Stromatolite formation by Anaerolineae-dominated microbial communities in hot spring travertine in North Sumatra, Indonesia

The travertine deposits could provide a prospective geomicrobiological snapshot of the early Earth considering its similarity with ancient stromatolites. Herein, we investigated the travertine carbonate deposits in a calcareous and sulfidic hot spring at Sipoholon, Northern Sumatra, Indonesia, with an emphasis on the formation of a laminated texture similar to one observed in the stromatolites. The travertine deposits were associated with microbial mats of different colors that change from yellow, pink, and green, lowering water temperature and H2S concentration along the water passages. A phylotype analysis of the Prokaryotic 16S rRNA gene revealed that chemolithoautotrophic sulfur and hydrogen oxidizing bacteria were abundant in the yellow sediment around vent sites. At the same time, photoheterotrophs, a linage of Anaerolineae, dominated the pink to green sediment at mid- to downstream. Laminated textures are developed in pink and green-colored sediments among these categories. Monitoring water chemical parameters and sediment texture for an Anaerolineae-dominated sample collected from a midstream site revealed that daily lamination was formed due to the daytime biofilm formation under the stable chemical composition of water conditions except for dissolved oxygen concentration. Daily laminations were also formed in other samples by microbial communities with lower Anaerolineae abundance. Further, the results showed that if mineral precipitation and microbial growth rates are appropriately balanced, cyanobacteria were not essential for the lamina development. This indicating that such a geochemical system could occur in early-time stromatolites even before the evolution of cyanobacteria. The microbial-mineral precipitation systems identified in this study provide significant insights into the formation of stromatolites, which can in turn shed light on the history of early Earth.