Adaptation of a strain of Spirulina platensis to grow in cobalt-and iodine-enriched media

Abstract

Cobalt- and iodide-enriched (adapted, tolerant) strains of the protein-rich cyanobacterium, Spirulina platensis, were produced by repeated sub-culturing in increasing concentrations of the two trace elements. The strains enriched with cobalt and iodide showed higher uptake of these elements than the controls. The LD₅₀ values for the parent and cobalt-adapted strains were 95 and 231 μmol 1^-1 Co²⁺, respectively. Likewise, the LD₅₀ values for parent and iodide-adapted strains were 12 and 42 mmol^-1 I^-1. The carotenoid : chlorophyll a ratio of the parent strains increased after cobalt addition. The cobalt-adapted strain showed a much higher ratio than the cobalt-grown parent (sensitive) cells which remained unchanged after cobalt addition. Intracellular Co²⁺ uptake by the cells was concentration-dependent and followed Michaelis-Menten kinetics with saturation in uptake occurring in the parent and adapted strains at 126 and 189 μmol 1^-1 Co², respectively. At saturating concentrations, the maximum Co²⁺ uptake was 39.73 and 158.43 nmol Co²⁺ mg^-1 protein, respectively for the parent and adapted strains. The adapted strain also showed greater cobalt adsorption. The K_mof intracellular Co²⁺ uptake was lower in the case of adapted cells as compared with the parent, whereas V_max showed an opposite trend. Thus, the adapted cells appear to be more efficient than the parent strain in intracellular uptake of cobalt. Differences between kinetic constants of both the strains suggest that the strains may be physiologically different. Likewise, iodide uptake was significantly higher in iodide-adapted cells than in controls.