Change in the microbial community of saline geothermal fluids amended with a scaling inhibitor: effects of heat extraction and nitrate dosage

Geothermal plants are often affected by corrosion caused by microbial metabolites such as H 2 S. In the Bad Blumau (Austria) geothermal system, an increase in microbially produced H 2 S was observed in the hot (107 °C) and scaling inhibitor-amended saline fluids and in fluids that had cooled down (45 °C). Genetic fingerprinting and quantification revealed the dominance, increasing abundance and diversity of sulfate reducers such as Desulfotomaculum spp. that accompanied the cooling and processing of the geothermal fluids. In addition, a δ 34 S isotopic signature showed the microbial origin of the H 2 S that has been produced either chemolithotrophically or chemoorganotrophically. A nitrate addition test in a test pipe as a countermeasure against the microbial H 2 S formation caused a shift from a biocenosis dominated by bacteria of the phylum Firmicutes to a community of Firmicutes and Proteobacteria . Nitrate supported the growth of nitrate-reducing sulfur-oxidizing Thiobacillus thioparus , which incompletely reduced nitrate to nitrite. The addition of nitrate led to a change in the composition of the sulfate-reducing community. As a result, representatives of nitrate- and nitrite-reducing SRB, such as Desulfovibrio and Desulfonatronum , emerged as additional community members. The interaction of sulfate-reducing bacteria and nitrate-reducing sulfur-oxidizing bacteria (NR-SOB) led to the removal of H 2 S, but increased the corrosion rate in the test pipe.