Eco-waste of Posidonia oceanica as a reservoir for halophilic cellulolytic bacteria with potentialities for plant growth promotion and waste bioconversion

Marine vascular plants such as Posidonia oceanica form symbiotic associations with microbiota to enhance nutrient acquisition, supporting plant growth and fitness. The beneficial symbiotic bacteria can theoretically persist in Posidonia oceanica leaves and balls deposed on sandy beaches as marine cellulosic wastes, opening up new opportunities for their valorization in biotechnological processes. The objective of this work was to characterize the bacterial communities associated with P. oceanica waste (POW) samples collected from four sandy beaches in northeastern Tunisia. Analysis of metataxonomic datasets showed high bacterial diversity and richness in POW samples with a prevalence of bacteria belonging to the phyla proteobacteria, firmicutes, and actinobacteria. Analysis of alpha and beta diversity metrics also indicated that POW samples show distinct bacterial profiles and abundances at taxa levels from species to phylum. Combined metataxonomic and chemical data revealed that low bacterial diversity correlated with low contents of cellulose and hemicellulose in POW samples, maybe indicating stressed microbial communities. Both culture-dependent and culture-independent techniques showed that POW harbored halophilic bacteria with tremendous potential for carbohydrate-active enzymes (CAZymes) production (cellulases, xylanases, laccases, peroxidases, etc.) and plant growth-promoting activities such as enhancement of nutrients supply/availability and resistance to salt stress by accumulating osmolytes. The selected enzyme-producing halotolerant PGPB, Nocardiopsis synnemataformans PT2, and Halobacillus trueperi PHm3 have been successfully applied in the bioconversion of POW into bioethanol. A maximum bioethanol yield of 41.17