Characterisation of the enzyme transport path between shipworms and their bacterial symbionts

Simon J Mcqueen-Mason,Simon M Cragg,Clare Steele-King,Luisa Elias,Paul Dupree,Federico Sabbadin,Giovanna Pesante,Katrin Besser,Marta Busse-Wicher,Yi Li,J Reuben Shipway,Adam A Dowle,Neil C Bruce

doi:10.1186/s12915-021-01162-6

Simon J Mcqueen-Mason, Simon M Cragg + Show 11 more

Open Access

https://doi.org/10.1186/s12915-021-01162-6

Copy DOI

Abstract

BackgroundShipworms are marine xylophagus bivalve molluscs, which can live on a diet solely of wood due to their ability to produce plant cell wall-degrading enzymes. Bacterial carbohydrate-active enzymes (CAZymes), synthesised by endosymbionts living in specialised shipworm cells called bacteriocytes and located in the animal’s gills, play an important role in wood digestion in shipworms. However, the main site of lignocellulose digestion within these wood-boring molluscs, which contains both endogenous lignocellulolytic enzymes and prokaryotic enzymes, is the caecum, and the mechanism by which bacterial enzymes reach the distant caecum lumen has remained so far mysterious. Here, we provide a characterisation of the path through which bacterial CAZymes produced in the gills of the shipworm Lyrodus pedicellatus reach the distant caecum to contribute to the digestion of wood.ResultsThrough a combination of transcriptomics, proteomics, X-ray microtomography, electron microscopy studies and in vitro biochemical characterisation, we show that wood-digesting enzymes produced by symbiotic bacteria are localised not only in the gills, but also in the lumen of the food groove, a stream of mucus secreted by gill cells that carries food particles trapped by filter feeding to the mouth. Bacterial CAZymes are also present in the crystalline style and in the caecum of their shipworm host, suggesting a unique pathway by which enzymes involved in a symbiotic interaction are transported to their site of action. Finally, we characterise in vitro four new bacterial glycosyl hydrolases and a lytic polysaccharide monooxygenase identified in our transcriptomic and proteomic analyses as some of the major bacterial enzymes involved in this unusual biological system.ConclusionBased on our data, we propose that bacteria and their enzymes are transported from the gills along the food groove to the shipworm’s mouth and digestive tract, where they aid in wood digestion.

Highlights

Shipworms are marine xylophagus bivalve molluscs, which can live on a diet solely of wood due to their ability to produce plant cell wall-degrading enzymes
The caecum is the main site of wood digestion and contains a large number of carbohydrate-active enzymes (CAZymes) of both endogenous and bacterial origin [21] despite harbouring few bacteria, belonging to ribotypes different from those found in the gills [22]
Micro-CT analysis and scanning electron microscopy In order to explore the internal anatomy of L. pedicellatus, a high-resolution three-dimensional rendering of an adult specimen was created using micro-CT (Fig. 3), of an animal extracted from the wood

Summary

Introduction

Shipworms are marine xylophagus bivalve molluscs, which can live on a diet solely of wood due to their ability to produce plant cell wall-degrading enzymes. Bacteria and fungi are well known for their ability to digest lignocellulosic materials, and many terrestrial and marine invertebrates (including species of insects, nematodes, molluscs and crustaceans) are able to produce endogenous cellulases In most of these invertebrates, the digestion of woody biomass is made more efficient by the presence of bacterial or fungal symbionts, with only the isopods Limnoria ssp. The ability of shipworms to feed on lignocellulose is dependent on the presence of endosymbiotic bacteria that live in the animal’s gills in specialised eukaryotic cells called bacteriocytes [11, 12] These bacteria provide the animal with fixed nitrogen to supplement its diet [13,14,15], as well as hydrolytic enzymes to help wood digestion and secondary metabolites such as antibiotics [16,17,18,19,20]. Despite numerous later studies, no further evidence supporting the existence of this “duct of Deshayes” has been published [26], suggesting that a yet unidentified mechanism may be responsible for the translocation of enzymes from the gills to the caecum

Methods

Results

Conclusion