Molecular insight into lignocellulose digestion by a marine isopod in the absence of gut microbes

Abstract

The digestion of lignocellulose is attracting attention both in terms of basic research into its metabolism by microorganisms and animals, and also as a means of converting plant biomass into biofuels. Limnoriid wood borers are unusual because, unlike other wood-feeding animals, they do not rely on symbiotic microbes to help digest lignocellulose. The absence of microbes in the digestive tract suggests that limnoriid wood borers produce all the enzymes necessary for lignocellulose digestion themselves. In this study we report that analysis of ESTs from the digestive system of Limnoria quadripunctata reveals a transcriptome dominated by glycosyl hydrolase genes. Indeed, >20% of all ESTs represent genes encoding putative cellulases, including glycosyl hydrolase family 7 (GH7) cellobiohydrolases. These have not previously been reported in animal genomes, but are key digestive enzymes produced by wood-degrading fungi and symbiotic protists in termite guts. We propose that limnoriid GH7 genes are important for the efficient digestion of lignocellulose in the absence of gut microbes. Hemocyanin transcripts were highly abundant in the hepatopancreas transcriptome. Based on recent studies indicating that these proteins may function as phenoloxidases in isopods, we discuss a possible role for hemocyanins in lignin decomposition. biofuels | cellulase | hemocyanin | phenoloxidase | wood degradation

Structural characterization of a unique marine animal family 7 cellobiohydrolase suggests a mechanism of cellulase salt tolerance

“…1B), indicating that LqCel7B functions in a high-solids loading environment. These studies not only provide direct evidence that LqCel7B gene expression is confined to the hepatopancreas, as previously suggested by in situ hybridization (13), but that the protein is subsequently translocated into the hindgut.…”

“…The L. quadripunctata transcriptome was dominated by a few GH families, with transcripts representing GH7s being particularly abundant, representing ∼12% of total ESTs sequenced. This was unexpected as GH7s were previously only reported from fungi and symbiont protists from termites, but not from animals (13).…”

“…branch from fungal and protist equivalents in phylogenetic analysis of protein sequences (13). The lack of a CBM is intriguing and together with the high abundance of LqCel7B in the gut lumen (Fig.…”

“…More recently, the hydrolytic capacity of the wood-boring crustacean Limnoria quadripunctata, which thrives on a diet of wood, was revealed. Unusually, Limnoria do not rely on gut microbes for wood digestion, but instead have a digestive system free of microbial populations, and digest lignocellulose using endogenous enzymes (13). At least three classes of GHs are necessary to deconstruct cellulose (7), and analysis of a cDNA library constructed from the L. quadripunctata digestive gland revealed candidate transcripts that putatively represent all three classes of hydrolases, namely cellobiohydrolases, endoglucanases, and β-glucosidases.…”

Efficient Cleavage of Lignin–Carbohydrate Complexes and Ultrafast Extraction of Lignin Oligomers from Wood Biomass by Microwave‐Assisted Treatment with Deep Eutectic Solvent

“…Biomass recalcitrance generally arises from lignin–carbohydrate complexes (LCCs), in which lignin is mainly covalently bonded to hemicellulose 10, 11. The traditional methods to break the LCC structure involve physical, chemical, physicochemical, and biological pretreatments such as steam explosion, ammonia fiber expansion, acid‐catalyzed prehydrolysis, and microorganism digestion 12, 13, 14, 15. Besides, organosolv and ionic liquid pretreatments are recognized as efficient methods for fractionation of lignocellulose components following the concepts of green chemistry and biorefinery 16, 17, 18, 19.…”