Microbially-enhanced composting of olive mill solid waste (wet husk): Bacterial and fungal community dynamics at industrial pilot and farm level M. Agnolucci ⇑ , C. Cristani, F. Battini, M. Palla, R. Cardelli, A. Saviozzi, M. Nuti Department of Agriculture, Food and Environment (DAFE), University of Pisa, Via del Borghetto 80, 56124 Pisa, Italy highlights " Community dynamics analysed by DGGE along composting wet husk at pilot and farm level. " Starters enhanced transformation and increased earlier bacterial diversity. " High diversity in thermic phase was a typical trait of wet husk transformation. article info Article history: Received 17 December 2012 Received in revised form 7 February 2013 Accepted 9 February 2013 Available online 16 February 2013 Keywords: Olive mill solid waste Wet husk Bacterial and fungal diversity Microbial dynamics PCR-DGGE abstract Bacterial and fungal community dynamics during microbially-enhanced composting of olive mill solid waste (wet husk), used as a sole raw material, were analysed in a process carried out at industrial pilot and at farm level by the PCR-DGGE profiling of the 16 and 26S rRNA genes. The use of microbial starters enhanced the biotransformation process leading to an earlier and increased level of bacterial diversity. The bacterial community showed a change within 15 days during the first phases of composting. Without microbial starters bacterial biodiversity increased within 60 days. Moreover, the thermophilic phase was characterized by the highest bacterial biodiversity. By contrast, the biodiversity of fungal communities in the piles composted with the starters decreased during the thermophilic phase. The biodiversity of the microbial populations, along with physico-chemical traits, evolved similarly at industrial pilot and farm level, showing different maturation times. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Composting and co-composting of olive mill solid waste (wet husk) is receiving an increasing attention (Alfano et al., 2008; Hachicha et al., 2009), and the resulting compost has recently been shown to improve nutraceutical traits of horticultural crops (Ulrichs et al., 2008) and to represent a fertilizer for short-term crops (Altieri and Esposito, 2010). Detoxification of fats, organic acids and polyphenols is achieved throughout the process, result- ing in an odourless product with a good germination and humifica- tion index (Echeverria et al., 2011). These results are due to the biotransformation activity of microorganisms, leading to a rapid succession of specialized bacterial populations during co-compost- ing (Federici et al., 2011). Various attempts have been carried out to identify the microbial species and to enumerate the physio- taxonomical groups (bacteria and fungi) during the three phases of husk composting or co-composting, namely the activation, ther- mophilic, and maturation phase, using cultivation-dependent methods. However, the results are erratic and provide underesti- mates due to the constraints of culture media and cultivation conditions (Principi et al., 2003; Bru-Adan et al., 2009), as well as to the presence of microbial communities in viable but non-cultur- able state. Culture-independent approaches are becoming prominent to study microbial communities structure and dynamics, and molec- ular methods such as PCR-DGGE have been used to analyze micro- bial biodiversity during the composting process of different matrices (Novinscak et al., 2009; Takaku et al., 2006; Zhang et al., 2011). According to such studies, the microbial communities are highly variable during the various phases of the composting pro- cess and tend to get stabilized at compost maturity. This approach has been recently used by Vivas et al. (2009) to analyze the bacte- rial community structure in the final vs. in the initial matrix of a mixture of fresh olive waste and sheep manure processed by co-composting or vermicomposting. They found that the bacterial diversity was markedly affected by vermicomposting but not by co-composting. The use of starter cultures to speed up the 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.02.022 ⇑ Corresponding author. Tel.: +39 0502216647; fax: +39 0502216641. E-mail address: magnolucci@agr.unipi.it (M. Agnolucci). Bioresource Technology 134 (2013) 10–16 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech