Abstract
BackgroundRecent research articles indicate that direct interspecies electron transfer (DIET) is an alternative metabolic route for methanogenic archaea that improves microbial methane productivity. It has been shown that multiple conductive materials such as biochar can be supplemented to anaerobic digesters to increase the rate of DIET. However, the industrial applicability, as well as the impact of such supplements on taxonomic profiles, has not been sufficiently assessed to date.ResultsSeven industrial biogas plants were upgraded with a shock charge of 1.8 kg biochar per ton of reactor content and then 1.8 kg per ton were added to the substrate for one year. A joint analysis for all seven systems showed a decreasing trend for the concentration of acetic acid (p < 0.0001), propionic acid (p < 0.0001) and butyric acid (p = 0.0022), which was significant in all cases. Quantification of the cofactor F420 using fluorescence microscopy showed a reduction in methanogenic archaea by up to a power of ten. Methanogenic archaea could grow within the biochar, even if the number of cells was 4 times less than in the surrounding sludge. 16S-rRNA gene amplicon sequencing showed a higher microbial diversity in the biochar particles than in the sludge, as well as an accumulation of secondary fermenters and halotolerant bacteria. Taxonomic profiles indicate microbial electroactivity, and show the frequent occurrence of Methanoculleus, which has not been described in this context before.ConclusionsOur results shed light on the interplay between biochar particles and microbial communities in anaerobic digesters. Both the microbial diversity and the absolute frequency of the microorganisms involved were significantly changed between sludge samples and biochar particles. This is particularly important against the background of microbial process monitoring. In addition, it could be shown that biochar is suitable for reducing the content of inhibitory, volatile acids on an industrial scale.
Highlights
Recent research articles indicate that direct interspecies electron transfer (DIET) is an alternative meta‐ bolic route for methanogenic archaea that improves microbial methane productivity
To give here an example: It was recommended that when using leftover food as a substrate, the amount of total volatile fatty acids (TVFAs) should remain below 4000 mg L −1 [15]
Taking into account that applied biochar particles may increase the rate of DIET, our results suggest that Methanoculleus may be involved in DIET
Summary
Recent research articles indicate that direct interspecies electron transfer (DIET) is an alternative meta‐ bolic route for methanogenic archaea that improves microbial methane productivity. Anaerobic digestion is a methane-yielding process carried out by a microbial biocenosis composed of bacteria and methanogenic archaea. As discussed in a recent review article, more recent articles show that electrons can be transported by conductive particles, direct cell contact or microbial nanowires [4]. This more direct way of electron transport is known as direct interspecies electron transfer (DIET) [4]. It has been shown that phenazine crystals can form long and needle-like conductive structures, which overgrew with methanogenic archaea during the respective experiments
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