Valorisation of olive oil waste material by microbial multispecies biocatalytic aggregates

Increased demands for olive oil production result in more than 30mio m3 of lignocellulosic wastes per year in the Mediterranean basin, calling for sustainable management of solid olive mill residues such as olive pomace (OP). The phytotoxic nature of fresh OP, caused by the high concentrations of polyphenols, lipids and organic acids, can be reduced by two distinctive approaches: either by burning the waste or by valorising it to meet the objectives of sustainability and circularity. Valorization can be achieved at two technical levels: (i) by composting aiming to produce organic fertilizers or (ii) by biotransformation aiming to produce added value compounds that can be used in polymer, food and pharmaceutical industries. One of the main problems of the highly organic OP material is that it is naturally only slowly biotransformed and biodegraded due to low nitrogen content, recalcitrant substances such as lignocellulose and antimicrobial compounds e.g. fatty acids and polyphenols. To overcome the problems and to increase the effectiveness of OP biodegradation and biotransformation we need to introduce the appropriate bacterial strains, either a single generalist strain that incorporates all the metabolic pathways needed to degrade the compounds found in pomace, or several specialist strains, which collectively act as a superorganism by contributing individual steps of the metabolic pathways involved in degradation. Based on the chemical composition of OP it is almost impossible to find a generalist strain that would be able to degrade such diverse substrates and survive also the high C:N ratio, low pH and the presence of antimicrobial substances. The only solution is, therefore, to use a synthetic consortium, composed of a set of different specialist strains, which highlights the second biggest problem as to how this consortium should be built. As it is known from different reports, it is important how various strains are distributed in space to have collaborative action on the degradation of the substrate. As the spatially ordered synthetic (SOS) consortia can be prepared by attaching the bacterial cells to each other, we are going to implement our innovative cell modification approach based on the changing of their surface charge. Such surface modification allows the cells to attach to the oppositely charged polymeric substrate such as insoluble plant material or other cells. Accordingly, the two research teams, one from Slovenia specialised in the development of synthetic communities, and another from Croatia experts in composting, aim to obtain naturally compatible cells through random aggregation as well as to construct (SOS) consortia synthetically that will be incorporated into carriers or attached on insoluble plant material to: (i) achieve a faster composting process, i.e. reducing OP quantity and transforming it into organic fertilizer and (ii) demonstrate the production of various added-value products out of the compounds present in OP material (e.g. polyhydroxyalkanoates and aromatic substances). To achieve these objectives we will select, construct and characterize the most efficient SOS consortia using different molecular biology methodologies as well as chemical analytic approaches. We will mainly try to assess the activity of these consortia by determining the various metabolic side products of the decomposition process as well as by molecular tools such as transcriptomics and metagenomics to determine community dynamics and their stability to efficiently optimize processes. The prepared immobilized consortia will be then also used in the pilot-scale composting reactors and the composting process will be optimized. The mature compost will be then tested with bioassays to determine phytotoxic and plant growth-promoting activity.