Screening of Phytophagous and Xylophagous Insects Guts Microbiota Abilities to Degrade Lignocellulose in Bioreactor

Responses of physiological, microbiome and lipid metabolism to lignocellulose wastes in gut of yellow mealworm (Tenebrio molitor)

There is limited research on physiological and degradation mechanisms of yellow mealworm, a novel organic waste converter, in processing lignocellulosic wastes. This study has selected two types of lignocellulosic wastes, distillers' grains (DG) and maize straw (MS), to feed yellow mealworms. This study investigated the effects of lignocellulosic wastes on the growth, antioxidant system, microbiome, and lipidome of yellow mealworms. The relative growth of lignocellulosic waste group was not significantly different from wheat bran. The antioxidant level was elevated in DG. MS was significantly enriched in cellulose-degrading bacteria in the gut and was accompanied by disturbances in lipid metabolism. The correlation coefficients were used to construct a network connecting diet, microbiota, and lipids. The correlation analysis indicated that two sphingolipids, hexylglyceramide and dihydroglyceramide, were strongly and positively linked with the dominating species. This study provides comprehensive information on physiological and mechanism of mealworms in process of treating lignocellulosic waste.

The role of insect gut microbiota in host fitness, detoxification and nutrient supplementation

Insects are incredibly diverse, ubiquitous and have successfully flourished out of the dynamic and often unpredictable nature of evolutionary processes. The resident microbiome has accompanied the physical and biological adaptations that enable their continued survival and proliferation in a wide array of environments. The host insect and microbiome's bidirectional relationship exhibits their capability to influence each other's physiology, behavior and characteristics. Insects are reported to rely directly on the microbial community to break down complex food, adapt to nutrient-deficit environments, protect themselves from natural adversaries and control the expression of social behavior. High-throughput metagenomic approaches have enhanced the potential for determining the abundance, composition, diversity and functional activities of microbial fauna associated with insect hosts, enabling in-depth investigation into insect-microbe interactions. We undertook a review of some of the major advances in the field of metagenomics, focusing on insect-microbe interaction, diversity and composition of resident microbiota, the functional capability of endosymbionts and discussions on different symbiotic relationships. The review aims to be a valuable resource on insect gut symbiotic microbiota by providing a comprehensive understanding of how insect gut symbionts systematically perform a range of functions, viz., insecticide degradation, nutritional support and immune fitness. A thorough understanding of manipulating specific gut symbionts may aid in developing advanced insect-associated research to attain health and design strategies for pest management.

Exploring Gut Microbiome Variations between Popillia japonica Populations of Azores

Popillia japonica (Coleoptera: Scarabaeidae), is an emerging invasive pest in Europe and America. In the Azores, this pest was first found on Terceira Island during the sixties and soon spread to other islands. The rate of infestation differs between islands, and we hypothesized that microbiome composition could play a role. Therefore, we sampled 3rd instar larvae and soil from sites with high and low infestation rates to analyze the microbiome using next-generation sequencing. We analyzed twenty-four 16S DNA libraries, which resulted in 3278 operational taxonomic units. The alpha and beta diversity of the soil microbiome was similar between sites. In contrast, the larvae from high-density sites presented a higher bacterial gut diversity than larvae from low-density sites, with biomarkers linked to plant digestion, nutrient acquisition, and detoxification. Consequently, larvae from high-density sites displayed several enriched molecular functions associated with the families Ruminococcaceae, Clostridiaceae and Rikenellaceae. These bacteria revealed a supportive function by producing several CAZyme families and other proteins. These findings suggest that the microbiome must be one drive for the increase in P. japonica populations, thus providing a checkpoint in the establishment and spread of this pest.

