Hydrogen cycling in a three-tiered food web growing on the methanogenic conversion of 3-chlorobenzoate

A defined 3-chlorobenzoate-degrading methanogenic consortium was constructed by recombining key organisms isolated from a 3-chlorobenzoate-degrading methanogenic sludge enrichment. The organisms comprise a three-tiered food chain which includes: (1) reductive dechlorination of 3-chlorobenzoate; (2) oxidation of benzoate to acetate, H2 and CO2; (3) removal of H2 plus CO2 by conversion into methane. The defined consortium, consisting of a dechlorinating organism (DCB-1), a benzoate degrader (BZ-1) and a lithotrophic methanogen (Methanospirillum strain PM-1) grew well in a basal salts medium supplemented with 3-chlorobenzoate (3.2 mM) as the sole energy source. The chlorine released from the aromatic ringe was recovered in stoichiometric amounts as the chloride ion. The reducing power required for reductive dechlorination was obtained from the hydrogen produced in the acetogenic oxidation of benzoate. One-third of the benzoate-derived hydrogen was recycled via the reductive dechlorination of 3-chlorobenzoate, indicating that the consortium operated as a food web rather than a food chain.