Hydrogen production by termite gut protists: characterization of iron hydrogenases of Parabasalian symbionts of the termite Coptotermes formosanus

Novel xylanase activity and biofuel production in Lacrimispora xylanolytica isolated from termite guts symbionts

Xylanase is crucial for converting lignocellulosic biomass into fermentable sugars, yet challenges such as thermostability, narrow pH activity ranges, and high production costs hindered recent advancements. The strain Ter3, isolated from termite guts and confirmed in both termite guts and nest soil via polymerase chain reaction-denaturing gradient gel electrophoresis, shares 99 % sequence similarity with Lacrimispora xylanolytica sy1. Ter3 produces 37-kilodalton extracellular endo-1,4-β-xylanase with an activity of 951.2 ± 25.9 U/mg at pH 6.5 and 50℃. Additionally, it generates 2,987.9 ± 31.1 mg/mL of ethanol, 37.6 ± 2.0 mg/L of butanol, and 41.5 ± 2.3 % hydrogen of total gas. Using molasses wastewater as a carbon source enhanced butanol production. Heterologous expression in Escherichia coli further increased specific activity of xylanase, reaching 8,960.1 ± 65.5 U/mg post-purification, surpassing that of the native enzyme. Ter3's robust xylanolytic, ethanologenic, and hydrogen-producing capabilities highlight its potential for biofuel production.

Termite Microbial Symbiosis as a Model for Innovative Design of Lignocellulosic Future Biorefinery: Current Paradigms and Future Perspectives

The hunt for renewable and alternative fuels has driven research towards the biological conversion of lignocellulosic biomass (LCB) into biofuels, including bioethanol and biohydrogen. Among the natural biomass utilization systems (NBUS), termites represent a unique and easy-to-access model system to study host–microbe interactions towards lignocellulose bioconversion/valorization. Termites have gained significant interest due to their highly efficient lignocellulolytic systems. The wood-feeding termites apply a unique and stepwise process for the hydrolysis of lignin, hemicellulose, and cellulose via biocatalytic processes; therefore, mimicking their digestive metabolism and physiochemical gut environments might lay the foundation for an innovative design of nature-inspired biotechnology. This review highlights the gut system of termites, particularly the wood-feeding species, as a unique model for future biorefinery. The gut system of termites is a treasure-trove for prospecting novel microbial species, including protists, bacteria, and fungi, having higher biocatalytic efficiencies and biotechnological potentials. The significance of potential bacteria and fungi for harnessing the enzymes appropriate for lignocellulosic biorefinery is also discussed. Termite digestomes are rich sources of lignocellulases and related enzymes that could be utilized in various industrial processes and biomass-related applications. Consideration of the host and symbiont as a single functioning unit will be one of the most crucial strategies to expedite developments in termite-modeled biotechnology in the future.

Weight and protozoa number but not bacteria diversity are associated with successful pair formation of dealates in the Formosan subterranean termite, Coptotermes formosanus

New colonies of Formosan subterranean termites are founded by monogamous pairs. During swarming season, alates (winged reproductives) leave their parental colony. After swarming, they drop to the ground, shed their wings, and male and female dealates find suitable nesting sites where they mate and become kings and queens of new colonies. The first generation of offspring is entirely dependent on the nutritional resources of the founder pair consisting of the fat and protein reserves of the dealates and their microbiota, which include the cellulose-digesting protozoa and diverse bacteria. Since termite kings and queens can live for decades, mate for life and colony success is linked to those initial resources, we hypothesized that gut microbiota of founders affect pair formation. To test this hypothesis, we collected pairs found in nest chambers and single male and female dealates from four swarm populations. The association of three factors (pairing status, sex of the dealates and population) with dealate weights, total protozoa, and protozoa Pseudotrichonympha grassii numbers in dealate hindguts was determined. In addition, Illumina 16S rRNA gene sequencing and the QIIME2 pipeline were used to determine the impact of those three factors on gut bacteria diversity of dealates. Here we report that pairing status was significantly affected by weight and total protozoa numbers, but not by P. grassii numbers and bacteria diversity. Weight and total protozoa numbers were higher in paired compared to single dealates. Males contained significantly higher P. grassii numbers and bacteria richness and marginally higher phylogenetic diversity despite having lower weights than females. In conclusion, this study showed that dealates with high body weight and protozoa numbers are more likely to pair and become colony founders, probably because of competitive advantage. The combined nutritional resources provided by body weight and protozoa symbionts of the parents are important for successful colony foundation and development.