A Metagenomic Based Approach on Abundance and Diversity of Bacterial Communities Across the Life Stages of Culicoides peregrinus (Diptera: Ceratopogonidae) a Vector of Bluetongue Virus

During larval rearing of Culicoides peregrinus Kieffer (Diptera: Ceratopogonidae) it was obligatory to add a small quantity of mud from larval habitat to nutrient broth in culture plates. This initiated microbial growth in rearing plates which facilitated growth and development of immature. The primary aim was to enumerate gut microbial communities across the different life stages of C. peregrinus. Amplicon sequencing of the V3-V4 hypervariable region (16S rDNA) was done on Illumina Miseq platform to detect gut bacterial communities at different life stages, while ITS regions (18S rRNA) were targeted for fungal communities of the 4th instar larvae. The major findings were: 1) Phylum Proteobacteria and Firmicutes were the most abundant throughout the life stages, along with the highest bacterial alpha diversity in the egg, 2) bacterial compositions were similar to laboratory reared and field collected adults, and 3) abundant fungal phyla associated with the larval gut were Ascomycota and Basidiomycota. Furthermore, analyses of the gut microbiome with METAGENassist might be indicative of their likely function in the natural habitat. Abundant gut-associated bacteria and/or fungal genera detected in the present study could be used as dietary supplements to establish laboratory colonies for further vectorial research. While, individual roles of the bacteria or fungi in paratransgenesis are warned for their possible utilization to frame the management strategy in upcoming works.

Effect of Alkaline and Mechanical Pretreatment of Wheat Straw on Enrichment Cultures from Pachnoda marginata Larva Gut

In order to partially mimic the efficient lignocellulose pretreatment process performed naturally in the gut system of Pachnoda marginata larvae, two wheat straw pretreatments were evaluated: a mechanical pretreatment via cutting the straw into two different sizes and an alkaline pretreatment with calcium hydroxide. After pretreatment, gut enrichment cultures on wheat straw at alkaline pH were inoculated and kept at mesophilic conditions over 45 days. The methanogenic community was composed mainly of the Methanomicrobiaceae and Methanosarcinaceae families. The combined pretreatment, size reduction and alkaline pretreatment, was the best condition for methane production. The positive effect of the straw pretreatment was higher in the midgut cultures, increasing the methane production by 192%, while for hindgut cultures the methane production increased only by 149% when compared to non-pretreated straw. Scanning electron microscopy (SEM) showed that the alkaline pretreatment modified the surface of the wheat straw fibers, which promoted biofilm formation and microbial growth. The enrichment cultures derived from larva gut microbiome were able to degrade larger 1 mm alkaline treated and smaller 250 µm but non-pretreated straw at the same efficiency. The combination of mechanical and alkaline pretreatments resulted in increased, yet not superimposed, methane yield.

Dietary Association with Midgut Microbiota Components of Eocanthecona furcellata (Wolff)

Eocanthecona furcellata is an important predatory stinkbug that attacks many lepidopteran pests. For mass-rearing, artificial diets are used to rear this predator in the laboratory; however, the fitness of the predators is reduced, and little is known about the cause. Since gut microbiota plays vital roles in the digestion and development of many hosts and can consequently affect host fitness, an understanding of the microbial community composition of E. furcellata may help to solve this unresolved problem. We compared the development and reproduction of E. furcellata reared on an artificial diet, and a natural (Spodoptera litura) or semi-natural (Tenebrio molitor) diet, and then the midgut microbiota were assessed using high-throughput 16S rRNA. The results of the high-throughput 16S rRNA show that the bacterial richness and diversity in the artificial diet gut samples increased considerably compared with the other samples. Proteobacteria and Firmicutes were the dominant phyla in E. furcellata. At the genus level, Serratia (however, the relative abundance was lower in the artificial diet gut samples), Enterococcus, and an uncultured bacterium genus of family Enterobacteriaceae, were dominant. The midgut microbiota components significantly differed among the diets, indicating that the gut bacteria had a dietary association with E. furcellata. This study provides a better understanding of midgut microbiota and the artificial diets that might affect them in E. furcellata.

