Characteristics of Sulfite Transport by Chlorella vulgaris

The rate of short-term accumulation of ³⁵S]sulfite in Chlorellavulgaris cells was found to be strongly dependent on the pHof the medium. The rate increased with decreased pH, and theincrease in rate closely paralleled the increase in the concentrationof the un-ionized form of sulfite. When the pH of the mediumwas increased, fast accumulation ceased immediately. The rateof accumulation showed a strong temperature dependence, withan apparent temperature coefficient of 1.93 per 10°C rise,between 10 and 25°C. Because pK_a values of sulfite shiftwith temperature, the rates were corrected by dividing by theconcentration of the un-ionized form of sulfite present at therespective temperatures. The temperature coefficient was thenfound to decrease to 1.45. When cells which had been allowedto accumulate ³⁵S]sulfite for 20 min were transferred to amedium containing no sulfite, more than 50% of the accumulated³⁵S] was released into the medium in 20 min. Our results arecompatible with a simple diffusion model of SO₂ transport intoChlorella cells. (Received September 26, 1996; Accepted January 20, 1997)