Avaliação da qualidade ambiental no entorno de áreas de mineração e análise da exposição da cianobactéria Synechococcus nidulans a oxiânions de arsênio

Abstract

The Iron Quadrangle is a region in Minas Gerais state rich in arsenic (As) in soils and sediments, where mining activity increases the risk of contamination by toxic elements. In the present study, the first step was to evaluate the concentrations of toxic metals in water and sediment in streams in the Rio Doce and Rio Das Velhas basin in the vicinity of mining areas, with attention focused on the risks of human contamination at these sites. The second step was to compare the phytoplankton community between impacted and reference sites to evaluate environmental quality. In a third step, the cyanobacterium Synechococcus nidulans (Pringsheim) Komárek, collected and isolated form a stream in a mining area, was used as a test organism to study As toxicity, bioaccumulation and speciation, in order to verify the possibility of using cyanobacteria as agents to remove As oxyanions from aqueous solution or to biotransform inorganic As into organic forms. S. nidulas was cultivated in culture media BG-11 under exposure to As(III) and As(V). The X-ray Absorption Spectroscopy (XAFS) was used to investigate intracellular As binding sites and Inductively Coupled Plasma Mass Spectrometry (HPLC-ICP-MS ) was used for As chemical speciation. In the water from sampled points, it was observed contamination by the toxic metals As and manganese (Mn). In addition to these elements, other toxic metals were found in the sediments, such as cadmium (Cd) in the Rio Doce basin. Because it is a rural area where direct use of water is common, the local population is at risk of contamination by chronic exposure. Among the groups of phytoplanktonic organisms, Bacillariophyceae and Zignematophyceae stood out with greater richness and abundance associated with reference sites and Euglenophyceae associated with higher nutrients concentrations sites. The differences in the composition and structure of the phytoplankton community indicated its potential as bioindicator of environmental quality. S. nidulans showed high resistance to As(III) and As(V) oxyanions. The EC50 were determined upon exposure to As(V) =2,642.97 mg/l and As(III)= 6.64 mg/l. In the present study, it was found that cyanobacteria are resistant to very high concentrations of As compared to those found in the sampled points. As(III) seems to easily interact with cell wall components than As (V), which results in greater cell penetration. Using XAFS technique, it was demonstrated that sulfhydryl groups are the primary binding site of As(III), forming As(GS)3 complex. In long-term exposure (30 days) to As(III) and As(V), S. nidulans reduced As(V) to As(III) and produced organic compounds of As, indicating the involvement of S. nidulans in As biogeochemistry and its potential to be tested as an agent of arsenic biotransformation. Thus, contributions were presented to clarify the dynamics of As penetration and intracellular behavior in S. nidulans, a cyanobacteria strain isolated from a site contaminated with As and other toxic elements.