Enrichment and characterisation of ethanol chain elongating communities from natural and engineered environments

Chain elongation is a microbial process in which an electron donor, such as ethanol, is used to elongate short chain carboxylic acids, such as acetic acid, to medium chain carboxylic acids. This metabolism has been extensively investigated, but the spread and differentiation of chain elongators in the environment remains unexplored. Here, chain elongating communities were enriched from several inocula (3 anaerobic digesters, 2 animal faeces and 1 caproic acid producing environment) using ethanol and acetic acid as substrates at pH 7 and 5.5. This approach showed that (i) the inoculum's origin determines the pH where native chain elongators can grow; (ii) pH affects caproic acid production, with average caproic acid concentrations of 6.4 ± 1.6  g·L^-1 at pH 7, versus 2.3 ± 1.8  g·L^-1 at pH 5.5; however (iii) pH does not affect growth rates significantly; (iv) all communities contained a close relative of the known chain elongator Clostridium kluyveri; and (v) low pH selects for communities more enriched in this Clostridium kluyveri-relative (57.6 ± 23.2% at pH 7, 96.9 ± 1.2% at pH 5.5). These observations show that ethanol-consuming chain elongators can be found in several natural and engineered environments, but are not the same everywhere, emphasising the need for careful inoculum selection during process development.