Molecular Analyses of Bacterial Community Dynamics During the Static Composting of Olive Mill Waste

Static composting of olive mill waste (OMW) can be regarded as an easy and convenient way for the disposal and valorisation of such agroindustrial by-product. We investigated the bacterial community dynamics during OMW composting by applying a culture-independent molecular approach based on denaturing gradient gel electrophoresis (DGGE) profiling and Illumina next generation sequencing (NGS)of amplified 16S rRNA genes.DGGE analysis indicated the presence of a very high bacterial diversity in both the OMW and the composting mixture and the occurrence of a rapid succession of different bacterial populations throughout the process. Indeed, only few populations were shared among all the samples, while most seemed peculiar of the different phases of biotransformation of OMW into mature compost. NGS analysis allowed the identification of these populations. Actinobacteriawere abundant in the OMW, declined at the beginning of the active/thermophilic phase and then represented the predominant populations during the curing/mesophilic phase. Interestingly, while in the OMW most Actinobacteria belonged to the family Microbacteriaceae, the compost in the curing phase featured a high abundance of Pseudonocardiaceae, mostly represented by the moderately halophilic Prauserella genus. As expected, the most abundant populations found in the active phase, when the temperature reached 60°C, were the Bacilli, particularly those belonging to the thermophilic, spore-forming Geobacillus genus. Finally, the compost in the maturing phase was mainly characterized by an even distribution of bacteria belonging to the orders Actinomycetales, Sphingobacteriales, Burkholderiales, Xanthomonadales and Rhizobiales.