Lemur Gut Microeukaryotic Community Variation Is Not Associated with Host Phylogeny, Diet, or Habitat

Identifying the major forces driving variation in gut microbiomes enhances our understanding of how and why symbioses between hosts and microbes evolved. Gut prokaryotic community variation is often closely associated with host evolutionary and ecological variables. Whether these same factors drive variation in other microbial taxa occupying the animal gut remains largely untested. Here, we present a one-to-one comparison of gut prokaryotic (16S rRNA metabarcoding) and microeukaryotic (18S rRNA metabarcoding) community patterning among 12 species of wild lemurs. Lemurs were sampled from dry forests and rainforests of southeastern Madagascar and display a range of phylogenetic and ecological niche diversity. We found that while lemur gut prokaryotic community diversity and composition vary with host taxonomy, diet, and habitat, gut microeukaryotic communities have no detectable association with any of these factors. We conclude that gut microeukaryotic community composition is largely random, while gut prokaryotic communities are conserved among host species. It is likely that a greater proportion of gut microeukaryotic communities comprise taxa with commensal, transient, and/or parasitic symbioses compared with gut prokaryotes, many of which form long-term relationships with the host and perform important biological functions. Our study highlights the importance of greater specificity in microbiome research; the gut microbiome contains many "omes" (e.g., prokaryome, eukaryome), each comprising different microbial taxa shaped by unique selective pressures.

Impact of Wood Age on Termite Microbial Assemblages

The decomposition of wood and detritus is challenging to most macroscopic organisms due to the recalcitrant nature of lignocellulose. Moreover, woody plants often protect themselves by synthesizing toxic or nocent compounds which infuse their tissues. Termites are essential wood decomposers in warmer terrestrial ecosystems and, as such, they have to cope with high concentrations of plant toxins in wood. In this paper, we evaluated the influence of wood age on the gut microbial (bacterial and fungal) communities associated with the termites Reticulitermes flavipes (Rhinotermitidae) (Kollar, 1837) and Microcerotermes biroi (Termitidae) (Desneux, 1905). We confirmed that the secondary metabolite concentration decreased with wood age. We identified a core microbial consortium maintained in the gut of R. flavipes and M. biroi and found that its diversity and composition were not altered by the wood age. Therefore, the concentration of secondary metabolites had no effect on the termite gut microbiome. We also found that both termite feeding activities and wood age affect the wood microbiome. Whether the increasing relative abundance of microbes with termite activities is beneficial to the termites is unknown and remains to be investigated. IMPORTANCE Termites can feed on wood thanks to their association with their gut microbes. However, the current understanding of termites as holobiont is limited. To our knowledge, no studies comprehensively reveal the influence of wood age on the termite-associated microbial assemblage. The wood of many tree species contains high concentrations of plant toxins that can vary with their age and may influence microbes. Here, we studied the impact of Norway spruce wood of varying ages and terpene concentrations on the microbial communities associated with the termites Reticulitermes flavipes (Rhinotermitidae) and Microcerotermes biroi (Termitidae). We performed a bacterial 16S rRNA and fungal ITS2 metabarcoding study to reveal the microbial communities associated with R. flavipes and M. biroi and their impact on shaping the wood microbiome. We noted that a stable core microbiome in the termites was unaltered by the feeding substrate, while termite activities influenced the wood microbiome, suggesting that plant secondary metabolites have negligible effects on the termite gut microbiome. Hence, our study shed new insights into the termite-associated microbial assemblage under the influence of varying amounts of terpene content in wood and provides a groundwork for future investigations for developing symbiont-mediated termite control measures.

Termite gas emissions select for hydrogenotrophic microbial communities in termite mounds

Organoheterotrophs are the dominant bacteria in most soils worldwide. While many of these bacteria can subsist on atmospheric hydrogen (H₂), levels of this gas are generally insufficient to sustain hydrogenotrophic growth. In contrast, bacteria residing within soil-derived termite mounds are exposed to high fluxes of H₂ due to fermentative production within termite guts. Here, we show through community, metagenomic, and biogeochemical profiling that termite emissions select for a community dominated by diverse hydrogenotrophic Actinobacteriota and Dormibacterota. Based on metagenomic short reads and derived genomes, uptake hydrogenase and chemosynthetic RuBisCO genes were significantly enriched in mounds compared to surrounding soils. In situ and ex situ measurements confirmed that high- and low-affinity H₂-oxidizing bacteria were highly active in the mounds, such that they efficiently consumed all termite-derived H₂ emissions and served as net sinks of atmospheric H₂ Concordant findings were observed across the mounds of three different Australian termite species, with termite activity strongly predicting H₂ oxidation rates (R ² = 0.82). Cell-specific power calculations confirmed the potential for hydrogenotrophic growth in the mounds with most termite activity. In contrast, while methane is produced at similar rates to H₂ by termites, mounds contained few methanotrophs and were net sources of methane. Altogether, these findings provide further evidence of a highly responsive terrestrial sink for H₂ but not methane and suggest H₂ availability shapes composition and activity of microbial communities. They also reveal a unique arthropod-bacteria interaction dependent on H₂ transfer between host-associated and free-living microbial communities.

