Biohydrogen-producing from bottom to top? Quali-quantitative characterization of thermophilic fermentative consortia reveals microbial roles in an upflow fixed-film reactor

Abstract

Abstract Understanding specificities of biohydrogen (bioH2)-producing systems is a key factor to enable efficient energetic exploitation of organic substrates in dark fermentation systems. The use of fixed-film reactors has been pointed as a determining approach to enhance bioH2 evolution due to maintaining high cell densities within reactors. However, limiting substrate availability by excess cell accumulation has been associated with low hydrogenogenic activity levels due to stimulating bioH2-consuming or non-producing pathways. A compartmentalized assessment was used to discover the dynamics of substrate conversion, cell retention patterns and the microbial characterization in different zones of a thermophilic (55\xa0°C) molasses-fed fixed-film reactor under an organic loading rate of 20.0\xa0kg-COD m−3 d−1. Results indicated that microbial populations (e.g. Thermoanaerobacterium genus) in the basal portion of the reactor governed bioH2 production by acetic- and butyric-type fermentation from sugars so that localized substrate availability (3.71\xa0±\xa01.13\xa0g-COD g−1VSS d−1) reached favorable levels for bioH2-producing populations. Lactate production by the Janthinobacterium genus occurred only as a secondary metabolic pathway in the bed-region, in which complementary bioH2 production by the co-fermentation of acetate and lactate (also by Thermoanaerobacterium genus) was identified. The obtained results provide insights to manage the operation, as well as the design of acidogenic systems, indicating where and why bioH2-producing populations prevail in fixed-film reactors.