CARBONATE-RICH CONES: A NEW MODERN ANALOGUE OF ANCIENT CONICAL STROMATOLITE FORMATION?

Ancient conical stromatolite formation is poorly understood and modern analogues are uncommon (and typically not CaCO₃). However, a team from the 2015 International GeoBiology Course investigated carbonate-rich microbial cones found in a hot spring pool near Little Hot Creek (LHC), Long Valley Caldera, California, in order to improve the understanding of how ancient conical stromatolites might form.

The cones rise up from a layered microbial mat on the east side of a 45° C pool with very low flow that is super-saturated with respect to CaCO₃. Cone structures are 8-30 mm in height, are rigid and do not deform when removed from the pool. Morphological characterization through environmental scanning electronic microscopy revealed that the cone structure is maintained by a matrix of intertwining microbial filaments around carbonate grains. This matrix gives rise to cone-filaments that are arranged vertically or horizontally, and provide further stability to the cone.

16S rRNA analysis of the microbial communities in the cone tip, cone bottom, and underlying mat was performed. The bacterial and archaeal community present in the cone bottom and underlying mat top are not significantly different and contain abundance of Euryarchaeota, Acidobacteria, Chloroflexi and Calothrix. Comparatively, the relative abundances of Leptolingbya, Phormidium and Isosphaera increased significantly at the cone tip. Metagenomic analyses of this section of the cone showed the potential for photosynthetic, chemotactic and gliding motility genes associated with Planctomycete and Isosphaera.

The results suggest that the formation of the cones could be attributed to differing functionality of the microbial communities in the cone tip compared to the cone bottom and underlying mat (specifically the growth of photosynthetic microorganisms, phototaxis and chemotaxis). A direct comparison between the cone covered mat and sites having no cones (i.e. extant hot spring mat) suggests that the microbial community may play a key role in the formation and lithification of these carbonate-rich cones. The cones in Little Hot Creek may constitute a new modern analogue to address how microbial mats become part of the rock record and subsequent interpretation of ancient environments through stromatolites and microbialites.