Abstract
Ancient putative microbial structures that appear in the rock record commonly serve as evidence of early life on Earth, but the details of their formation remain unclear. The study of modern microbial mat structures can help inform the properties of their ancient counterparts, but modern mineralizing mat systems with morphological similarity to ancient structures are rare. Here, we characterize partially lithified microbial mats containing cm-scale dendrolitic coniform structures from a geothermal pool (“Cone Pool”) at Little Hot Creek, California, that if fully lithified, would resemble ancient dendrolitic structures known from the rock record. Light and electron microscopy revealed that the cm-scale ‘dendrolitic cones’ were comprised of intertwined microbial filaments and grains of calcium carbonate. The degree of mineralization (carbonate content) increased with depth in the dendrolitic cones. Sequencing of 16S rRNA gene libraries revealed that the dendrolitic cone tips were enriched in OTUs most closely related to the genera Phormidium, Leptolyngbya, and Leptospira, whereas mats at the base and adjacent to the dendrolitic cones were enriched in Synechococcus. We hypothesize that the consumption of nutrients during autotrophic and heterotrophic growth may promote movement of microbes along diffusive nutrient gradients, and thus microbialite growth. Hour-glass shaped filamentous structures present in the dendrolitic cones may have formed around photosynthetically-produced oxygen bubbles—suggesting that mineralization occurs rapidly and on timescales of the lifetime of a bubble. The dendrolitic-conical structures in Cone Pool constitute a modern analog of incipient microbialite formation by filamentous microbiota that are morphologically distinct from any structure described previously. Thus, we provide a new model system to address how microbial mats may be preserved over geological timescales.
Highlights
Ancient fossilized putative microbial structures appear in the rock record as morphologically distinctive indicators of early life on Earth.[1,2] Tufts in fossilized Archaean structures have been interpreted as Cyanobacteria and evidence for oxygenic photosynthesis.[3]
Concentrations of dissolved total CO2 (TCO2), dissolved oxygen, and Ca2+, K+, Mg2+, and Na+ were similar between the Cone Pool and Outflow Pool (Table 1)
Based on visits to Cone Pool prior to and after the sampling trip of the present study (June 2015), dendrolitic cones may be an ephemeral feature of the microbial mat: dendrolitic cones were visible from the first visit (December 2013) until September 2015, were not visible on return to the site in June 2016
Summary
Ancient fossilized putative microbial structures appear in the rock record as morphologically distinctive indicators of early life on Earth.[1,2] Tufts in fossilized Archaean structures have been interpreted as Cyanobacteria and evidence for oxygenic photosynthesis.[3] as structures that have undergone lithification and post-depositional diagenetic alteration, most ancient microbialites lack preserved microfossils or unobscured information concerning their formation and the chemistry of their environments.[4,5] An understanding of the processes that control the formation of microbialites and determining the geochemistry of these structures will help to more accurately interpret the ambient ancient environmental, geochemical and physical conditions. We present a detailed physical, geochemical, and genomic characterization of semi-lithified carbonate, dendrolitic conical microbial structures (i.e. dendrolitic cones) from a geothermal pool (Cone Pool) at Little Hot Creek (LHC) spring, in the Long Valley Caldera, California, USA (Fig. 1). Carbonate-rich dendrolitic cones from Little Hot Creek JA Bradley et al
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