Determining the microbial community composition of the sampling site. (A) A picture of the field site in one of the Río Tinto springs showing details of the filamentous biofilms. (B) Estimation of the active microorganisms in the sampling site by using a prokaryotic acidophile microarray (PAM). Histograms show the signal intensities of each sample (biofilm and planktonic cells) after PAM micrarray hybridizations performed by simultaneous two-color labeled total RNA sample incubation (biofilm+planktonic) of each campaign. yellow bars, probe LEP439 for the L. ferrooxidans strain printed on the DNA microarray for transcriptomic studies. (C) Estimation of the relative proportion of rRNA in each sampling campaign by the analysis of Log2 ratio of biofilm/planktonic fluorescent signals (colored figure: red, dominate in biofilm; green, dominate in plankton; yellow, similar proportion in both places). The names of the microorganisms and the probes, as well as their phylogenetic adscription are indicated. H-GC, High GC content Actinobacteria.