Methanogenesis in the Digestive Tracts of the Tropical Millipedes Archispirostreptus gigas (Diplopoda, Spirostreptidae) and Epibolus pulchripes (Diplopoda, Pachybolidae)

Methanogens represent the final decomposition step in anaerobic degradation of organic matter, occurring in the digestive tracts of various invertebrates. However, factors determining their community structure and activity in distinct gut sections are still debated. In this study, we focused on the tropical millipede species Archispirostreptus gigas (Diplopoda, Spirostreptidae) and Epibolus pulchripes (Diplopoda, Pachybolidae), which release considerable amounts of methane. We aimed to characterize relationships between physicochemical parameters, methane production rates, and methanogen community structure in the two major gut sections, midgut and hindgut. Microsensor measurements revealed that both sections were strictly anoxic, with reducing conditions prevailing in both millipedes. Hydrogen concentration peaked in the anterior hindgut of E. pulchripes. In both species, the intestinal pH was significantly higher in the hindgut than in the midgut. An accumulation of acetate and formate in the gut indicated bacterial fermentation activities in the digestive tracts of both species. Phylogenetic analysis of 16S rRNA genes showed a prevalence of Methanobrevibacter spp. (Methanobacteriales), accompanied by a small fraction of so-far-unclassified "Methanomethylophilaceae" (Methanomassiliicoccales), in both species, which suggests that methanogenesis is mostly hydrogenotrophic. We conclude that anoxic conditions, negative redox potential, and bacterial production of hydrogen and formate promote gut colonization by methanogens. The higher activities of methanogens in the hindgut are explained by the higher pH of this compartment and their association with ciliates, which are restricted to this compartment and present an additional source of methanogenic substrates. IMPORTANCE Methane (CH₄) is the second most important atmospheric greenhouse gas after CO₂ and is believed to account for 17% of global warming. Methanogens are a diverse group of archaea and can be found in various anoxic habitats, including digestive tracts of plant-feeding animals. Termites, cockroaches, the larvae of scarab beetles, and millipedes are the only arthropods known to host methanogens and emit large amounts of methane. Millipedes are ranked as the third most important detritivores after termites and earthworms, and they are considered keystone species in many terrestrial ecosystems. Both methane-producing and non-methane-emitting species of millipedes have been observed, but what limits their methanogenic potential is not known. In the present study, we show that physicochemical gut conditions and the distribution of symbiotic ciliates are important factors determining CH₄ emission in millipedes. We also found close similarities to other methane-emitting arthropods, which might be associated with their similar plant-feeding habits.

Bacterial and archaeal symbioses with protists

Most of the genetic, cellular, and biochemical diversity of life rests within single-celled organisms - the prokaryotes (bacteria and archaea) and microbial eukaryotes (protists). Very close interactions, or symbioses, between protists and prokaryotes are ubiquitous, ecologically significant, and date back at least two billion years ago to the origin of mitochondria. However, most of our knowledge about the evolution and functions of eukaryotic symbioses comes from the study of animal hosts, which represent only a small subset of eukaryotic diversity. Here, we take a broad view of bacterial and archaeal symbioses with protist hosts, focusing on their evolution, ecology, and cell biology, and also explore what functions (if any) the symbionts provide to their hosts. With the immense diversity of protist symbioses starting to come into focus, we can now begin to see how these systems will impact symbiosis theory more broadly.

Molecular characterization of Trichomonas gypaetinii isolated from the upper alimentary tract of Steller's sea eagles (Haliaeetus pelagicus) and white-tailed sea eagles (Haliaeetus albicilla) in Hokkaido, Japan

Recent phylogenetic and morphologic studies of Trichomonas spp. suggests that there are more than 3 species that infect the upper alimentary tract of wild birds, which include T. gallinae, T. stableri, and T. gypaetinii. In this study, investigations were conducted on the prevalence of trichomonads in the upper alimentary tract of 12 Steller's sea eagles (Haliaeetus pelagicus) and 18 white-tailed sea eagles (H. albicilla). All birds were rescued from the wild and kept at a rehabilitation facility in Hokkaido, Japan, for variable durations and did not show any symptoms of trichomonosis. The ITS1-5.8SrRNA-ITS2 (ITS) genomic region of Trichomonas spp. was detected from 29 samples by PCR, and flagellates were confirmed from 4 samples by culture. Morphologic observations and measurement recordings were conducted under a light microscope on trophozoites obtained from the cultured isolates. Genomic sequences of the ITS, 18S ribosomal RNA (18S rRNA), Fe-hydrogenase, and RNA polymerase II largest subunit (Rpb1) regions were determined by direct sequencing, and phylogenetic analyses were conducted with previously published sequences of Trichomonas spp. All isolates were concluded as T. gypaetinii based on morphologic and molecular characterizations of the ITS and 18S rRNA genes. This is the first study to isolate T. gypaetinii from Haliaeetus eagles and further provide novel sequences of the Fe-hydrogenase and Rpb1 genes of T. gypaetinii. Both genomic regions also confirmed that T. gypaetinii belong to independent clusters from other Trichomonas spp.

