midgut regions, averaging 40 and 110 mV, respectively. Correlation plots between hydrogenase amino acid sequences and 16S rRNA gene sequences indicated that closely related hydrogenases belonged to closely related taxa whereas distantly related hydrogenases did not necessarily belong to distantly related taxa. 177 of 178 [FeFe]-hydrogenase gene transcripts clustered in 12 Clostridiales-affiliated operational taxonomic units, the majority of which were indicative of heretofore unknown hydrogenases. 79% and 21% of 86 Group 4 [NiFe]-hydrogenase gene transcripts were affiliated to Enterobacteriaceae and Aeromonadaceae, respectively. The collective results (i) suggest that fermenters must cope with variable and moderately oxidative redox conditions along the alimentary canal, (ii) demonstrate that heretofore undetected hydrogenases are present in the earthworm gut, and (iii) corroborate previous findings implicating Clostridiaceae and Enterobacteriaceae as active fermentative taxa in earthworm gut content.
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