Bitter friends are not always toxic: The loss of acetic acid bacteria and the absence of Komagataeibacter in the gut microbiota of the polyphagous fly Anastrepha ludens could inhibit its development in Psidium guajava in contrast to A. striata and A. fraterculus that flourish in this host

The gut microbiota is key for the homeostasis of many phytophagous insects, but there are few studies comparing its role on host use by stenophagous or polyphagous frugivores. Guava (Psidium guajava) is a fruit infested in nature by the tephritids Anastrepha striata and A. fraterculus. In contrast, the extremely polyphagous A. ludens infests guava only under artificial conditions, but unlike A. striata and the Mexican A. fraterculus, it infests bitter oranges (Citrus x aurantium). We used these models to analyze whether the gut microbiota could explain the differences in host use observed in these flies. We compared the gut microbiota of the larvae of the three species when they developed in guava and the microbiota of the fruit pulp larvae fed on. We also compared the gut microbiota of A. ludens developing in C. x aurantium with the pulp microbiota of this widely used host. The three flies modified the composition of the host pulp microbiota (i.e., pulp the larvae fed on). We observed a depletion of Acetic Acid Bacteria (AAB) associated with a deleterious phenotype in A. ludens when infesting P. guajava. In contrast, the ability of A. striata and A. fraterculus to infest this fruit is likely associated to a symbiotic interaction with species of the Komagataeibacter genus, which are known to degrade a wide spectrum of tannins and polyphenols. The three flies establish genera specific symbiotic associations with AABs. In the case of A. ludens, the association is with Gluconobacter and Acetobacter, but importantly, it cannot be colonized by Komagataeibacter, a factor likely inhibiting its development in guava.

Biodegradation of Gramineous Lignocellulose by Locusta migratoria manilensis (Orthoptera: Acridoidea)

Exploring an efficient and green pretreatment method is an important prerequisite for the development of biorefinery. It is well known that locusts can degrade gramineous lignocellulose efficiently. Locusts can be used as a potential resource for studying plant cell wall degradation, but there are few relative studies about locusts so far. Herein, some new discoveries were revealed about elucidating the process of biodegradation of gramineous lignocellulose in Locusta migratoria manilensis. The enzyme activity related to lignocellulose degradation and the content of cellulose, hemicellulose, and lignin in the different gut segments of locusts fed corn leaves were measured in this study. A series of characterization analyses were conducted on corn leaves and locust feces, which included field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction pattern (XRD), and thermogravimetric (TG) analysis. These results showed that the highest activities of carboxymethyl cellulase (CMCase), filter paper cellulase (FPA), and xylanase were obtained in the foregut of locusts, which strongly indicated that the foregut was the main lignocellulose degradation segment in locusts; furthermore, the majority of nutritional components were absorbed in the midgut of locusts. The activity of CMCase was significantly higher than that of xylanase, and manganese peroxidase (MnPase) activity was lowest, which might be due to the basic nutrition of locusts being cellulose and hemicellulose and not lignin based on the results of FE-SEM, FTIR, XRD, and TG analysis. Overall, these results provided a valuable insight into lignocellulosic degradation mechanisms for understanding gramineous plant cell wall deconstruction and recalcitrance in locusts, which could be useful in the development of new enzymatic pretreatment processes mimicking the locust digestive system for the biochemical conversion of lignocellulosic biomass to fuels and chemicals.

A chromosome-level genome assembly and intestinal transcriptome of Trypoxylus dichotomus (Coleoptera: Scarabaeidae) to understand its lignocellulose digestion ability

Lignocellulose, as the key structural component of plant biomass, is a recalcitrant structure, difficult to degrade. The traditional management of plant waste, including landfill and incineration, usually causes serious environmental pollution and health problems. Interestingly, the xylophagous beetle, Trypoxylus dichotomus, can decompose lignocellulosic biomass. However, the genomics around the digestion mechanism of this beetle remain to be elucidated. Here, we assembled the genome of T. dichotomus, showing that the draft genome size of T. dichotomus is 636.27 Mb, with 95.37% scaffolds anchored onto 10 chromosomes. Phylogenetic results indicated that a divergent evolution between the ancestors of T. dichotomus and the closely related scarabaeid species Onthophagus taurus occurred in the early Cretaceous (120 million years ago). Through gene family evolution analysis, we found 67 rapidly evolving gene families, within which there were 2 digestive gene families (encoding Trypsin and Enoyl-(Acyl carrier protein) reductase) that have experienced significant expansion, indicating that they may contribute to the high degradation efficiency of lignocellulose in T. dichotomus. Additionally, events of chromosome breakage and rearrangement were observed by synteny analysis during the evolution of T. dichotomus due to chromosomes 6 and 8 of T. dichotomus being intersected with chromosomes 2 and 10 of Tribolium castaneum, respectively. Furthermore, the comparative transcriptome analyses of larval guts showed that the digestion-related genes were more commonly expressed in the midgut or mushroom residue group than the hindgut or sawdust group. This study reports the well-assembled and annotated genome of T. dichotomus, providing genomic and transcriptomic bases for further understanding the functional and evolutionary mechanisms of lignocellulose digestion in T. dichotomus.

