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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 &lt;1 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.

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