UPTAKE AND TURNOVER OF ACETATE IN HYPERSALINE ENVIRONMENTS

Acetate uptake and turnover rates were determined for the heterotrophi c community in hypersaline environments (saltern crystallizer ponds, t he Dead Sea) dominated by halophilic Archaea. Acetate was formed from glycerol, which is potentially the major available carbon source for n atural communities of halophilic Archaea. Values of [K-t + S-n] (the s um of the substrate affinity and the substrate concentration present i n situ) for acetate measured in saltern crystallizer ponds were around 4.5-11.5 mu M, while in the Dead Sea during a Dunaliella bloom values up to 12.8 mu M were found. Maximal theoretical rates (V-max) of acet ate uptake in saltern crystallizer ponds were 12-56 nmol 1(-1) h(-1), with estimated turnover times for acetate (T-t) between 127-730 h at 3 5 degrees C. V-max values measured in the Dead Sea were between 0.8 an d 12.8 nmol 1(-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. C omparisons with pure cultures of halophilic Archaea grown under differ ent conditions showed that the natural communities were not adapted fo r preferential use of acetate. Both in natural brines and in pure cult ures of halophilic Archaea, acetate incorporation rates rapidly decrea sed above the optimum pH value, probably since acetate enters the cell only in its unionized form. The low affinity for acetate, together wi th low potential utilization rates result in the long acetate turnover times, which explains the accumulation of acetate observed when low c oncentrations of glycerol are supplied as a nutrient to natural commun ities of halophilic Archaea.