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3.1. Growth of mixed cultures under carbon, nitrate and phosphate limitation
Fig. 1 shows the mean concentrations of glucose, nitrate and phosphate measured in the carbon-, nitrate- and phosphate-limited mixed continuous cultures. In the four carbon-limited cultures, glucose (Fig. 1A) was always below the limit of detection (15 μM). In the other cultures, residual glucose (supplied at 0.7 mM in the growth medium) was sometimes present. It should be noted that in all cultures,the growth medium also contained acetate, citrate,benzoate and salicylate as additional carbon and energy sources. Nitrate (Fig. 1B) was detectable (0.5 mM) in the surface water used for inoculation, but in the four nitratelimited cultures, nitrate was no longer detectable (650μM) from the first sampling 68 h after switching on the medium supply pump. In the carbon- and phosphate-limited cultures the average nitrate concentration was lower than 2.5 mM during the whole period, indicating that significant bacterial consumption of nitrate occurred also in these cultures. Fig. 1C shows the residual concentration of phosphate in the cultures. No phosphate could be detected under phosphate limitation. The amount of phosphate supplied in the medium (20 μM) was just above the limit of detection (10 μM). In the carbon- and nitrate-limited chemostats phosphate (10 mM) was evidently supplied to the cultures in excess. Since nitrate (Fig. 1B) and occasionally glucose (Fig. 1A) could be detected in the phosphatelimited cultures and since the molar ratio of phosphate per carbon supplied was only 0.0014:1, growth of the mixed bacterial culture on the medium with proportionally less phosphate was concluded to be controlled by the amount of phosphate. Fig. 2 provides an additional indication that the cultures that received less phosphate were limited by this macronutrient. The specific cell-bound activity of APase was consistently about 10-fold lower in the other cultures that contained 10 mM of phosphate in the growth medium.