Lessons From Insect Fungiculture: From Microbial Ecology to Plastics Degradation

Anthropogenic activities have extensively transformed the biosphere by extracting and disposing of resources, crossing boundaries of planetary threat while causing a global crisis of waste overload. Despite fundamental differences regarding structure and recalcitrance, lignocellulose and plastic polymers share physical-chemical properties to some extent, that include carbon skeletons with similar chemical bonds, hydrophobic properties, amorphous and crystalline regions. Microbial strategies for metabolizing recalcitrant polymers have been selected and optimized through evolution, thus understanding natural processes for lignocellulose modification could aid the challenge of dealing with the recalcitrant human-made polymers spread worldwide. We propose to look for inspiration in the charismatic fungal-growing insects to understand multipartite degradation of plant polymers. Independently evolved in diverse insect lineages, fungiculture embraces passive or active fungal cultivation for food, protection, and structural purposes. We consider there is much to learn from these symbioses, in special from the community-level degradation of recalcitrant biomass and defensive metabolites. Microbial plant-degrading systems at the core of insect fungicultures could be promising candidates for degrading synthetic plastics. Here, we first compare the degradation of lignocellulose and plastic polymers, with emphasis in the overlapping microbial players and enzymatic activities between these processes. Second, we review the literature on diverse insect fungiculture systems, focusing on features that, while supporting insects' ecology and evolution, could also be applied in biotechnological processes. Third, taking lessons from these microbial communities, we suggest multidisciplinary strategies to identify microbial degraders, degrading enzymes and pathways, as well as microbial interactions and interdependencies. Spanning from multiomics to spectroscopy, microscopy, stable isotopes probing, enrichment microcosmos, and synthetic communities, these strategies would allow for a systemic understanding of the fungiculture ecology, driving to application possibilities. Detailing how the metabolic landscape is entangled to achieve ecological success could inspire sustainable efforts for mitigating the current environmental crisis.

Enrichment of Anaerobic Microbial Communities from Midgut and Hindgut of Sun Beetle Larvae (Pachnoda marginata) on Wheat Straw: Effect of Inoculum Preparation

The Pachnoda marginata larva have complex gut microbiota capable of the effective conversion of lignocellulosic biomass. Biotechnological utilization of these microorganisms in an engineered system can be achieved by establishing enrichment cultures using a lignocellulosic substrate. We established enrichment cultures from contents of the midgut and hindgut of the beetle larva using wheat straw in an alkaline medium at mesophilic conditions. Two different inoculation preparations were used: procedure 1 (P1) was performed in a sterile bench under oxic conditions using 0.4% inoculum and small gauge needles. Procedure 2 (P2) was carried out under anoxic conditions using more inoculum (4%) and bigger gauge needles. Higher methane production was achieved with P2, while the highest acetic acid concentrations were observed with P1. In the enrichment cultures, the most abundant bacterial families were Dysgonomonadaceae, Heliobacteriaceae, Ruminococcaceae, and Marinilabiliaceae. Further, the most abundant methanogenic genera were Methanobrevibacter, Methanoculleus, and Methanosarcina. Our observations suggest that in samples processed with P1, the volatile fatty acids were not completely converted to methane. This is supported by the finding that enrichment cultures obtained with P2 included acetoclastic methanogens, which might have prevented the accumulation of acetic acid. We conclude that differences in the inoculum preparation may have a major influence on the outcome of enrichment cultures from the P. marginata larvae gut.

