From Biofilms to Siliceous Stromatolites: The Interplay Between Microbial Activity, Hot Spring Chemistry and Growth Rate

Siliceous stromatolites with active surficial biofilms are found in a subsidiary pool of Obsidian Hot Spring, Yellowstone National Park. The structures appear as small domes (2-5 cm in diameter) and reside in 0-30 cm water depth near the edge of the spring at temperatures approaching 75 C. Some of the structures have accreted up to 15 cm of laminated silica, whereas others stand only a few cm tall. The internal laminae are 50-100 um thick and are defined by alternating sets of silicified tubes—one set is oriented horizontally, while the next set stands sub-vertically. The tubes are 4-5 um diameter and can be over 100 um long. The vertically oriented laminae sets contain gaps that resemble bubbles that likely grew within the mat and pushed the tubes aside or were present while the tubes actively grew around them. It is interesting to note that the external architecture of the structures, although small, resembles the architecture of much larger carbonate platforms, with a leading edge “berm” and a central “lagoon”.

The siliceous stromatolites were collected from the hot spring with a biofilm growing on the surface. The mineralized portion underlying the active biofilm contains about 1 % organic carbon. 16sDNA analyses indicate that the outer microbial mat contains primarily cyanobacteria most closely related to Chlorogloeopsis sp. The layers just below the surface mat also contain primarily cyanobacteria with different affinities. In contrast to the exterior surface, the interior of the stromatolite contains abundant proteobacteria, primarily from the beta group. The stromatolites are >92% silica and harden with drying. We will discuss the results of 14-C ages acquired in a section through these structures and describe a mechanism, whereby alternate wetting and drying yield the structure reminiscent of Precambrian stromatolites.