Hydrogen is the central free intermediate during lignocellulose degradation by termite gut symbionts

Abstract

The key role of free hydrogen in the digestion of lignocellulose by wood-feeding lower termites and their symbiotic gut microbiota has been conceptually outlined in the past decades but remains to be quantitatively analyzed in situ. Using Reticulitermes santonensis, Zootermopsis nevadensis and Cryptotermes secundus, we determined metabolite fluxes involved in hydrogen turnover and the resulting distribution of H 2 in the microliter-sized gut. High-resolution hydrogen microsensor profiles revealed pronounced differences in hydrogen accumulation among the species (from o1 kPa to the saturation level). However, flux measurements indicated that the hydrogen pool was rapidly turned over in all termites, irrespective of the degree of accumulation. Microinjection of radiotracers into intact guts confirmed that reductive acetogenesis from CO 2 dominated hydrogen consumption, whereas methanogenesis played only a minor role. Only negligible amounts of H 2 were lost by emission, documenting an overall equilibrium between hydrogen production and consumption within the gut. Mathematical modeling revealed that production dominates in the gut lumen and consumption in the gut periphery for R. santonensis and Z. nevadensis, explaining the large accumulation of H 2 in these termites, whereas the moderate hydrogen accumulation in C. secundus indicated a more balanced radial distribution of the two processes. Daily hydrogen turnover rates were 9-33 m 3 H 2 per m 3 hindgut volume, corresponding to 22-26% of the respiratory activity of the termites. This makes H 2 the central free intermediate during lignocellulose degradation and the termite gut-with its high rates of reductive acetogenesis-the smallest and most efficient natural bioreactor currently known. The ISME Journal (2007) 1, 551-565; doi:10.1038/ismej.2007 IntroductionTermites inhabit about two-thirds of Earth's terrestrial surface and play important roles in the global carbon cycle (Lee and Wood, 1971;Wood and Sands, 1978;Collins and Wood, 1984;Sanderson, 1996;Bignell and Eggleton, 2000). Recently, wood-feeding termites received additional attention because lignocellulose degradation by their symbiotic gut microbiota presumably produces large amounts of hydrogen, an emerging 'clean' energy carrier (Dunn, 2002).Lignocellulose is the most abundant renewable resource of the biosphere. It consists mainly of cellulose and hemicelluloses, both of which are degraded with high efficiency during gut passage by the termite (65-99%, reviewed in Breznak and Brune, 1994). Although termites secrete their own cellulases and hemicellulases into foregut and midgut (Inoue et al., 1997;Watanabe and Tokuda, 2001;Tokuda et al., 2004Tokuda et al., , 2005, it is generally accepted that most of the polysaccharide degradation in lower termites occurs in the enlarged hindgut (Inoue et al., 1997;Tokuda et al., 2005), a bioreactorlike compartment with increased retention time and tightly packed with microorganisms (Breznak and Brune, 1994;Brune, 1998).In lower termites, the hindgut microbiota compr...

Bacteroidales ectosymbionts of gut flagellates shape the nitrogen-fixing community in dry-wood termites

“…Rather, the hydrogen concentrations in the gut of the four termite species seem to reflect the types of flagellates present: Kalotermes and Incisitermes species harbor large hypermastigid flagellates (Joenia and Trichonympha, respectively), which are absent in Neotermes and Cryptotermes species (Yamin, 1979), and probably form the bulk of the hydrogen produced in the gut (Messer and Lee, 1989;Inoue et al, 2007). This is in agreement also with previous reports, where Reticulitermes and Zootermopsis species (harboring Trichonympha flagellates) showed high hydrogen partial pressures, and Cryptotermes secundus (lacking Trichonympha flagellates) showed only little accumulation of hydrogen (Ebert and Brune, 1997;Pester and Brune, 2007).…”

“…Microsensor calibration and the experimental setup for hydrogen measurement in termite guts have been described in detail elsewhere (Ebert and Brune, 1997;Pester and Brune, 2007).…”

RNA-seq reveals cooperative metabolic interactions between two termite-gut spirochete species in co-culture

“…Fermenting gut microbes, including T. azotonutricium, provide homoacetogens like T. primitia a source of H 2 (Pester and Brune, 2007). In the culture media, an initial amount of H 2 gas (80%), which roughly mimics physiological conditions within the gut, was supplied in the headspace but was not replenished during growth.…”

“…During the course of lignocellulose fermentation, the free intermediate hydrogen (H 2 ) is produced and accumulates to concentrations near saturation (Pester and Brune, 2007), before being consumed by homoacetogenic gut bacteria and methanogenic archaea (Breznak and Switzer, 1986;Brauman et al, 1992). It has been estimated that H 2 þ CO 2 acetogenic bacteria supply their termite host with up to a third of its acetate supply (Breznak and Switzer, 1986).…”

Acetogenesis from H2plus CO2and nitrogen fixation by an endosymbiotic spirochete of a termite-gut cellulolytic protist

“…The host termite uses the produced acetate as a major carbon and energy source. The produced H 2 is a key metabolic intermediate that fuels many bacteria in the gut (9). The gut bacteria are also important for the nutrition of the host termite, carrying out both CO 2 -reducing acetogenesis (hereafter, reductive acetogenesis) and nitrogen fixation (6,10).…”