Regulation of cellular manganese and manganese transport rates in the unicellular alga Chlamydomonas1

The cellular accumulation and uptake kinetics of manganese by Chlamydomonas sp. were studied in model chelate buffer systems. Cellular manganese concentrations and uptake rates were related to the computed free manganese ion concentration and were independent of the total or chelated manganese concentration. Cellular manganese was constant at about 1 mmol liter−1 of cellular volume at free manganese ion concentrations of 10−7.6−10−6.3 mol liter−1 and decreased below this range. Manganese uptake rates followed saturation kinetics and Vmax, but not Ks(10−7.0 mol liter−1), varied with the free manganese ion concentration in the growth medium. Vmax appeared to be under negative feedback control and increased with decreasing manganese ion concentration. Variations of up to 30‐fold in this parameter seemed to be instrumental in limiting the variation in cellular manganese to a sixfold range (0.18–1.0 mmol liter−1) despite a 1,000‐fold variation in free manganese ion concentration in the growth medium (10−9.5–10−6.3 mol liter−1).