The oldest lithified cyanobacteria found in sedi� ments date back to 3.6 Ga, and largescale formation of stromatolitic structures associated with copious cyanobacterial mats is known at 2 and 1 Ga (1, 2). Today's cyanobacteria have been expelled to extreme habitats, such as hot springs where thermophilic com� munities can live. Many hot springs vent in the Bar� gusin basin of the Baikal Rift Zone (3-5).They are alkaline waters with high silica concentrations and a nitrogendominated gas phase. The waterbearing rocks are sheared outsized granites or granodiorites. Microbial populations have been recognized at all venting sites (6) (more than ten within the Barguzin basin), but travertine occurs at three sites only (Alla, Garga, and Gusikha) (7).The thickest carbonate (trav� ertine) framework at the Garga spring has been attrib� uted to the microbial activity (8). Yet, minerals depos� ited immediately in the hotspring environments of the basin have never been described. We investigated microbial populations of five springs (Alla, Kuchiger, and Umkhei on the right side of the Barguzin valley, and Garga and Uro on its left side) and discovered bacteriamediated precipitation of amorphous silica and calcite in the Alla and Garga springs, just where travertine is formed. Our objective was to study the morphology of minerals and the laws of their forma� tion in microbial communities. Water samples collected in 2007 and 2008 from the emergent sources and down the stream were filtered through (0.45 µm) membrane filters. Eh and pH, as well as O2 and HS - concentrations, were measured at the site by an Anion 4100 meter and MERCK tests (Germany), respectively. The water chemistry was analyzed by several methods to provide cross valida� tion: atomemission spectrometry, ICPMS, capillary electrophoresis, and atomic absorption. For transpor� tation, the microbial mat samples were packed with caution to preserve their structure, and were split into layers and dried in the laboratory. The element abun� dances in the organic and mineral components of the mats were determined by SRXRF method. The com� munity composition of the mats, and the mineral mor� phology and phase composition were studied on "Carl Zeiss" light microscopes (Axiolab 40 Pol and Axio Imager, Germany) and on a Leo Oxford 1430VP scan� ning electron microscope (SEM) (Germany), analysts S.V. Letov and A.T. Titov. The predominant ions in the water of the Alla springs are bicarbonate, silica and sodium. The emer