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

Abstract

Symbiotic associations of cellulolytic eukaryotic protists and diverse bacteria are common in the gut microbial communities of termites. Besides cellulose degradation by the gut protists, reductive acetogenesis from H 2 plus CO 2 and nitrogen fixation by gut bacteria play crucial roles in the host termites' nutrition by contributing to the energy demand of termites and supplying nitrogen poor in their diet, respectively. Fractionation of these activities and the identification of key genes from the gut community of the wood-feeding termite Hodotermopsis sjoestedti revealed that substantial activities in the gut-nearly 60% of reductive acetogenesis and almost exclusively for nitrogen fixation-were uniquely attributed to the endosymbiotic bacteria of the cellulolytic protist in the genus Eucomonympha. The rod-shaped endosymbionts were surprisingly identified as a spirochete species in the genus Treponema, which usually exhibits a characteristic spiral morphology. The endosymbionts likely use H 2 produced by the protist for these dual functions. Although H 2 is known to inhibit nitrogen fixation in some bacteria, it seemed to rather stimulate this important mutualistic process. In addition, the single-cell genome analyses revealed the endosymbiont's potentials of the utilization of sugars for its energy requirement, and of the biosynthesis of valuable nutrients such as amino acids from the fixed nitrogen. These metabolic interactions are suitable for the dual functions of the endosymbiont and reconcile its substantial contributions in the gut.endosymbiosis | spirochetes | single-cell genomics | adaptive evolution | metabolic interaction

Genome Evolution and Nitrogen Fixation in Bacterial Ectosymbionts of a Protist Inhabiting Wood-Feeding Cockroaches

“…The prevalence of nitrogen fixation by bacterial symbionts of protists supports this hypothesis, but the amount of nitrogen supplied to protists by their symbionts, the mechanism, the type of molecule supplied, and the dependency of the protists on this nitrogen supply are not known. Free-living hindgut bacteria probably also fix nitrogen (38)(39)(40), but most of the nitrogen fixation activity in the hindguts is likely performed by bacterial symbionts of protists (37,41), a conclusion supported by our analysis of free-living bacteria in the hindguts of Cryptocercus (Fig. 6D).…”

“…Nitrogen fixation has been hypothesized from molecular and genomic data to be a key process in termite gut symbioses (16,(35)(36)(37)41). Our NanoSIMS analysis of in vivo 15 N-labeled hindgut microbes provides direct imaging evidence of a specific hindgut microbe playing a role in nitrogen fixation in its natural environment and represents a significant advance for these uncultivated microbes.…”

“…Under these conditions, such genomic features are typical for endosymbionts (19)(20)(21), including those of Bacteroidales (16), because the symbionts undergo population bottlenecks during vertical transmission resulting in relaxed selection and genetic drift. Not all endosymbionts have reduced genomes (41), but this raises the question of whether the genomes of ectosymbionts evolve in a fundamentally different way. Much like endosymbionts, the extracellular bacteria associated with the guts of plataspid and acanthosomatid stinkbugs (Plataspidae and Acanthosomatidae) exhibit AT-biased, reduced genomes and accelerated rates of molecular evolution (72,73).…”

“…For instance, a few cultivated bacteria have been shown to fix nitrogen, but these are likely not the dominant nitrogen fixers in the hindgut (37)(38)(39)(40). As well, in using cultivationindependent methods, results of PCR, acetylene reduction, and 15 N 2 incorporation assays suggest that the majority of nitrogen fixation in the hindguts of Hodotermopsis sjoestedti is performed by spirochete endosymbionts of Eucomonympha (41).…”

The Gut Microbiota of Termites: Digesting the Diversity in the Light of Ecology and Evolution

“…They are phylogenetically highly diverse and comprise various monophyletic groups of termite-specific lineages (e.g., 39,82). Individual lineages differ in abundance between host groups (22,92), occur either as free-swimming cells or are associated with the surface of flagellates (9,47) or the fiber fraction (65), and may comprise different functional guilds (see below).…”