Production of D-lactate, acetate, and pyruvate from glycerol in communities of halophilic archaea in the Dead Sea and in saltern crystallizer ponds

When glycerol is added to cultures of halophilic archaea, especially representatives of the genera Haloferax and Haloarcula, massive amounts of acids are formed. HPLC and enzymatic analyses of supernatants of Haloferax cultures grown in the presence of glycerol showed that all produced D-lactate and acetate. Cultures of two Haloarcula species tested produced pyruvate and acetate from glycerol. In all cases only a small fraction of the added glycerol was converted to organic acids. Both lactate, pyruvate, and acetate can be used as substrates for the growth of many halophilic archaea, including those that produce them, and acid production is possibly an overflow phenomenon, due to the limited capacity of the enzymatic systems responsible for their dissimilation. To test whether lactate is formed also by natural communities of halophilic archaea at low glycerol concentrations such as may be encountered in situ, we incubated samples from the Dead Sea and from the saltern crystallizer ponds at Eilat with 1.5-3 μM [U-¹⁴C]glycerol. After depletion of the glycerol, around 10% of the label was found in lactate and acetate in both brine samples. In addition, pyruvate was formed in Dead Sea water. Upon further incubation of the Dead Sea samples after depletion of the glycerol, pyruvate disappeared rapidly, while acetate and lactate concentrations decreased only very slowly. In saltern brines the lactate formed was degraded after depletion of the glycerol, but the concentration of labelled acetate decreased only very slowly.