Uptake and turnover of acetate in hypersaline environments

Acetate uptake and turnover rates were determined for the heterotrophic community in hypersaline environments (saltern crystallizer ponds, the Dead Sea) dominated by halpphilic Archaea. Acetate was formed from glycerol, which is potentially the major available carbon source for natural communities of halophilic Archaea. Values of [K_t + S_n] (the sum of the substrate affinity and the substrate concentration present in situ) for acetate measured in saltern crystallizer ponds were around 4.5-11.5 μM, while in the Dead Sea during a Dunaliella bloom values up to 12.8 μM were found. Maximal theoretical rates (V_max) of acetate uptake in saltern crystallizer ponds were 12-56 nmol l^-1 h^-1, with estimated turnover times for acetate (T_t) between 127-730 h at 35°C. V_max values measured in the Dead Sea were between 0.8 and 12.8 nmol l^-1 h^-1, with turnover times in the range of 320-2190 h. V_max values for acetate were much lower than those for glycerol. Comparisons with pure cultures of halophilic Archaea grown under different conditions showed that the natural communities were not adapted for preferential use of acetate. Both in natural brines and in pure cultures of halophilic Archaea, acetate incorporation rates rapidly decreased above the optimum pH value, probably since acetate enters the cell only in its unionized form. The low affinity for acetate, together with low potential utilization rates result in the long acetate turnover times, which explains the accumulation of acetate observed when low concentrations of glycerol are supplied as a nutrient to natural communities of halophilic Archaea.