Termites in the woodwork

Taxonomic and Functional Annotation of Termite Degraded Butea monosperma (Lam.) Kuntze (Flame of the Forest)

Background:

Butea monosperma is an economically and medicinally important plant that grows all over India, however, the plant is highly susceptible to termite attack. The present study unravelled the bacterial community composition and their functional attributions from the termite degraded Butea.

Methods:

Total genomic DNA from termite degraded Butea monosperma samples was extracted and subjected to sequencing on Illumina's Miseq. The raw and unassembled reads obtained from high-throughput sequencing were used for taxonomic and functional profiling using different online and stand-alone softwares. Moreover, to ascertain the effect of different geographical locations and environmental factors, comparative analysis was performed using four other publically available metagenomes.

Results:

The higher abundance of Actinobacteria (21.27%), Proteobacteria (14.18%), Firmicutes (10.46%), and Bacteroidetes (4.11%) was found at the phylum level. The genus level was dominated by Bacillus (4.33%), Gemmatimonas (3.13%), Mycobacterium (1.82%), Acidimicrobium (1.69%), Thermoleophilum (1.23%), Nocardioides (1.44%), Terrimonas and Acidithermus (1.09%) and Clostridium (1.05%). Functional annotation of the termite degraded B. monosperma metagenome revealed a high abundance of ammonia oxidizers, sulfate reducers, dehalogenators, nitrate reducers, sulfide oxidizers, xylan degraders, nitrogen fixers and chitin degraders.

Conclusion:

The present study highlights the significance of the inherent microbiome of the degraded Butea shaping the microbial communities for effective degradation of biomass and different environmental toxicants. The unknown bacterial communities present in the sample can serve as enzyme sources for lignocelluloses degradation for biofuel production.

Variation in the Gut Microbiota of Termites (Tsaitermes ampliceps) Against Different Diets

Termites are well recognized for their thriving on recalcitrant lignocellulosic diets through nutritional symbioses with gut-dwelling microbiota; however, the effects of diet changes on termite gut microbiota are poorly understood, especially for the lower termites. In this study, we employed high-throughput 454 pyrosequencing of 16S V1-V3 amplicons to compare gut microbiotas of Tsaitermes ampliceps fed with lignin-rich and lignin-poor cellulose diets after a 2-week-feeding period. As a result, the majority of bacterial taxa were shared across the treatments with different diets, but their relative abundances were modified. In particular, the relative abundance was reduced for Spirochaetes and it was increased for Proteobacteria and Bacteroides by feeding the lignin-poor diet. The evenness of gut microbiota exhibited a significant difference in response to the diet type (filter paper diets < corn stover diets < wood diets), while their richness was constant, which may be related to the lower recalcitrance of this biomass to degradation. These results have important implications for sampling and analysis strategies to probe the lignocellulose degradation features of termite gut microbiota and suggest that the dietary lignocellulose composition could cause shifting rapidly in the termite gut microbiota.

Mining biomass-degrading genes through Illumina-based de novo sequencing and metagenomic analysis of free-living bacteria in the gut of the lower termite Coptotermes gestroi harvested in Vietnam

The 5.6 Gb metagenome of free-living microbial flora in the gut of the lower termite Coptotermes gestroi, harvested in Vietnam, was sequenced using Illumina technology. Genes related to biomass degradation were mined for a better understanding of biomass digestion in the termite gut and to identify lignocellulolytic enzymes applicable to biofuel production. The sequencing generated 5.4 Gb of useful reads, containing 125,431 ORFs spanning 78,271,365 bp, 80% of which was derived from bacteria. The 12 most abundant bacterial orders were Spirochaetales, Lactobacillales, Bacteroidales, Clostridiales, Enterobacteriales, Pseudomonades, Synergistales, Desulfovibrionales, Xanthomonadales, Burkholderiales, Bacillales, and Actinomycetales, and 1460 species were estimated. Of more than 12,000 ORFs with predicted functions related to carbohydrate metabolism, 587 encoding hydrolytic enzymes for cellulose, hemicellulose, and pectin were identified. Among them, 316 ORFs were related to cellulose degradation, and included β-glucosidases, 6-phospho-β-glucosidases, licheninases, glucan endo-1,3-β-D-glucosidases, endoglucanases, cellulose 1,4-β-cellobiosidases, glucan 1,3-β-glucosidases, and cellobiose phosphorylases. In addition, 259 ORFs were related to hemicellulose degradation, encoding endo-1,4-β-xylanases, α-galactosidases, α-N-arabinofuranosidases, xylan 1,4-β-xylosidases, arabinan endo-1,5-α-L-arabinosidases, endo-1,4-β-mannanases, and α-glucuronidases. Twelve ORFs encoding pectinesterases and pectate lyases were also obtained. To our knowledge, this is the first successful application of Illumina-based de novo sequencing for the analysis of a free-living bacterial community in the gut of a lower termite C. gestroi and for mining genes related to lignocellulose degradation from the gut bacteria.

