The microbial ecology associated with siliceous sinters was studied in five geochemically diverse Icelandic geothermal systems. Bacterial 16S rRNA clone libraries were constructed from water-saturated precipitates from each site resulting in a total of 342 bacterial clone sequences and 43 species level phylotypes. In near-neutral, saline (2.6-4.7% salinity) geothermal waters where sinter growth varied between 10 and ~300 kg year(-1) m(-2), 16S rRNA gene analyses revealed very low (no OTUs could be detected) to medium (9 OTUs) microbial activity. The most dominant phylotypes found in these waters belong to marine genera of the Proteobacteria. In contrast, in alkaline (pH = 9-10), meteoric geothermal waters with temperature = 66-96°C and <1-20 kg year(-1)m(-2) sinter growth, extensive biofilms (a total of 34 OTUs) were observed within the waters and these were dominated by members of the class Aquificae (mostly related to Thermocrinis), Deinococci (Thermus species) as well as Proteobacteria. The observed phylogenetic diversity (i.e., number and composition of detected OTUs) is argued to be related to the physico-chemical regime prevalent in the studied geothermal waters; alkaliphilic thermophilic microbial communities with phylotypes related to heterotrophic and autotrophic microorganisms developed in alkaline high temperature waters, whereas halophilic mesophilic communities dominated coastal geothermal waters.