Arthropod-Microbiota Integration: Its Importance for Ecosystem Conservation

Recent reports indicate that the health of our planet is getting worse and that genuine transformative changes are pressing. So far, efforts to ameliorate Earth's ecosystem crises have been insufficient, as these often depart from current knowledge of the underlying ecological processes. Nowadays, biodiversity loss and the alterations in biogeochemical cycles are reaching thresholds that put the survival of our species at risk. Biological interactions are fundamental for achieving biological conservation and restoration of ecological processes, especially those that contribute to nutrient cycles. Microorganism are recognized as key players in ecological interactions and nutrient cycling, both free-living and in symbiotic associations with multicellular organisms. This latter assemblage work as a functional ecological unit called "holobiont." Here, we review the emergent ecosystem properties derived from holobionts, with special emphasis on detritivorous terrestrial arthropods and their symbiotic microorganisms. We revisit their relevance in the cycling of recalcitrant organic compounds (e.g., lignin and cellulose). Finally, based on the interconnection between biodiversity and nutrient cycling, we propose that a multicellular organism and its associates constitute an Ecosystem Holobiont (EH). This EH is the functional unit characterized by carrying out key ecosystem processes. We emphasize that in order to meet the challenge to restore the health of our planet it is critical to reduce anthropic pressures that may threaten not only individual entities (known as "bionts") but also the stability of the associations that give rise to EH and their ecological functions.

Novel Synergistic Mechanism for Lignocellulose Degradation by a Thermophilic Filamentous Fungus and a Thermophilic Actinobacterium Based on Functional Proteomics

Effective artificial microbial consortia containing microorganisms with desired biological functions have the potential to optimize the lignocellulose-based bioindustry. Thermobifida fusca was a dominant actinobacterium in high-temperature corn stalk composts, but it was unable to grow alone in corn stalk solid medium. Interestingly, T. fusca showed good growth and secreted enzymes when cocultured with Thermomyces lanuginosus. T. lanuginosus grew firstly during the initial stage, whereas T. fusca dominated the system subsequently during cocultivation. The secretome indicated that T. lanuginosus mainly degraded xylan by expressing a GH11 xylanase (g4601.t1, GenBank AAB94633.1; with relative secretion of 4.95 ± 0.65%). T. fusca was induced by xylan mainly to secrete a xylanase from GH11 family (W8GGR4, GenBank AHK22788.1; with relative secretion of 8.71 ± 3.83%) which could rapidly degrade xylan to xylo-oligosaccharide (XOS) and xylose within 2 min, while high concentrations (>0.5%, w/v) of XOS or xylose suppressed the growth of T. fusca; which may be the reason why T. fusca unable to grow alone in corn stalk solid medium. However, T. lanuginosus could utilize the XOS and xylose produced by xylanases secreted by T. fusca. During the synergistic degradation of lignocellulose by T. lanuginosus and T. fusca, xylan was rapidly consumed by T. lanuginosus, the residual cellulose could specifically induced T. fusca to express a GH10 xylanase with a CBM2 domain (Q47KR6, GenBank AAZ56956.1; with relative secretion of 5.03 ± 1.33%) and 6 cellulases (2 exocellulases and 4 endocellulases). Moreover, T. lanuginosus increased the secretion of cellulases from T. fusca by 19-25%. The order of T. lanuginosus and T. fusca was consistent with the multilayered structures of lignocellulose and could be regulated by different concentrations of XOS and xylose. The novel synergism of T. lanuginosus and T. fusca gave a new sight for revealing more synergetic relationships in natural environments and exploring efficient microbial inoculants and enzyme cocktails for lignocellulose degradation.

An Insight into Diversity and Functionalities of Gut Microbiota in Insects

The gut microbiota has long been of research interests due to its nutritional importance for many insects. It has been demonstrated that diversity of gut microbiota in insects can be modulated by many factors, including habitats, feeding preference, etc. Besides, the community structure of gut microbiota could also be altered during the different life stages of host insects. With development of conventional culture-dependent technologies and advanced culture-independent technologies, comprehensive and deep understanding of the functions of gut microbiota and their relationship with host insects were achieved, especially for the nutrient metabolic process mediated by them. In this review, we summarized the gut microbiota composition, major methods for gut microbiota characterization, and vital nutrient metabolic process mediated by gut microbiota in different insects. The increasing knowledge on the modulation of gut microbiota will help us for the comprehension of the contribution of gut microbiota to the nutritional metabolism of insects, prompting their growth and health.