The complexities of hydrolytic enzymes from the termite digestive system

The main challenge in second generation bioethanol production is the efficient breakdown of cellulose to sugar monomers (hydrolysis). Due to the recalcitrant character of cellulose, feedstock pretreatment and adapted hydrolysis steps are needed to obtain fermentable sugar monomers. The conventional industrial production process of second-generation bioethanol from biomass comprises several steps: thermochemical pretreatment, enzymatic hydrolysis and sugar fermentation. This process is undergoing continuous optimization in order to increase the bioethanol yield and reduce the economic cost. Therefore, the discovery of new enzymes with high lignocellulytic activity or new strategies is extremely important. In nature, wood-feeding termites have developed a sophisticated and efficient cellulose degrading system in terms of the rate and extent of cellulose hydrolysis and exploitation. This system, which represents a model for digestive symbiosis has attracted the attention of biofuel researchers. This review describes the termite digestive system, gut symbionts, termite enzyme resources, in vitro studies of isolated enzymes and lignin degradation in termites.

Advanced biorefinery in lower termite-effect of combined pretreatment during the chewing process

BACKGROUND

Currently the major barrier in biomass utilization is the lack of an effective pretreatment of plant cell wall so that the carbohydrates can subsequently be hydrolyzed into sugars for fermentation into fuel or chemical molecules. Termites are highly effective in degrading lignocellulosics and thus can be used as model biological systems for studying plant cell wall degradation.

RESULTS

We discovered a combination of specific structural and compositional modification of the lignin framework and partial degradation of carbohydrates that occurs in softwood with physical chewing by the termite, Coptotermes formosanus, which are critical for efficient cell wall digestion. Comparative studies on the termite-chewed and native (control) softwood tissues at the same size were conducted with the aid of advanced analytical techniques such as pyrolysis gas chromatography mass spectrometry, attenuated total reflectance Fourier transform infrared spectroscopy and thermogravimetry. The results strongly suggest a significant increase in the softwood cellulose enzymatic digestibility after termite chewing, accompanied with utilization of holocellulosic counterparts and an increase in the hydrolysable capacity of lignin collectively. In other words, the termite mechanical chewing process combines with specific biological pretreatment on the lignin counterpart in the plant cell wall, resulting in increased enzymatic cellulose digestibility in vitro. The specific lignin unlocking mechanism at this chewing stage comprises mainly of the cleavage of specific bonds from the lignin network and the modification and redistribution of functional groups in the resulting chewed plant tissue, which better expose the carbohydrate within the plant cell wall. Moreover, cleavage of the bond between the holocellulosic network and lignin molecule during the chewing process results in much better exposure of the biomass carbohydrate.

CONCLUSION

Collectively, these data indicate the participation of lignin-related enzyme(s) or polypeptide(s) and/or esterase(s), along with involvement of cellulases and hemicellulases in the chewing process of C. formosanus, resulting in an efficient pretreatment of biomass through a combination of mechanical and enzymatic processes. This pretreatment could be mimicked for industrial biomass conversion.

Developing symbiotic consortia for lignocellulosic biofuel production

The search for petroleum alternatives has motivated intense research into biological breakdown of lignocellulose to produce liquid fuels such as ethanol. Degradation of lignocellulose for biofuel production is a difficult process which is limited by, among other factors, the recalcitrance of lignocellulose and biological toxicity of the products. Consolidated bioprocessing has been suggested as an efficient and economical method of producing low value products from lignocellulose; however, it is not clear whether this would be accomplished more efficiently with a single organism or community of organisms. This review highlights examples of mixtures of microbes in the context of conceptual models for developing symbiotic consortia for biofuel production from lignocellulose. Engineering a symbiosis within consortia is a putative means of improving both process efficiency and stability relative to monoculture. Because microbes often interact and exist attached to surfaces, quorum sensing and biofilm formation are also discussed in terms of consortia development and stability. An engineered, symbiotic culture of multiple organisms may be a means of assembling a novel combination of metabolic capabilities that can efficiently produce biofuel from lignocellulose.

