Isolation and characterization of yeasts associated with plants growing in heavy-metal-and arsenic-contaminated soils

Yeasts were quantified and isolated from the rhizospheres of 5 plant species grown at 2 sites of a Mexican region contaminated with arsenic, lead, and other heavy metals. Yeast abundance was about 102 CFU/g of soil and 31 isolates were obtained. On the basis of the phylogenetic analysis of 26S rRNA and internal transcribed spacer fragment, 6 species were identified within the following 5 genera: Cryptococcus (80.64%), Rhodotorula (6.45%), Exophiala (6.45%), Trichosporon (3.22%), and Cystobasidium (3.22%). Cryptococcus spp. was the predominant group. Pectinases (51.6%), proteases (51.6%), and xylanases (41.9%) were the enzymes most common, while poor production of siderophores (16.1%) and indole acetic acid (9.67%) was detected. Isolates of Rhodotorula mucilaginosa and Cystobasidium sloffiae could promote plant growth and seed germination in a bioassay using Brassica juncea. Resistance of isolates by arsenic and heavy metals was as follows: As³⁺ ≥ 100 mmol/L, As⁵⁺ ≥ 30 mmol/L, Zn²⁺ ≥ 2 mmol/L, Pb²⁺ ≥ 1.2 mmol/L, and Cu²⁺ ≥ 0.5 mmol/L. Strains of Cryptococcus albidus were able to reduce arsenate (As⁵⁺) into arsenite (As³⁺), but no isolate was capable of oxidizing As³⁺. This is the first study on the abundance and identification of rhizosphere yeasts in a heavy-metal- and arsenic-contaminated soil, and of the reduction of arsenate by the species C. albidus.