Caracterização de biofilmes epilíticos ocorrentes na bacia hidrográfica do rio Guaporé

Abstract

Epilithic biofilms dominate microbial life in streams and rivers, lead to crucial ecosystem processes and contribute to global biogeochemical fluxes. However, the intensification of human activities continually exposes water resources to different sources of contamination and may cause alterations in the organic and mineral composition of these communities. In this context, the chemical characterization of biofilms is of great importance to better understand their dynamics in the aquatic environment. The present study aimed to characterize epilithic biofilms occurring in a agricultural catchment of Rio Grande do Sul State in terms of organic and mineral composition. Nine sampling points were selected in the sub-catchment of Capingui River, in the sub-catchment of Marau Stream, both tributaries of the Guaporé River, and in the main course of the Guaporé River catchment. Epilithic biofilms were sampled in the fall and spring, from the collection and manual brushing of the material adhered to fragments of submerged rocks. The organic composition was evaluated by the analyzes of total organic carbon and total nitrogen, percentage of minerals and organic material, pyrolysis coupled to gas chromatography and mass spectroscopy, attenuated total reflectance infrared and 2D nuclear magnetic resonance. The mineral composition of the biofilms was obtained from the analysis of granulometry, pseudo-total and total elements contents and X-ray diffractometry. The results demonstrate that epilithic biofilms are constituted mainly by minerals (62.14 to 84.45%). The collection points with less influence of the anthropic action presented greater amount of biofilm on the rocks. On the other hand, the concentrations of organic carbon (36.0 to 44.6%) and organic nitrogen (5.2 to 7.2%) showed little variation within the biofilm biomass. Spectrometric techniques have demonstrated that epilithic biofilms are mainly composed of polysaccharides, nitrogen-containing compounds, lipids and aromatic and phenolic structures, which have functional groups characteristic of alcohol, ester, ether and amide. The quantitative composition of these biomolecules varied due to the variation of the contamination with minerals and consequent variation of the organic material contents. The oxidation of the organic matter caused the increase of the percentage of particles in the clay fraction, evidencing that the clay fraction present in the biofilms has a strong physical-chemical interaction with the organic material. However, the highest proportion of particles accumulated in biofilms is found in silt size (67.18 to 83.44%). Epilithic biofilms bioaccumulate expressive amounts of elements, especially Al, Fe, Mn, Ca and Mg, originating from the source material, and mainly from the anthropic activities. The X-ray diffractometry analysis highlights the presence of quartz and illite and the possibility of mineral neoformation in epilithic biofilms. Thus, the connectivity of the erosive processes with the fluvial network of the Guaporé River is responsible for the adsorption of organic compounds and the contamination of the biofilms with elements and nutrients of the cultivated areas. The spectrometric techniques used allowed to deepen the investigation on the chemical constitution of the epilithic biofilms.