TABLE 4.

Effects of extracellular Zn²⁺ on growth, magnetite synthesis, and ferric reductase activity^a

Zn in medium (μM)	Growth (optical density at 660 nm)	Avg no. of magnetosomes/ cell	Activity (Ub)	Zinc in cell (μg/g of protein)
1 (control)	0.085	18.6	11.6	50.9
20	0.080	12.1	3.03	559
75	0.080	6.4	1.68	1,726
200	0.065	5.3	1.42	NDc

^aCells were grown for 72 h in the presence of various amounts of ZnSO₄. The average numbers of magnetosomes in the given samples were obtained by counting of electron-dense particles in electron micrographs from a total of about 100 cells in each sample. Ferric iron reductase activity was determined by measuring the increase in the A₅₆₂ (7). Zinc concentrations in the cells were determined with an induced coupled plasma atomic spectroscopy system as described in Materials and Methods. 

^bOne unit is 1 nmol of Fe²⁺-ferrozine formed per min per mg of protein. 

^cND, not determined.