Division of functional roles for termite gut protists revealed by single-cell transcriptomes

The microbiome in the hindgut of wood-feeding termites comprises various species of bacteria, archaea, and protists. This gut community is indispensable for the termite, which thrives solely on recalcitrant and nitrogen-poor wood. However, the difficulty in culturing these microorganisms has hindered our understanding of the function of each species in the gut. Although protists predominate in the termite gut microbiome and play a major role in wood digestion, very few culture-independent studies have explored the contribution of each species to digestion. Here, we report single-cell transcriptomes of four protists species comprising the protist population in worldwide pest Coptotermes formosanus. Comparative transcriptomic analysis revealed that the expression patterns of the genes involved in wood digestion were different among species, reinforcing their division of roles in wood degradation. Transcriptomes, together with enzyme assays, also suggested that one of the protists, Cononympha leidyi, actively degrades chitin and assimilates it into amino acids. We propose that C. leidyi contributes to nitrogen recycling and inhibiting infection from entomopathogenic fungi through chitin degradation. Two of the genes for chitin degradation were further revealed to be acquired via lateral gene transfer (LGT) implying the importance of LGT in the evolution of symbiosis. Our single-cell-based approach successfully characterized the function of each protist in termite hindgut and explained why the gut community includes multiple species.

Parallel reductive genome evolution in Desulfovibrio ectosymbionts independently acquired by Trichonympha protists in the termite gut

Several Trichonympha protist species in the termite gut have independently acquired Desulfovibrio ectosymbionts in apparently different stages of symbiosis. Here, we obtained the near-complete genome sequence of Desulfovibrio phylotype ZnDsv-02, which attaches to the surface of Trichonympha collaris cells, and compared it with a previously obtained genome sequence of 'Candidatus Desulfovibrio trichonymphae' phylotype Rs-N31, which is almost completely embedded in the cytoplasm of Trichonympha agilis. Single-nucleotide polymorphism analysis indicated that although Rs-N31 is almost clonal, the ZnDsv-02 population on a single host cell is heterogeneous. Despite these differences, the genome of ZnDsv-02 has been reduced to 1.6 Mb, which is comparable to that of Rs-N31 (1.4 Mb), but unlike other known ectosymbionts of protists with a genome similar in size to their free-living relatives. Except for the presence of a lactate utilization pathway, cell-adhesion components and anti-phage defense systems in ZnDsv-02, the overall gene-loss pattern between the two genomes is very similar, including the loss of genes responsive to environmental changes. Our study suggests that genome reduction can occur in ectosymbionts, even when they can be transmitted horizontally and obtain genes via lateral transfer, and that the symbiont genome size depends heavily on their role in the symbiotic system.

Heterologous Hydrogenase Overproduction Systems for Biotechnology-An Overview

Hydrogenases are complex metalloenzymes, showing tremendous potential as H2-converting redox catalysts for application in light-driven H2 production, enzymatic fuel cells and H2-driven cofactor regeneration. They catalyze the reversible oxidation of hydrogen into protons and electrons. The apo-enzymes are not active unless they are modified by a complicated post-translational maturation process that is responsible for the assembly and incorporation of the complex metal center. The catalytic center is usually easily inactivated by oxidation, and the separation and purification of the active protein is challenging. The understanding of the catalytic mechanisms progresses slowly, since the purification of the enzymes from their native hosts is often difficult, and in some case impossible. Over the past decades, only a limited number of studies report the homologous or heterologous production of high yields of hydrogenase. In this review, we emphasize recent discoveries that have greatly improved our understanding of microbial hydrogenases. We compare various heterologous hydrogenase production systems as well as in vitro hydrogenase maturation systems and discuss their perspectives for enhanced biohydrogen production. Additionally, activities of hydrogenases isolated from either recombinant organisms or in vivo/in vitro maturation approaches were systematically compared, and future perspectives for this research area are discussed.

