The effect of Cd
2+ and a combination of Cd
2+ with non-inhibiting levels of metals like Ca
2+, Zn
2+ and Hg
2+ on the uptake of ammonia and phosphate by
Anacystis nidulans IU625 (ATCC 27144) was investigated. Increasing concentrations of Cd
2+ resulted in the subsequent inhibition of both NH
4+ and PO
43- uptake. The uptake kinetics was studied by monitoring short-term uptake rates at varying substrate levels. The values for
Vmax (maximum uptake capacity) and
Km (half saturation constant) were obtained using the double reciprocal plot. The NH
4+ uptake as such, was found to be more sensitive to the heavy metal than the PO
43- uptake as fairly low levels of Cd
2+ were sufficient to inhibit the process significantly. However, the uptake kinetics for both NH4+ and PO
43- was altered if Cd
2+ was present in combination with other divalent cations such as Ca
2+, Zn
2+ and Hg
2+. Zn
2+ acted antagonistically to Cd
2+ toxicity. Similarly, the Ca
2+-Cd
2+ combination protected against inhibition by Cd
2+ ions. Hg
2+ behaved differently: the Cd
2+-Hg
2+ combination resulted in increased inhibition.
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