A simple agar plate assay for screening siderophore producer yeasts

Yeasts produce hydroxamate-type siderophores (iron-binding compounds) in response to Fe-stress conditions. Because these siderophores are important to the biocontrol of postharvest diseases of apple and pears, a method for screening siderophore producer yeast was developed. The screening method was carried out in special Petri dishes with eight or nine wells (25-mm diameter). These wells were filled with siderophore production medium and seeded with yeasts isolated from epiphytic apple microflora. After yeasts grew (24-48 h), holes (2-mm diameter) were made in the agar of each well. Holes were filled with an acid solution of ferric perchlorate. After 10-15 min, reddish halos appeared in the bottom of the plate and their intensities were compared with standards. Standards were prepared in the same special dish with rhodotorulic acid solutions (concentrations between 0.05 and 1 g/l) plus 2% agar. When agar solidified into wells, holes were made and filled with ferric perchlorate solution. Color intensities of reddish halos were proportional to siderophore concentration and the detection limit was 0.1 g/l. It was possible to correlate the production of siderophore in solid medium with the results obtained in liquid medium. The methodology was also a useful tool for making a preliminary assessment of the influence of different factors on the siderophore production.

Effect of nitrogen source and pH on siderophore production by Rhodotorula strains and their application to biocontrol of phytopathogenic moulds

The production of rhodotorulic acid, a siderophore synthesized by Rhodotorula strains, was improved with the objective of achieving the biocontrol of phytopathogenic moulds. Rhodotorulic acid increased up to 60% in the presence of urea as a nitrogen source, pH near to 8 and a C:N ratio of 8:1. The siderophore-containing spent medium showed in vitro antifungal activity against important plant pathogens including Botrytis cinerea, which causes grey mould on a wide variety of host plants including numerous commercial crops. The antifungal activity was related to siderophore concentration.

Extraction of lithium from spodumene by bioleaching

The recovery of lithium from spodumene (6.9% Li2O) by bioleaching was investigated. This process was carried out using heterotrophic micro-organisms previously isolated from the mineral. Penicillium purpurogenum, Aspergillus niger and Rhodotorula rubra were assayed separately. Two different media were used for bioleaching; one of them (M2 medium) was highly limited in Mg2+, Fe2+ and K+. The assays were carried out in 500 ml Erlenmeyer flasks with 1 g of ground mineral at 50-80 mesh and 150 ml of leaching medium. Lithium extracted and accumulated in biomass during 30 d of bioleaching with P. purpurogenum was 6.35 mg % dry weight (d.w.) in M1 medium and 10.8 mg % d.w. in M2 medium, while in the leach liquor, Li concentration was 1.06 ppm (M1 medium) and 1.26 ppm (M2 medium). Results of leaching on day 30 with R. rubra were 5.87 mg % d.w. and 16.7 mg % d.w. of lithium accumulated in biomass in M1 medium and M2 medium, respectively. In leach liquor, lithium was 0.5 ppm (M1 medium) and 1.53 ppm (M2 medium). Aspergillus niger was able to accumulate 1.60 mg % d.w. (M1 medium) and 5.1 mg % d.w. of lithium (M2 medium) in biomass. Lithium in leach liquor was 0.37 ppm (M1 medium) and 0.75 ppm (M2 medium). Chemical analysis of the leach liquor showed gluconic and citric acids. It was possible to detect capsular exopolymers in the yeast. These metabolic products seem to be related to leaching but a more important factor for enhancing this process may be microbial adaptation to a low nutrient environment.