Genome analyses of uncultured TG2/ZB3 bacteria in 'Margulisbacteria' specifically attached to ectosymbiotic spirochetes of protists in the termite gut

We investigated the phylogenetic diversity, localisation and metabolism of an uncultured bacterial clade, Termite Group 2 (TG2), or ZB3, in the termite gut, which belongs to the candidate phylum 'Margulisbacteria'. We performed 16S rRNA amplicon sequencing analysis and detected TG2/ZB3 sequences in 40 out of 72 termite and cockroach species, which exclusively constituted a monophyletic cluster in the TG2/ZB3 clade. Fluorescence in situ hybridisation analysis in lower termites revealed that these bacteria are specifically attached to ectosymbiotic spirochetes of oxymonad gut protists. Draft genomes of four TG2/ZB3 phylotypes from a small number of bacterial cells were reconstructed, and functional genome analysis suggested that these bacteria hydrolyse and ferment cellulose/cellobiose to H2, CO2, acetate and ethanol. We also assembled a draft genome for a partner Treponema spirochete and found that it encoded genes for reductive acetogenesis from H2 and CO2. We hypothesise that the TG2/ZB3 bacteria we report here are commensal or mutualistic symbionts of the spirochetes, exploiting the spirochetes as H2 sinks. For these bacteria, we propose a novel genus, 'Candidatus Termititenax', which represents a hitherto uncharacterised class-level clade in 'Margulisbacteria'. Our findings add another layer, i.e., cellular association between bacteria, to the multi-layered symbiotic system in the termite gut.

A Geometric Analysis of the Regulation of Inorganic Nutrient Intake by the Subterranean Termite Reticulitermes flavipes Kollar

Most studies on termite food selection have focused on a single nutrient per choice, however, termites, like all animals, must balance multiple nutrients in their diet. While most studies that use multi-nutrient approaches focus on macromolecules, the ability to balance the intake of inorganic nutrients is also vital to organisms. In this study, we used the geometric framework to test the effects of multiple inorganic nutrients on termite feeding. We presented the subsets of Reticulitermes flavipes colonies with food enriched with varying in levels of KCl, MgSO₄, and FePO₄. Each trial varied two of the three nutrients while the third nutrient was kept constant. The amount of food consumed was measured over two weeks. The termites' feeding patterns during the study suggested that they fed until they reached a limit for MgSO₄. This result suggests that the termites were using the rule of compromise such that the termites would over consume KCl or FePO₄ in order to avoid overeating MgSO₄. Thus, the termite colonies are able to regulate the intake of inorganic nutrients, and by doing so, adjust their intake from multiple resources in order to maintain an intake target.

Genome of 'Ca. Desulfovibrio trichonymphae', an H2-oxidizing bacterium in a tripartite symbiotic system within a protist cell in the termite gut

The cellulolytic protist Trichonympha agilis in the termite gut permanently hosts two symbiotic bacteria, ‘Candidatus Endomicrobium trichonymphae' and ‘Candidatus Desulfovibrio trichonymphae'. The former is an intracellular symbiont, and the latter is almost intracellular but still connected to the outside via a small pore. The complete genome of ‘Ca. Endomicrobium trichonymphae' has previously been reported, and we here present the complete genome of ‘Ca. Desulfovibrio trichonymphae'. The genome is small (1 410 056 bp), has many pseudogenes, and retains biosynthetic pathways for various amino acids and cofactors, which are partially complementary to those of ‘Ca. Endomicrobium trichonymphae'. An amino acid permease gene has apparently been transferred between the ancestors of these two symbionts; a lateral gene transfer has affected their metabolic capacity. Notably, ‘Ca. Desulfovibrio trichonymphae' retains the complex system to oxidize hydrogen by sulfate and/or fumarate, while genes for utilizing other substrates common in desulfovibrios are pseudogenized or missing. Thus, ‘Ca. Desulfovibrio trichonymphae' is specialized to consume hydrogen that may otherwise inhibit fermentation processes in both T. agilis and ‘Ca. Endomicrobium trichonymphae'. The small pore may be necessary to take up sulfate. This study depicts a genome-based model of a multipartite symbiotic system within a cellulolytic protist cell in the termite gut.

Lower Termite Associations with Microbes: Synergy, Protection, and Interplay

Lower-termites are one of the best studied symbiotic systems in insects. Their ability to feed on a nitrogen-poor, wood-based diet with help from symbiotic microbes has been under investigation for almost a century. A unique microbial consortium living in the guts of lower termites is essential for wood-feeding. Host and symbiont cellulolytic enzymes synergize each other in the termite gut to increase digestive efficiency. Because of their critical role in digestion, gut microbiota are driving forces in all aspects of termite biology. Social living also comes with risks for termites. The combination of group living and a microbe-rich habitat makes termites potentially vulnerable to pathogenic infections. However, the use of entomopathogens for termite control has been largely unsuccessful. One mechanism for this failure may be symbiotic collaboration; i.e., one of the very reasons termites have thrived in the first place. Symbiont contributions are thought to neutralize fungal spores as they pass through the termite gut. Also, when the symbiont community is disrupted pathogen susceptibility increases. These recent discoveries have shed light on novel interactions for symbiotic microbes both within the termite host and with pathogenic invaders. Lower termite biology is therefore tightly linked to symbiotic associations and their resulting physiological collaborations.

