Remoção e alocação de metais pesados por bactérias diazotróficas nodulíferas da espécie Cupriavidus necator

Abstract

Bacteria of the Cupriavidus genus are capable of fixating atmospher ic nitrogen and tolerate high concentrations of heavy metals. In this work, the strains of Cupriavidus necator UFLA 01-659, UFLA 01-663 and UFLA 02-71 were tested regarding their capacity for tolerating and removing cadmium (Cd), copper (Cu) and zinc (Zn) in liquid medium. Studies with scan and transmission electron microscopy were conducted in order to visualize growth standards, exopolysaccharide production and allocation of metals in the cellular biomass. Kinetic and biosorption isotherm experiments were conducted with live and dead cells from strain UFLA 01-659 regarding the aforementioned metals. The data were adjusted to the Langmuir and Freudlich models. The metal allocation study with live biomass cells, in biosorption experiment, was conducted by transmission electron microscopy. Of the tested strains, UFLA 01-659 presented the highest values of minimum inhibiting concentration (of 5 mM of Cd, 4.95 mM of Cu and 14.66 mM of Zn) and metal removal (9.0, 4.6 and 3.2 mg L -1 of Cd, Cu and Zn, respectively). The presence of metals induced the formation of biofilms and the decrease in cell size, which were organized in an aggregated manner, with variation of this behavior between strains and metals. In comparison to literature, the indexes reaches with Cd wer e the most expressive. The allocation of the metals removed in culture medium occurred in the membrane and cell walls (all metals), and in the interior of the cell (Cu and Zn). Metal biosorption had very few correlations with the time of incubation. The increase in the concentration of all metals induced an increase in its biosorption, with the data adjusted to the Freudlich model (P<0.001). The biomass of C. necator UFLA 01-659 removed approximately 52% of Cd in solution, and lower values for zinc and copper (36 and 20%) on the biosorption experiments. Only copper biosorption presented difference between live and dead cells, probably due to the allocation of this material between the wall and cytoplasmic membrane. Copper and zinc were also allocated in the interior of the cell, differently from cadmium. Due to the C. necator strain UFLA 01-659 presenting high capacity for the removal of the metals Cd, Cu and Zn by means of incorporation to its biomass and by absorption to the wall and plasmatic membrane in liquid culture medium. In biosorption experiments, this strain can be considered promising in the ex situ bioremediation of heavy metals.