AEM Accepts, published online ahead of print on 3 August 2007

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Appl. Environ. Microbiol. doi:10.1128/AEM.00181-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Methanogen-free lambs: establishment and development of ruminal hydrogenotrophs

Gérard Fonty^*, Keith Joblin, Michel Chavarot, Remy Roux, Graham Naylor, and Fabien Michallon

Unité de Microbiologie, INRA, Centre de Clermont-Ferrand-Theix, 63177 Aubière cedex, France; Grasslands Research Centre, AgResearch, Private Bag 11008, Palmerston North 4442, New Zealand

^* To whom correspondence should be addressed. Email: gerard.FONTY{at}univ-bpclermont.fr.

The aim of this work was to determine whether reductive acetogenesis can provide an alternative to methanogenesis in the rumen. Gnotobiotic lambs were inoculated with a functional rumen microbiota lacking methanogens and reared to maturity on a fibrous diet. Lambs with a methanogen-free rumen grew well, and feed intake and and ruminal volatile fatty acid concentrations in lambs lacking ruminal methanogens were lower but not markedly dissimilar to those in conventional lambs reared on the same diet. A high population density (10⁷-10⁸ cells g^-1) of ruminal acetogens slowly developed in methanogen-free lambs. Sulfate- and fumarate-reducing bacteria were present but their population densities were highly variable. In methanogen-free lambs, hydrogen capture from fermentation was low (28-46 %) in comparison with that in lambs containing ruminal methanogens (>90%). Reductive acetogenesis was not a significant part of ruminal fermentation in conventional lambs but contributed 21-25 % to fermentation in methanogen-free meroxenic animals. Ruminal H₂ utilization was lower in lambs lacking ruminal methanogens, but when a methanogen-free lamb was inoculated with a methanogen, ruminal H₂ utilization was similar to that in conventional lambs. H₂ utilization in lambs containing a normal ruminal microflora was age-dependent and increased with the animal age. The animal-age effect was less marked in lambs lacking ruminal methanogens. Addition of fumarate to rumen contents from methanogen-free lambs increased H₂ utilization. These findings provide the first evidence from animal studies that reductive acetogens can sustain a functional rumen and replace methanogens as a sink for H₂ in the rumen.

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