Metabolomic profiling of 13C-labelled cellulose digestion in a lower termite: insights into gut symbiont function

Termites consume an estimated 3–7 billion tonnes of lignocellulose annually, a role in nature which is unique for a single order of invertebrates. Their food is digested with the help of microbial symbionts, a relationship that has been recognized for 200 years and actively researched for at least a century. Although DNA- and RNA-based approaches have greatly refined the details of the process and the identities of the participants, the allocation of roles in space and time remains unclear. To resolve this issue, a pioneer study is reported using metabolomics to chart the in situ catabolism of ¹³C-cellulose fed to the dampwood species Hodotermopsis sjostedti. The results confirm that the secretion of endogenous cellulases by the host may be significant to the digestive process and indicate that a major contribution by hindgut bacteria is phosphorolysis of cellodextrins or cellobiose. This study provides evidence that essential amino acid acquisition by termites occurs following the lysis of microbial tissue obtained via proctodaeal trophallaxis.

Comprehensive phylogenetic diversity of [FeFe]-hydrogenase genes in termite gut microbiota

Phylogenetic diversity of [FeFe]-hydrogenase (HydA) in termite guts was assessed by pyrosequencing PCR amplicons obtained using newly designed primers. Of 8,066 reads, 776 hydA phylotypes, defined with 97% nucleotide sequence identity, were recovered from the gut homogenates of three termite species, Hodotermopsis sjoestedti, Reticulitermes speratus, and Nasutitermes takasagoensis. The phylotype coverage was 92–98%, and the majority shared only low identity with database sequences. It was estimated that 194–745 hydA phylotypes existed in the gut of each termite species. Our results demonstrate that hydA gene diversity in the termite gut microbiota is much higher than previously estimated.

Identification of Endosymbiotic Methanogen and Ectosymbiotic Spirochetes of Gut Protists of the Termite Coptotermes formosanus

Prokaryotic associations with gut protists of the termite Coptotermes formosanus were investigated based on 16S rRNA gene sequences. An endosymbiotic methanogen of Spirotrichonympha leidyi phylogenetically grouped with endosymbionts of other gut protists in the genus Methanobrevibacter, seemed to be unrelated to the host protist phylogeny. Three different lineages of ectosymbiotic spirochetes in the genus Treponema were identified in single cells of Holomastigotoides mirabile, indicating their simultaneous occurrence. Although these symbionts represented mere minor populations in the gut, their phylogenetic assignments suggest a common symbiotic relationship involving H(2) metabolism.

Patterns of [FeFe] hydrogenase diversity in the gut microbial communities of lignocellulose-feeding higher termites

Hydrogen is the central free intermediate in the degradation of wood by termite gut microbes and can reach concentrations exceeding those measured for any other biological system. Degenerate primers targeting the largest family of [FeFe] hydrogenases observed in a termite gut metagenome have been used to explore the evolution and representation of these enzymes in termites. Sequences were cloned from the guts of the higher termites Amitermes sp. strain Cost010, Amitermes sp. strain JT2, Gnathamitermes sp. strain JT5, Microcerotermes sp. strain Cost008, Nasutitermes sp. strain Cost003, and Rhyncotermes sp. strain Cost004. Each gut sample harbored a more rich and evenly distributed population of hydrogenase sequences than observed previously in the guts of lower termites and Cryptocercus punctulatus. This accentuates the physiological importance of hydrogen for higher termite gut ecosystems and may reflect an increased metabolic burden, or metabolic opportunity, created by a lack of gut protozoa. The sequences were phylogenetically distinct from previously sequenced [FeFe] hydrogenases. Phylogenetic and UniFrac comparisons revealed congruence between host phylogeny and hydrogenase sequence library clustering patterns. This may reflect the combined influences of the stable intimate relationship of gut microbes with their host and environmental alterations in the gut that have occurred over the course of termite evolution. These results accentuate the physiological importance of hydrogen to termite gut ecosystems.

