Mercury removal during growth of mercury tolerant and self-aggregating Yarrowia spp.

Abstract

Ecotoxicological implications of mercury (Hg) pollution of hydrosphere require effective Hg-removal strategies as antidote to the environmental problems. Mercury-tolerant yeasts, Yarrowia spp. Idd1 and Idd2 strains, were studied for intracellular accumulation and extracellular micro-precipitation of Hg during growth stage of the yeast strains. In a liquid medium containing 870 (±23.6) µg of bioavailable Hg2+, 419.0 µg Hg2+ (approx.) was taken up by the wet biomasses of the yeast strains after 48 h post-inoculation. Large portion of the adsorbed Hg was found in cell wall (approx. 49–83 %) and spheroplast (approx. 62–89 %). Negligible quantities of Hg were present in the mitochondria (0.02–0.02 %), and appreciable amount of Hg was observed in nuclei and cell debris (15.2–65.3 %) as evidence of bioaccumulation. Extracellular polymeric substances (EPS) produced by the growing Yarrowia cells was a complex of protein, carbohydrates and other substances, immobilizing 43.8 (±0.7)–58.7 (±1.0) % of initial Hg in medium as micro-precipitates, while 10.13 ± 0.4–39.2 ± 4.3 % Hg content was volatilized. Transmission electron microscopy coupled with X-ray energy dispersive spectrophotometry confirmed the cellular removal of Hg and formation of EPS-Hg complex colloids in the surrounding bulk solution as micro-precipitates in form of extracellular Hg-nanoparticles. Hg mass balance in the bio-sequestration experiment revealed excellent Hg removal (>97 %) from the medium (containing ≤16 μg ml−1 Hg2+) by the yeast strains via bioaccumulation, volatilization and micro-precipitation. The yeast strains are also effectively applicable in biological purification technology for Hg contaminated water because of their high self-aggregation activity and separatability from the aquatic environments.[Figure not available: see fulltext.]