Widespread occurrence of endogenous cellulase production and glycosyl hydrolase in grapsoid crabs along the land-sea transition indicates high potential for mineralisation of mangrove production

Abstract

Previous studies on mangrove carbon utilisation and mineralisation were focused on mangrove sesarmid crabs (Grapsoidea: Sesarmidae) because they are initial processors of mangrove organic carbon. Grapsoid crabs from other families and habitats were generally ignored although they also perform various levels of herbivory. It remains unclear how the herbivorous crabs utilise the production from mangrove or vascular plants because of the high C/N ratio and high structural carbon content of these materials. In this study, production of cellulase and expression of hepatopancreatic glycosyl hydrolase of grapsoid crabs were investigated in a more extensive way in terms of their taxa (15 species) and habitats (i.e., land margin, mangrove, mudflat, and subtidal area). Generally, low gastric endoglucanase (1.46-2.98 μmol min−1 ml−1) and β-glucosidase (0.06-0.61 μmol min−1 ml−1) activity was detected in 15 grapsoid crabs distributed along the land-sea transition. The activity of both enzymes was significantly affected by the taxonomic affiliation and habitat of the crabs: endoglucanase in sesarmids > varunids > grapsids; and land margin, mangrove and limnic habitats > mudflat and subtidal area. For β-glucosidase: sesarmids > varunids and grapsids; and land and limnic > mangrove and subtidal > mudflat. Our detection of gastric cellulases in mudflat and subtidal grapsoid crabs has rarely been reported. These results show that there is a need to re-evaluate the role of crabs from these habitats in mangrove/vascular plant carbon mineralisation. Further, low cellulase activity could be a common trait among crabs along the land-sea transition and the importance of cellulase in vascular plant carbon utilisation could have been previously over-rated. Glycosyl hydrolase family 9 (GH9) genes were identified in the hepatopancreatic transcriptome of all 15 species, indicating endogenous production of cellulase. In addition, the presence of other digestive carbohydrate-active enzymes (CAZymes), including GH5_10, GH13, GH13_24, and GH16, suggests that a capacity for the utilisation of carbon sources other than cellulose by the grapsoid crabs, which may enable sufficient assimilation of vascular plant carbon despite the low cellulase activity.

Metagenomic analysis of gut microbiome illuminates the mechanisms and evolution of lignocellulose degradation in mangrove herbivorous crabs

“…The detritivores enriched genes mainly related to hemicellulose and pectin degradation. The variations in CAZyme composition are also higher among the detritivorous species that partially reflect their diverse microhabitat preference (e.g., more muddy sediment that contains more organic matter versus sandy substratum that contains more animal meiofauna), and hence variations in organic matter content in diet, which is consistent with previous reports on higher cellulase activities in crab species having a greater plant biomass in diet [20,76]. This provides further support to our hypothesis that endosymbiotic cellulolytic microbes play an important role in lignocellulose degradation in most crab species but their abundance is strongly correlated with dietary preference, and thus enhancing their capacity for digestion of mangrove leaves and plant detritus.…”

“…Crustaceans can encode endogenous Carbohydrates Active enZymes (CAZymes) for lignocellulose degradation, which are hypothesized to be responsible for the degradation of algal cell wall, a major component in their omnivorous diet [15,16]. Transcriptomic and enzyme assay studies have confirmed the production of endogenous cellulolytic enzymes in herbivorous decapod species [17][18][19], including sesarmid crabs [20,21]. However, it remains largely unclear that if mangrove herbivorous crabs possess complete metabolic pathways to degrade lignocellulose, or if they require a symbiotic microbiome to complement the digestion process as the situation observed in terrestrial isopods [22] and termites [23][24][25][26], due to limited clues.…”

Comparative analysis of gut microbiome of mangrove brachyuran crabs revealed patterns of phylosymbiosis and codiversification

“…Similarly, members of Mycoplasmataceae and Vibrio are also known to possess cellulolytic genes (Fraune & Zimmer, 2008;Gao et al, 2010;Rasmussen et al, 2021). The host endogenous genomic toolkit in the mangrove herbivorous crabs (Lee & Lee, 2022;Miyake & Baba, 2022) together with the core cellulolytic microbiome appear to form a dual-lignocellulolytic system to achieve efficient digestion of plant materials and hence maximize the nutritional gain for both (Hu et al, 2013;Hui et al, 2024;Salem et al, 2020).…”

“…Hu et al, 2018;Newsome et al, 2020). Endogenous cellulolytic enzymes were found in mangrove crab transcriptome (Lee & Lee, 2022), but the study on the role of microbes is limited. Tongununui et al (2021) characterized the gut microbe composition of three mangrove-associated sesarmid species using 16S sequencing, and they detected cellulase and nitrogenase activities in the intestine.…”

“…Hu et al., 2018; Newsome et al., 2020). Endogenous cellulolytic enzymes were found in mangrove crab transcriptome (Lee & Lee, 2022), but the study on the role of microbes is limited. Tongununui et al.…”