Abstract
Background: This study focuses on the processes occurring during acidogenic step of anaerobic digestion, especially resulting from nutritional interactions between dark fermentation (DF) bacteria and lactic acid bacteria (LAB). Previously, we have confirmed that DF microbial communities fed on molasses are able to convert lactate and acetate to butyrate in batch experiments. The aims of the study were: (i) to recognize biodiversity of DF microbial communities able and unable to convert lactate and acetate to butyrate and (ii) to define the conditions for the transformation in static batch experiments.Results: Sucrose stimulated bacterial growth, especially LAB. In the samples where the microbial communities fermented media containing carbohydrates the two main tendencies were observed: (i) a low pH (pH≤4), lactate and ethanol as the main fermentation products, microbial communities dominated with Lactobacillus, Bifidobacterium, Leuconostoc and Fructobacillus was characterised by a low biodiversity; (ii) pH in the range 5.0-6.0, butyrate dominated among the fermentation products, the microbial communities composed mailny of Clostridium (especially Clostridium sensu stricto 12), Lactobacillus, Bifidobacterium and Prevotella. The biodiversity increased with the ability to convert acetate and lactate to butyrate. The microbial communities processing exclusively lactate and acetate showed the highest biodiversity and was dominated by Clostridium (especially Clostridium sensu stricto 12). LAB were reduced, other genera such as Terrisporobacter, Lachnoclostridium, Paraclostridium or Sutterella were found. Butyrate was the main metabolite and pH was 7. WGS analysis of the selected butyrate-producing microbial communities independently on the substrate, revealed C. tyrobutyricum as a dominant Clostridium species. Conclusions: The batch tests revealed dynamics of metabolic activity and composition of DF microbial communities dependent on fermentation conditions. The results expand our knowledge on lactate to butyrate conversion by DF microbial communities. The relevant factor for conversion of lactate and acetate to butyrate in the presence of carbohydrates is pH in the range 5-6 and the balance between LAB (especially Lactobacillus), lactate and acetate producers (Bifidobacterium) and butyrate producers (mainly Clostridium) as well Prevotella. The pH below 4 and ethanol concentration might be the signalling factors responsible for metabolic shift of the dark fermentation microbial communities towards lactate fermentation.
Dynamics of Dark Fermentation Microbial Communities in the Light of Lactate and Butyrate Production
