Extremely acidophilic streamer filaments are dynamic biofilms and a source of small organic acids. (A) Scheme showing the main functions related to biofilm formation, dispersion and maintenance that have been detected by environmental transcriptomic studies. In addition, two different metabolic states can be inferred: 1) under microaerophilic conditions in the biofilm, cells activate the fermentative pathways with a net production of acetate, and 2) free living cells under aerophilic conditions showed a high active TCA cycle for energetic metabolism. Dashed arrows, quorum sensing activities; dots, metabolites (acetate, formate, etc.). Long arrows indicate the water stream. (B) Role of acetate in the extremely acidophilic microbial community. The microorganisms identified by PAM (Fig. 1) are located as a function of their key metabolic role. Sugars from the biofilm (shadowed and hatched area) and other complex organic matter are decomposed by heterotrophs like Fungae and Actinobacteria. Secondary fermenters could render acetate that can be used by iron reducers to produce Fe2+ which, in turn, is the energy source for iron oxidizers. The released acetate may inhibit the growth of chemolitotrophs like Leptospirillum spp. and Acidithiobacillus spp. (-), while it can be used by Firmicutes, Sulfobacillus spp., Alpha- and Betaproteobacteria as nutrient.