Response of mixed cultures of chlorella vulgaris and heterotrophic bacteria to variation of pH

The effects of hydrogen ion concentration on the growth of heterotrophic bacteria and alga Chlorella vulgaris, one of the dominant species in waste stabilization ponds were investigated in fed batch cultures. Glucose used as a source of organic carbon was fed daily in granular form and pH was controlled between 3.0 and 11.5 by addition of either HCl or NaOH. Heterotrophic bacterial densities were highest between pH 4.0 and 5.5, but the growth rate of Chlorella vulgaris was optimum at pH 4.5∼8.0. Chlorella vulgaris was sensitive to alkaline pH values, with maximum growth rates, μ_max, of 2.03, 1.12 and 0.75 d⁻¹ at pH 7.0, 9.0 and 10.0, respectively. The saturation constants, K_s, for the same pH values were 181, 157 and 52 mg/l, respectively. The optimum total biomass yield was 0.381 mg POC/mg glucose and was observed at pH 5.5∼8.0, but the magnitude was strongly influenced by the pre-culture pH conditions. Glucose metabolism was significantly affected at alkaline conditions than at acidic conditions. These results also suggest that in a mixed culture with bacteria, algae are largely responsible for glucose metabolism, with apparent saturation constants at pH 7.0 of 27 mg/l and 181 mg/l for bacteria and Chlorella vulgaris, respectively.