Microbial diversity associated with the hydrothermal shrimp Rimicaris exoculata gut and occurrence of a resident microbial community

Rimicaris exoculata dominates the megafauna of several Mid-Atlantic Ridge hydrothermal sites. Its gut is full of sulphides and iron-oxide particles and harbours microbial communities. Although a trophic symbiosis has been suggested, their role remains unclear. In vivo starvation experiments in pressurized vessels were performed on shrimps from Rainbow and Trans-Atlantic Geotraverse sites in order to expel the transient gut contents. Microbial communities associated with the gut of starved and reference shrimps were compared using 16S rRNA gene libraries and microscopic observations (light, transmission and scanning electron microscopy and FISH analyses). We show that the gut microbiota of shrimps from both sites included mainly Deferribacteres, Mollicutes, Epsilon- and Gammaproteobacteria. For the first time, we have observed filamentous bacteria, inserted between microvilli of gut epithelial cells. They remained after starvation periods in empty guts, suggesting the occurrence of a resident microbial community. The bacterial community composition was the same regardless of the site, except for Gammaproteobacteria retrieved only in Rainbow specimens. We observed a shift in the composition of the microbiota of long-starved specimens, from the dominance of Deferribacteres to the dominance of Gammaproteobacteria. These results reinforce the hypothesis of a symbiotic relationship between R. exoculata and its gut epibionts.

Parallel metatranscriptome analyses of host and symbiont gene expression in the gut of the termite Reticulitermes flavipes

BACKGROUND

Termite lignocellulose digestion is achieved through a collaboration of host plus prokaryotic and eukaryotic symbionts. In the present work, we took a combined host and symbiont metatranscriptomic approach for investigating the digestive contributions of host and symbiont in the lower termite Reticulitermes flavipes. Our approach consisted of parallel high-throughput sequencing from (i) a host gut cDNA library and (ii) a hindgut symbiont cDNA library. Subsequently, we undertook functional analyses of newly identified phenoloxidases with potential importance as pretreatment enzymes in industrial lignocellulose processing.

RESULTS

Over 10,000 expressed sequence tags (ESTs) were sequenced from the 2 libraries that aligned into 6,555 putative transcripts, including 171 putative lignocellulase genes. Sequence analyses provided insights in two areas. First, a non-overlapping complement of host and symbiont (prokaryotic plus protist) glycohydrolase gene families known to participate in cellulose, hemicellulose, alpha carbohydrate, and chitin degradation were identified. Of these, cellulases are contributed by host plus symbiont genomes, whereas hemicellulases are contributed exclusively by symbiont genomes. Second, a diverse complement of previously unknown genes that encode proteins with homology to lignase, antioxidant, and detoxification enzymes were identified exclusively from the host library (laccase, catalase, peroxidase, superoxide dismutase, carboxylesterase, cytochrome P450). Subsequently, functional analyses of phenoloxidase activity provided results that were strongly consistent with patterns of laccase gene expression. In particular, phenoloxidase activity and laccase gene expression are mostly restricted to symbiont-free foregut plus salivary gland tissues, and phenoloxidase activity is inducible by lignin feeding.

CONCLUSION

To our knowledge, this is the first time that a dual host-symbiont transcriptome sequencing effort has been conducted in a single termite species. This sequence database represents an important new genomic resource for use in further studies of collaborative host-symbiont termite digestion, as well as development of coevolved host and symbiont-derived biocatalysts for use in industrial biomass-to-bioethanol applications. Additionally, this study demonstrates that: (i) phenoloxidase activities are prominent in the R. flavipes gut and are not symbiont derived, (ii) expands the known number of host and symbiont glycosyl hydrolase families in Reticulitermes, and (iii) supports previous models of lignin degradation and host-symbiont collaboration in cellulose/hemicellulose digestion in the termite gut. All sequences in this paper are available publicly with the accession numbers FL634956-FL640828 (Termite Gut library) and FL641015-FL645753 (Symbiont library).