Influence of As (V) on the diversity of biofilms formed on different substrata

Abstract

Microorganisms, including bacteria, present in natural and artificial aquatic environments, tend to attach to and grow on immersed surfaces developing a biofilm. Many problems in drinking water networks such as corrosion, persistence of pathogenic species and increased resistance to biocides are due to the presence of biofilms. Arsenic is a contaminant widely distributed in the Argentinean underground water. Despite arsenic’s toxicity, a number of microorganisms are capable of growing in arsenic environments playing an important role in the process of arsenic mobilization. The aim of this work was to study the influence of As (V) on the bacterial planktonic community and biofilms structures grown on different drinking water distribution materials. To simulate a water distribution system, two tanks with a closed loop of polypropylene (PP) tubes were built and filled with drinking water. As(V) (5 mg L-1) was added in one of the tanks. Coupons of four materials were place in the loops for biofilm formation: commercial iron (Fe), commercial zinc (Zn), copper (Cu) and PP. Bacterial planktonic and sessile communities were analysed by culture (heterotrophic plate counts) and molecular (DNA extraction, PCR, sequencing, DGGE) techniques. Bacterial counts on Fe and Zn were higher than those obtained on Cu and PP and, except for Cu, they were higher in the presence of 5mg.L-1 of As(V). Culturable Astolerant bacteria able to grow in the presence of high As(V) concentration (up to 1g.L-1) were obtained from all the biofilms except Cu-biofilms, which grew in the presence of up to 300mg.L-1 As(V). It was possible to isolate and identify 60 colonies corresponding: 40% to the Class Bacilli, 40% α-Proteobacteria (both Classes were found in all the biofilms), 10% Actinobacteria (detected in biofilms formed on Fe in the absence of As, Cu and PP in the presence of As), 8% β-Proteobacteria (found on Fe, Zn and PP biofilms in the presence of As) and 2% γ-Proteobacteria (detected only in biofilms formed on Zn in the absence of As). The DGGE profiles of the planktonic bacterial communities were qualitative and quantitative affected by the presence of arsenic. In general, the planktonic community developed in the water without As showed higher richness and diversity indices, indicating that the presence of a toxic element induced a selection of the species in the water with Arsenic. In the case of the sessile communities, the trends were not so clear. The clustering analysis of the sessile communities showed that the nature of the substrata was a more important factor for the establishment of the community than the presence of arsenic in water.