Analysis of extensive [FeFe] hydrogenase gene diversity within the gut microbiota of insects representing five families of Dictyoptera

We have designed and utilized degenerate primers in the phylogenetic analysis of [FeFe] hydrogenase gene diversity in the gut ecosystems of roaches and lower termites. H(2) is an important free intermediate in the breakdown of wood by termite gut microbial communities, reaching concentrations in some species exceeding those measured for any other biological system. The primers designed target with specificity the largest group of enzymatic H domain proteins previously identified in a termite gut metagenome. "Family 3" hydrogenase sequences were amplified from the guts of lower termites, Incisitermes minor, Zootermopsis nevadensis, and Reticulitermes hesperus, and two roaches, Cryptocercus punctulatus and Periplaneta americana. Subsequent analyses revealed that all termite and Cryptocercus sequences were phylogenetically distinct from non-termite-associated hydrogenases available from public databases. The abundance of unique sequence operational taxonomic units (as many as 21 from each species) underscores the previously demonstrated physiological importance of H(2) to the gut ecosystems of these wood-feeding insects. The diversity of sequences observed might be reflective of multiple niches that the enzymes have been evolved to accommodate. Sequences cloned from Cryptocercus and the lower termite samples, all of which are wood feeding insects, clustered closely with one another in phylogenetic analyses to the exclusion of alleles from P. americana, an omnivorous cockroach, also cloned during this study. We present primers targeting a family of termite gut [FeFe] hydrogenases and provide results that are consistent with a pivotal role for hydrogen in the termite gut ecosystem and point toward unique evolutionary adaptations to the gut ecosystem.

Genomic analysis reveals multiple [FeFe] hydrogenases and hydrogen sensors encoded by treponemes from the H(2)-rich termite gut

We have completed a bioinformatic analysis of the hydrogenases encoded in the genomes of three termite gut treponeme isolates: hydrogenotrophic, homoacetogenic Treponema primitia strains ZAS-1 and ZAS-2, and the hydrogen-producing, sugar-fermenting Treponema azotonutricium ZAS-9. H(2) is an important free intermediate in the breakdown of wood by termite gut microbial communities, reaching concentrations in some species exceeding those measured for any other biological system. The spirochetes encoded 4, 8, and 5 [FeFe] hydrogenase-like proteins, identified by their H domains, respectively, but no other recognizable hydrogenases. The [FeFe] hydrogenases represented many sequence families previously proposed in an analysis of termite gut metagenomic data. Each strain encoded both putative [FeFe] hydrogenase enzymes and evolutionarily related hydrogen sensor/transducer proteins likely involved in phosphorelay or methylation pathways, and possibly even chemotaxis. A new family of [FeFe] hydrogenases (FDH-Linked) is proposed that may form a multimeric complex with formate dehydrogenase to provide reducing equivalents for reductive acetogenesis in T. primitia. The many and diverse [FeFe] hydrogenase-like proteins encoded within the sequenced genomes of the termite gut treponemes has enabled the discovery of a putative new class of [FeFe] hydrogenase proteins potentially involved in acetogenesis and furthered present understanding of many families, including sensory, of H domain proteins beyond what was possible through the use of fragmentary termite gut metagenome sequence data alone, from which they were initially defined.

Composition of intake sugars and emission of gases from paper sludges by Coptotermes formosanus Shiraki

Paper sludge is a by-product of the pulping process and is landfilled or incinerated for disposal. In this study, we evaluated ingestion and digestibility of carbohydrates, by the termite Coptotermes formosanus, in two kinds of sludges: sludge C from the chemical pulp mill and sludge M from the mechanical pulp mill. The no-choice tests using the termite for three weeks showed that the mass loss of sludge C was significantly higher than that of the control samples: a bleached pulp and red pine wood. It is considered that the higher inorganic content of sludge C resulted in the higher mass loss when the same amount of carbohydrates was taken by the pulp- or wood-fed termite. Although the inorganic content of sludge M was almost the same as that of sludge C, the higher lignin content in sludge M is thought to have resulted in the lower mass loss. Analysis of sugar composition in the faecal materials of the termite showed that about 73% of glucose and 81% of xylose in sludge C were digested. It was concluded that the digestibility of these sugars in sludge C was the same as that of the control samples despite containing high amounts of inorganic compounds. However, the hydrogen conversion rate by the termites that were fed sludge was lower than that of the termites that were fed pulp in the no-choice test for three days: one mole of glucose from the sludge and pulp was converted to 0.51 and 0.80 moles of hydrogen, respectively.

