Small Oxygen Contamination Favors Propionate-producing Actinobacteria over Caproate-producing Clostridia in Lactate-Based Anaerobic Fermentation

Mixed microbial cultures have become a preferred choice of biocatalyst for chain elongation reactors due to their ability to convert complex substrates into medium-chain carboxylates. However, the complexity of the effects of process parameters on the microbial metabolic networks is a drawback that makes the task of optimizing product selectivity challenging. Here, we studied the effects of small air contaminations on the microbial community dynamics and the product formation in lactate- and acetate-based anaerobic fermentation. Two stirred tank reactors and two bubble column reactors were operated with H2/CO2 gas recirculation for 139 days and 116 days, respectively, at pH 6.0 and 32? with a hydraulic retention time of 14 days. One reactor of each type had periods with air contamination (between 90 ± 28 and 471 ± 33 mL O2 d-1 L-1, lasting from 4 to 32 days), while the other reactors were kept anoxic. During air contamination, caproate and CH4 production was strongly inhibited, whereas production of n-butyrate remained unaffected. During an air contamination event with O2 concentrations up to 14.3 %, facultative anaerobes of the genus Rummeliibacillus became predominant and only n-butyrate was produced. However, at small air contamination rates and with O2 below the detection level, Coriobacteriia and Actinobacteria gained a competitive advantage over Clostridia and Methanobacteria and led to increased propionate production rates of 0.75 – 1.84 mmol d-1 L-1 depending on the reactor (control reactors 0.10 – 0.47 mmol d-1 L-1). Moreover, i-butyrate production was observed, but only when Methanobacteria abundances were low and, consequently, H2 availability was high. After air contamination was stopped, caproate and CH4 production recovered, with caproate production rates of 1.4 – 1.8 mmol d-1 L-1 (control 0.9 – 2.0 mmol d-1 L-1). The results underline the importance of keeping strictly anaerobic conditions in fermenters when caproate is the target product. Beyond that, it is suggested that small aeration rates could enter in the repertoire of controllable process parameters to help shape the reactor microbiome and thus exploit the metabolic properties of aero-tolerant microorganisms.