Ecological perspectives of hydrogen fermentation by microbial consortia: What we have learned and the way forward

Research on hydrogen production by dark fermentation has improved the management and performance of bioreactors, although yield and long-term stability challenges still exist. To understand the causes of these challenges we propose to investigate the ecological properties of microbial consortia residing in hydrogen-producing bioreactors. In this study, we analyzed scientific literature (until 2016) on hydrogen fermentation and investigated the relationships between bioreactor operational conditions and microbial composition, as well as co-occurrences of microbial groups through multivariate and network analyses, respectively. The results of the analyses highlight ecological aspects that may need to be considered when aiming for increased performance and stability of bioreactors, such as: (i) the relationship between key parameters and their influence on microbial diversity (e.g. positive relationship between richness with H2 yield and higher diversity of certain inocula), (ii) the positive relationship of the presence of specific microbial families and genera (often overlooked) with increased H2 yield (e.g. Tissierellaceae, Oxalobacteraceae and Tepidimicrobium), (iii) the importance of specific groups and their interactions (e.g. potential cooperative interactions between H2 producers, biofilm producers and facultative anaerobic bacteria) and (iv) the importance of such interactions on the systemic properties of the consortia (e.g. increased network robustness with higher diversity). Beyond the ecological implications of our analyses, we also discuss the limitations of current methods for characterizing microbial communities and the potential for the application of modern methodologies such as high throughput sequencing and proteomics to re-evaluate the diversity and functional information thus far published in helping to disentangle the ecological phenomena that occur within hydrogenogenic consortia.