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

Although it is well documented that the lack of nitrogen in the diet of wood-feeding termites is compensated by the nitrogen-fixing capacity of their gut microbiota, the bacteria responsible for this activity are largely unknown. Here, we analyzed the diversity and expression of nitrogenase genes (homologs of nifH) in four species of dry-wood termites (Kalotermitidae), which thrive on a particularly nitrogen-poor resource. Although each species harbored a highly diverse suite of termite-specific homologs in their microliter-sized hindgut, only a core set related to nifH genes of Treponema and Azoarcus spp., 'Azobacteroides pseudotrichonymphae', the first member of the Bacteroidales identified as a diazotroph, and termite-gut-specific anfH genes of hitherto unknown origin were preferentially expressed. Transcription patterns corroborated that the populations of active diazotrophs differ fundamentally between termite genera. Capillary-picked suspensions of the flagellates Devescovina arta and Snyderella tabogae revealed that their bacterial ectosymbionts each possess two paralogs of nifH, which apparently have been acquired consecutively during evolution of Bacteroidales, but only one of them (anfH) is actively expressed. Transcription patterns correlated neither with the molybdenum content of the diet nor with intestinal hydrogen concentrations, measured with microsensors. We propose that the nitrogen-fixing community in different dry-wood termites is shaped by the symbionts of their specific flagellate populations. Our findings suggest that the diazotrophic nature of 'Armantifilum devescovinae' has an important role in the nitrogen metabolism of dry-wood termites and is the driving force of co-evolution with its flagellate host.

Toward the functional analysis of uncultivable, symbiotic microorganisms in the termite gut

Termites thrive on dead plant matters with the aid of microorganisms resident in their gut. The gut microbiota comprises protists (single-celled eukaryotes), bacteria, and archaea, most of which are unique to the termite gut ecosystem. Although this symbiosis has long been intriguing researchers of both basic and applied sciences, its detailed mechanism remains unclear due to the enormous complexity and the unculturability of the microbiota. In the effort to overcome the difficulty, recent advances in omics, such as metagenomics, metatranscriptomics, and metaproteomics have gradually unveiled the black box of this symbiotic system. Genomics targeting a single species of the unculturable microbial members has also provided a great progress in the understanding of the symbiotic interrelationships among the gut microorganisms. In this review, the symbiotic system organized by wood-feeding termites and their gut microorganisms is outlined, focusing on the recent achievement in omics studies of this multilayered symbiotic system.

Strict cospeciation of devescovinid flagellates and Bacteroidales ectosymbionts in the gut of dry-wood termites (Kalotermitidae)

The surface of many termite gut flagellates is colonized with a dense layer of bacteria, yet little is known about the evolutionary relationships of such ectosymbionts and their hosts. Here we investigated the molecular phylogenies of devescovinid flagellates (Devescovina spp.) and their symbionts from a wide range of dry-wood termites (Kalotermitidae). From species-pure flagellate suspensions isolated with micropipettes, we obtained SSU rRNA gene sequences of symbionts and host. Phylogenetic analysis showed that the Devescovina spp. present in many species of Kalotermitidae form a monophyletic group, which includes also the unique devescovinid flagellate Caduceia versatilis. All members of this group were consistently associated with a distinct lineage of Bacteroidales, whose location on the cell surface was confirmed by fluorescence in situ hybridization. The well-supported congruence of the phylogenies of devescovinids and their ectosymbionts documents a strict cospeciation. In contrast, the endosymbionts of the same flagellates ('Endomicrobia') were clearly polyphyletic and must have been acquired independently by horizontal transfer from other flagellate lineages. Also the Bacteroidales ectosymbionts of Oxymonas flagellates present in several Kalotermitidae belonged to several distantly related lines of descent, underscoring the general perception that the evolutionary history of flagellate-bacteria symbioses in the termite gut is complex.

Genome of an endosymbiont coupling N2 fixation to cellulolysis within protist cells in termite gut

Termites harbor diverse symbiotic gut microorganisms, the majority of which are as yet uncultivable and their interrelationships unclear. Here, we present the complete genome sequence of the uncultured Bacteroidales endosymbiont of the cellulolytic protist Pseudotrichonympha grassii, which accounts for 70% of the bacterial cells in the gut of the termite Coptotermes formosanus. Functional annotation of the chromosome (1,114,206 base pairs) unveiled its ability to fix dinitrogen and recycle putative host nitrogen wastes for biosynthesis of diverse amino acids and cofactors, and import glucose and xylose as energy and carbon sources. Thus, nitrogen fixation and cellulolysis are coupled within the protist's cells. This highly evolved symbiotic system probably underlies the ability of the worldwide pest termites Coptotermes to use wood as their sole food.

Symbioses of flagellates and prokaryotes in the gut of lower termites

The microbial community in the gut of phylogenetically lower termites, comprising both flagellated protists and prokaryotes, has fascinated many scientists because of the symbiotic relationships that are responsible for the efficient degradation of lignocellulose. However, the complex nature of this microbial community and the formidable unculturability of most members have hampered detailed microbial studies. Comprehensive phylogenetic descriptions of the community members in the past decade still provide little information about their functions because the community contains diverse novel microbial species. Recent advances in molecular approaches have shed new light on species-specific spatial distributions, particularly the cellular associations of flagellated protists and prokaryotes, their functional interactions and coevolutionary relationships. These advances have gradually unveiled how this symbiotic complex functions to efficiently utilize lignocellulose.