Avaliação da toxicidade de arsênio e cromo em microrganismos utilizados como bioinsumos na agricultura

Abstract

Soil contamination by metals and metalloids represents an important stress factor for microorganisms of agricultural interest, potentially compromising their survival and establishment in agricultural systems. In this context, the present study aimed to evaluate the toxic effects of arsenic, in two distinct chemical forms, on the growth and cell viability of microorganisms used as bioinputs, as well as to compare these effects with those caused by chromium. The microorganisms Azospirillum brasilense Abv06, Bacillus aryabhattai CMAA 1363, Bradyrhizobium japonicum SEMIA 5079, and Saccharomyces cerevisiae were evaluated under different concentrations of arsenite, arsenate, and hexavalent chromium in in vitro assays. Microbial growth was assessed by measuring optical density at 600 nm (OD₆₀₀), while cell viability was determined by colony-forming unit (CFU) counts. Growth data were fitted using nonlinear regression with the four-parameter log-logistic model (LL.4), which allowed the characterization of different response levels of the microorganisms to the tested concentrations. The results demonstrated that toxicity varied according to the chemical form of the contaminant, its concentration, exposure time, and the characteristics of each microorganism. Arsenate showed low toxicity to most of the microorganisms evaluated, whereas arsenite caused a greater reduction in growth and, under certain conditions, high cell mortality, with B. japonicum showing particularly high sensitivity. The other microorganisms exhibited variable responses to arsenite depending on concentration and exposure time. Chromium, in turn, exhibited more severe toxicity, causing strong growth inhibition and high mortality in some of the microorganisms evaluated. The combined analysis of optical density and CFU counts demonstrated that reduced growth was not always associated with an immediate loss of cell viability, indicating the occurrence of sublethal responses during exposure to the contaminants. Therefore, the results demonstrate that the response of microorganisms of agricultural interest to arsenic and chromium depends on the chemical form of the contaminant and the biological characteristics of each organism. These findings highlight the importance of considering both growth and cell viability when assessing microbial tolerance under contamination conditions.