Isolamento de microrganismos tolerantes a elementos vestigiais capazes de mobilizar nutrientes

Abstract

Abandoned mines are a major problem worldwide, with inherited impacts both upstream and downstream, whose dimension is more than just environmental, as they also influence sociological and economic spheres. Therefore, it is urgent to find viable alternatives to the recover affected soils. However, these extreme environments may also provide a panoply of microorganisms that may be useful in agricultural soils. The inoculation of soils with microorganisms is a technique increasingly addressed in the field of research. Plant growth assays using very specific bacteria – plant growth promoting bacteria (PGPB) – have been tested. In soils with small amounts of phosphorus, especially if rich in iron and aluminum oxides and hydroxides, most of the macronutrient is adsorbed and unavailable to plants. The same is true in alkaline soils with large amounts of calcium. Organic forms of phosphorus are also directly unavailable to plants. Calcium phytate, one of the most abundant forms of organic phosphorus, is insoluble. Soil microorganisms can help plant to acquire phosphorus from the soil by means of several mechanisms, and stimulate plant growth by improving the microbial-plant nutritional status and resistance to plant pathogens. Same bacteria may also tolerate high concentrations of trace elements. The hypothesis tested in this work was that isolates from contaminated mine soils would have a large tolerance to trace elements present in those soils. For this purpose, the tolerance of phosphorus-solubilizing bacteria isolated from the rhizosphere of plants from an abandoned Portuguese mine (S. Domingos mine) to four trace elements (arsenic, lead, copper and zinc) was tested and the ability to promote plant growth was evaluated using maize as the model plant, due to the economic importance of this crop in the national context and because its growth is rapid, which from the point of view of the laboratory tests becomes an asset. The total number of bacteria able to grow in rich media or in minimal media with the different sources of phosphorus was evaluated. The results showed that the greater value in rich media was obtained in the uncontaminated soil and the smallest value in the mine soil with the lowest pH and with large total contents of lead and arsenic. Isolates that formed a halo in calcium phosphate or calcium phytate were then tested in the presence of different levels of copper, zinc, lead and arsenic. The solubilization efficiency was diverse, and the tolerance to different concentrations of trace elements also varied between isolates, with better results obtained with arsenic, generally. The study in hydroponics showed the capacity of bacterial isolates to promote plant growth highlighting the importance of the interaction plant-bacteria. In conclusion, it was possible, under laboratory conditions, to obtain isolates with ability to tolerate different trace elements, in different concentrations, and able to promote plant growth. The results suggest that it may be possible to use some of the isolates under field conditions as inoculants for agricultural soils.