Degradation of lactate by an anaerobic mixed culture in a fluidized-bed reactor

Abstract

In order to obtain syntrophic lactate degradation in a low-sulfate medium, first a methanogenic consortium consisting of Methanobrevibacter arboriphilus, Methanoculleus sp. and Methanothrix sp. was established in a fluidized-bed reactor with porous glass beads as a substratum and H 2/CO 2/acetate as energy and carbon sources. Then the reactor was inoculated with a Desulfovibrio sp. strain L1 which had been isolated from a bioreactor degrading organic acids including lactate. Racemic lactate was fed to the reactor and in the absence of H 2 the substrate was rapidly oxidized to acetate, CO 2 and hydrogen. After 1 day of operation suddenly propionate was produced by a very fast growing contaminating Clostridium sp., which was isolated thereafter. Growing on lactate the Clostridium sp. has higher μ max and K s values than Desulfovibrio sp. strain L1. Based on these data it is concluded that under non-sterile conditions of full-scale anaerobic bioreactors, lactate degradation takes place via propionate as long as the concentration of lactate is high, while at low lactate concentrations and in the presence of methanogens the oxidative decarboxylation of lactate to acetate is the preferred pathway.