Effect of metals on a siderophore producing bacterial isolate and its implications on microbial assisted bioremediation of metal contaminated soils.

A bacterial isolate producing siderophore under iron limiting conditions, was isolated from mangroves of Goa. Based on morphological, biochemical, chemotaxonomical and 16S rDNA studies, the isolate was identified as Bacillus amyloliquefaciens NAR38.1. Preliminary characterization of the siderophore indicated it to be catecholate type with dihydroxy benzoate as the core component. Optimum siderophore production was observed at pH 7 in mineral salts medium (MSM) without any added iron with glucose as the carbon source. Addition of NaCl in the growth medium showed considerable decrease in siderophore production above 2% NaCl. Fe(+2) and Fe(+3) below 2 µM and 40 µM concentrations respectively, induced siderophore production, above which the production was repressed. Binding studies of the siderophore with Fe(+2) and Fe(+3) indicated its high affinity towards Fe(+3). The siderophore concentration in the extracellular medium was enhanced when MSM was amended with essential metals Zn, Co, Mo and Mn, however, decreased with Cu, while the concentration was reduced with abiotic metals As, Pb, Al and Cd. Significant increase in extracellular siderophore production was observed with Pb and Al at concentrations of 50 µM and above. The effect of metals on siderophore production was completely mitigated in presence of Fe. The results implicate effect of metals on the efficiency of siderophore production by bacteria for potential application in bioremediation of metal contaminated iron deficient soils especially in the microbial assisted phytoremediation processes.

Siderophore-producing bacteria from a sand dune ecosystem and the effect of sodium benzoate on siderophore production by a potential isolate.

Bioremediation in natural ecosystems is dependent upon the availability of micronutrients and cofactors, of which iron is one of the essential elements. Under aerobic and alkaline conditions, iron oxidizes to Fe(+3) creating iron deficiency. To acquire this essential growth-limiting nutrient, bacteria produce low-molecular-weight, high-affinity iron chelators termed siderophores. In this study, siderophore-producing bacteria from rhizosphere and nonrhizosphere areas of coastal sand dunes were isolated using a culture-dependent approach and were assigned to 8 different genera with the predominance of Bacillus sp. Studies on the ability of these isolates to grow on sodium benzoate revealed that a pigmented bacterial culture TMR2.13 identified as Pseudomonas aeruginosa showed growth on mineral salts medium (MSM) with 2% of sodium benzoate and produced a yellowish fluorescent siderophore identified as pyoverdine. This was inhibited above 54 µM of added iron in MSM with glucose without affecting growth, while, in presence of sodium benzoate, siderophore was produced even up to the presence of 108 µM of added iron. Increase in the requirement of iron for metabolism of aromatic compounds in ecosystems where the nutrient deficiencies occur naturally would be one of the regulating factors for the bioremediation process.