Carbonate-rich dendrolitic cones: insights into a modern analog for incipient microbialite formation, Little Hot Creek, Long Valley Caldera, California

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.

Living Dendrolitic Microbial Mats in Hamelin Pool, Shark Bay, Western Australia

“…The living dendrolitic structures discovered in Hamelin Pool (Figure 9a) are different from previously described tufted mats from the same area [21] (no vertical profile available), from the Little Hot Creek geothermal spring in California [54] (Figure 9b), from pinnacle mats in Yellowstone Park hot springs by Walter et al [20] (Figure 9c), and in other geothermal areas, such as Ohaaki Pool and Kirihoro hot pool, New Zealand [80] ( [83], Figure 11a), and in a solar pond ("Pamelup Pond", Lake Preston, Western Australia) ( [84], Figure 19a); [85] (Figure 9d), as discussed below.…”

“…Dendrolitic cones in geothermal hot springs of California, suggested as a modern analog for dendrolites in [54], were described as having "mm-scale protrusions upon them, creating an arborescent appearance" ( [54], Figure 1d). They commonly lacked laminations or other internal structures ( [54], Figure 3a) (Figure 9b). Composed of interlocking filaments, the main tuft-constructing organisms were Phormidium, Leptolyngbya, and Leptospira, whereas mats at the base and adjacent to the dendrolitic cones were enriched in Synechococcus.…”

“…The term "dendrolite" was rarely used when describing these travertine deposits [53]. Dendrolitic branching cones, which could be called shrubs, were recently described in a geothermal pool [54]. Detailed microbiological characterization of these structures supported the authors' hypothesis that biological activity may be a controlling factor in their formation.…”

Probing the geological source and biological fate of hydrogen in Yellowstone hot springs

“…PCR amplification and sequencing of small subunit rRNA genes was conducted as previously described (Colman et al ., ; Bradley et al ., ) and sequences were processed following previously described protocols (Schloss et al ., ; Lindsay et al ., ). For detailed methods see supplemental materials.…”

Microscale Biosignatures and Abiotic Mineral Authigenesis in Little Hot Creek, California

“…The precipitates occur in close association with microbial mats and biofilms in the spring. The phage community structure, geochemistry, isotopic characteristics, mineralogy, microbial community of LHC sediments, and microbial mat structures have been described in some detail previously ( Breitbart et al, 2004 ; Vick et al, 2010 ; Bradley et al, 2017 ; Wilmeth et al, in review). The mineral precipitate microbial communities, physical structure, and mineralogy have not been characterized, affording an opportunity to investigate the generation of silica- and carbonate-based biosignatures.…”