February 27, 2013 |
A termite’s own biology with help from microorganisms called protists, are keys to the insect’s digestion of woody material, according to a Purdue University scientist.
Michael Scharf, the O. Wayne Rollins/Orkin Endowed Chair in Urban Entomology, studies termite digestion to improve biofuels production and find better ways to control termites. The U.S. Department of Agriculture estimates the cost of controlling termites and repairing damaged homes is $2 billion each year in the United States.
Much of the study on how termites break down woody materials, which focused on the symbiotic relationship between the insect and the bacteria living in its gut, found that bacteria apparently have little, if anything, to do with termite digestion.
Scharf and collaborators at the University of Florida wanted to see how diet affected those bacteria. If the bacteria play a role in digestion, the type of materials the insect eats should affect the composition of the bacterial community living in the termite gut.
More than 4,500 different species of bacteria were cataloged in termite guts. When multiple colonies of termites were independently fed diets of wood or paper, however, those bacteria were unaffected.
“You would think diet would cause huge ecological shifts in bacterial communities, but it didn’t. We didn’t detect any statistical differences,” Scharf said.
What they did see were far more significant changes in gene expression in the termites and the protists that live in the insects’ guts along with the bacteria.
“The bacteria communities seem very stable, but the host and the protozoa gene expression are changing a lot based on diet,” Scharf said.
The scientists looked at 10,000 gene sequences from the termites and protists to determine which genes were expressed based on differing diets. Termites and protists fed woody and lignin-rich diets changed expression of about 500 genes, leading Scharf to believe those genes might be important for breaking down lignin, a rigid material in plant cell walls that isn’t easily broken down when making biofuels.
“We see much more of the playing field now,” Scharf said.
Understanding which genes are involved in digestion should help researchers track down the enzymes that actually break down woody materials in termite digestion. Those enzymes may be tools scientists could use to better break down biomass and extract sugars during biofuel production.
The National Science Foundation, the Consortium for Plant Biotechnology Inc. and the U.S. Department of Energy funded the research.
The findings were detailed in three papers published in the journals Molecular Ecology, Insect Molecular Biology, and Insect Biochemistry and Molecular Biology.
The Hindgut Lumen Prokaryotic Microbiota of the Termite Reticulitermes Flavipes and its Responses to Dietary Lignocellulose Composition
Drion G. Boucias, Yunpeng Cai, Yijun Sun, Verena-Ulrike Lietze, Ruchira Sen, Rhitoban Raychoudhury, Michael E. Scharf
Reticulitermes flavipes (Isoptera: Rhinotermitidae) is a highly eusocial insect that thrives on recalcitrant lignocellulosic diets through nutritional symbioses with gut-dwelling prokaryotes and eukaryotes. In the R. flavipes hindgut, there are up to 12 eukaryotic protozoan symbionts; the number of prokaryotic symbionts has been estimated in the hundreds. Despite its biological relevance, this diverse community, to date, has been investigated only by culture- and cloning-dependent methods. Moreover, it is unclear how termite gut microbiomes respond to diet changes and what roles they play in lignocellulose digestion. This study utilized high-throughput 454 pyrosequencing of 16S V5-V6 amplicons to sample the hindgut lumen prokaryotic microbiota of R. flavipes and to examine compositional changes in response to lignin-rich and lignin-poor cellulose diets after a 7-day feeding period. Of the ~475,000 high-quality reads that were obtained, 99.9% were annotated as bacteria and 0.11% as archaea. Major bacterial phyla included Spirochaetes (24.9%), Elusimicrobia (19.8%), Firmicutes (17.8%), Bacteroidetes (14.1%), Proteobacteria (11.4%), Fibrobacteres (5.8%), Verrucomicrobia (2.0%), Actinobacteria (1.4%) and Tenericutes (1.3%). The R. flavipeshindgut lumen prokaryotic microbiota was found to contain over 4,761 species-level phylotypes. However, diet-dependent shifts were not statistically significant or uniform across colonies, suggesting significant environmental and/or host genetic impacts on colony-level microbiome composition. These results provide insights into termite gut microbiome diversity and suggest that (i) the prokaryotic gut microbiota is much more complex than previously estimated, and (ii) environment, founding reproductive pair effects and/or host genetics influence microbiome composition.
Comparative Metatranscriptomic Signatures of Wood and Paper Feeding in the Gut of the Termite Reticulitermes flavipes (Isoptera: Rhinotermitidae)
R. Raychoudhury, R. Sen, Y. Cai, Y. Sun, V-U Lietze, D. G. Boucias, M.E. Scharf
Termites are highly eusocial insects that thrive on recalcitrant materials like wood and soil and thus play important roles in global carbon recycling and also in damaging wooden structures. Termites, such as Reticulitermes flavipes (Rhinotermitidae), owe their success to their ability to extract nutrients from lignocellulose (a major component of wood) with the help of gut-dwelling symbionts. With the aim to gain new insights into this enzymatic process we providedR. flavipes with a complex lignocellulose (wood) or pure cellulose (paper) diet and followed the resulting differential gene expression on a custom oligonucleotide-microarray platform. We identified a set of expressed sequence tags (ESTs) with differential abundance between the two diet treatments and demonstrated the source (host/symbiont) of these genes, providing novel information on termite nutritional symbiosis. Our results reveal: (1) the majority of responsive wood- and paper-abundant ESTs are from host and symbionts, respectively; (2) distinct pathways are associated with lignocellulose and cellulose feeding in both host and symbionts; and (3) sets of diet-responsive ESTs encode putative digestive and wood-related detoxification enzymes. Thus, this study illuminates the dynamics of termite nutritional symbiosis and reveals a pool of genes as potential targets for termite control and functional studies of termite-symbiont interactions.
Lignin-Associated Metagene Expression in a Lignocellulose-Digesting Termite
Amit Sethi, Jeffrey M. Slack, Elena S. Kovaleva, George W. Buchman, Michael E. Scharf
Lignin is a component of plant biomass that presents a significant obstacle to biofuel production. It is composed of a highly stable phenylpropanoid matrix that, upon degradation, releases toxic metabolites. Termites have specialized digestive systems that overcome the lignin barrier in wood lignocellulose to efficiently release fermentable simple sugars; however, how termites specifically degrade lignin and tolerate its toxic byproducts remains unknown. Here, using the termite Reticulitermes flavipes and its symbiotic (protozoan) gut fauna as a model system, we used high throughput Roche 454-titanium pyrosequencing and proteomics approaches to (i) experimentally compare the effects of diets containing varying degrees of lignin complexity on host-symbiont digestome composition, (ii) deeply sample host and symbiont lignocellulase diversity, and (iii) identify promising lignocellulase candidates for functional characterization. In addition to revealing over 9,500 differentially expressed transcripts related to a wide range of physiological processes, our findings reveal two detoxification enzyme families not generally considered in connection with lignocellulose digestion: aldo-keto reductases and catalases. Recombinant versions of two host enzymes from these enzyme families, which apparently play no roles in cellulose or hemicellulose digestion, significantly enhance lignocellulose saccharification by cocktails of host and symbiont cellulases. These hypothesis-driven results provide important new insights into (i) dietary lignin as a xenobiotic challenge, (ii) the complex mechanisms used by termites to cope with their lignin-rich diets, and (iii) novel lignin-targeted enzymatic approaches to enhance biofuel and biomaterial production.