Prokaryotic Diversity of the Composting Thermophilic Phase: the Case of Ground Coffee-Compost.

Waste biomass coming from a local coffee company, which supplied burnt ground coffee after an incorrect roasting process, was employed as a starting material in the compostingplant of the Experimental Station of the University of Naples Federico II at Castel Volturno (CE).The direct molecular characterization of compost using 13C-NMR spectra, which was acquiredthrough cross-polarization magic-angle spinning, showed a hydrophobicity index of 2.7% and analkyl/hydroxyalkyl index of 0.7%. Compost samples that were collected during the early "activethermophilic phase" (when the composting temperature was 63 oC) were analyzed for the prokaryotic community composition and activities. Two complementary approaches, i.e., genomic andpredictive metabolic analysis of the 16S rRNA V3-V4 amplicon and culture-dependent analysis,were combined to identify the main microbial factors that characterized the composting process. Thewhole microbial community was dominated by Firmicutes. The predictive analysis of the metabolicfunctionality of the community highlighted the potential degradation of peptidoglycan and the abilityof metal chelation, with both functions being extremely useful for the revitalization and fertilizationof agricultural soils. Finally, three biotechnologically relevant Firmicutes members, i.e., Geobacillusthermodenitrificans subsp. calidus, Aeribacillus pallidus, and Ureibacillus terrenus (strains CAF1, CAF2,and CAF5, respectively) were isolated from the "active thermophilic phase" of the coffee composting.All strains were thermophiles growing at the optimal temperature of 60 oC. Our findings contributeto the current knowledge on thermophilic composting microbiology and valorize burnt ground coffeeas waste material with biotechnological potentialities.