Character, Analysis, and Preservation of Biogenicity in Terrestrial Siliceous Stromatolites from Geothermal Settings

Living to Lithified: Construction and Preservation of Silicified Biomarkers

Whole microorganisms are rarely preserved in the fossil record but actively silicifying environments like hot springs provide an opportunity for microbial preservation, making silicifying environments critical for the study of microbial life through time on Earth and possibly other planetary bodies. Yet, the changes that biosignatures may undergo through lithification and burial remain unconstrained. At Steep Cone Geyser in Yellowstone National Park, we collected microbial material from (1) the living system across the active outflows, (2) the silicified areas adjacent to flows, and (3) lithified and buried material to assess the preservation of biosignatures and their changes across the lithification transect. Five biofabrics, built predominantly by Cyanobacteria Geitlerinema, Pseudanabaenaceae, and Leptolyngbya with some filamentous anoxygenic phototrophs contributions, were identified and tracked from the living system through the process of silicification/lithification. In the living systems, δ30Si values decrease from +0.13‰ in surficial waters to -2‰ in biomat samples, indicating a kinetic isotope effect potentially induced by increased association with actively growing biofabrics. The fatty acids C16:1 and iso-C14:0 and the hydrocarbon C17:0 were disentangled from confounding signals and determined to be reliable lipid biosignatures for living biofabric builders and tenant microorganisms. Builder and tenant microbial biosignatures were linked to specific Cyanobacteria, anoxygenic phototrophs, and heterotrophs, which are prominent members of the living communities. Upon lithification and burial, silicon isotopes of silicified biomass began to re-equilibrate, increasing from δ30Si -2‰ in living biomats to -0.55‰ in lithified samples. Active endolithic microbial communities were identified in lithified samples and were dominated by Cyanobacteria, heterotrophic bacteria, and fungi. Results indicate that distinct microbial communities build and inhabit silicified biofabrics through time and that microbial biosignatures shift over the course of lithification. These findings improve our understanding of how microbial communities silicify, the biomarkers they retain, and transitionary impacts that may occur through lithification and burial.

Mineralogical and microbial characterization of alkali hot spring microbial mats and deposits in Pong Dueat Pa Pae hot spring, Northern Thailand

Hot spring environments encompass broad physicochemical ranges, in which temperature and pH account for crucial factors shaping hot spring microbial community and diversity. However, the presence of photosynthetic microbial mats adjacent to boiling hot spring vents, where fluid temperatures extend beyond photosynthetic capability, questions the microbial profiles and the actual temperatures of such adjacent mats. Therefore, this study aims to characterize thermophilic microbial communities at Pong Dueat Pa Pae hot spring using next-generation sequencing, including investigating hot spring mineralogy. Results suggest that Pong Dueat Pa Pae hot spring precipitates comprise mainly silica which also acts as the main preservative medium for microbial permineralization. Molecular results revealed the presence of cyanobacterial and Chloroflexi species in the thick, orange and green subaerial mats surrounding the vents, suggesting the mats would be at least 30 °C cooler than source vents despite constantly receiving geyser splashes. Bacterial abundance was considerably higher than archaeal (97.9% versus 2.1%). Cyanobacterial (mainly Synechococcus and Leptolygbya) and Chloroflexi species (mainly Roseiflexus) accounted for almost half (40.04%) of the bacterial community, while DHVEG-6 and Thaumarchaeota comprised dominant members (> 90%) of the archaeal fraction. This study updates and provides insights into thermophilic microbial community composition and mineralogy of hot springs in Thailand.

Stromatolitic digitate sinters form under wide-ranging physicochemical conditions with diverse hot spring microbial communities

Digitate siliceous hot spring deposits are a form of biomediated sinter that is relatively common in the Taupo Volcanic Zone (TVZ), New Zealand, and elsewhere on Earth. Such deposits have gained prominence recently because of their morphological similarity to opaline silica rocks of likely hot spring origin found by the Spirit rover on Mars and the consequent implications for potential biosignatures there. Here, we investigate the possible relationship between microbial community composition and morphological diversity among digitate structures from actively forming siliceous hot spring sinters depositing subaerially in shallow discharge channels and around pool rims at several physicochemically distinct geothermal fields in the TVZ. The TVZ digitate sinters range in morphologic subtype from knobby to spicular, and are shown to be microstromatolites that grow under varied pH ranges, temperatures, and water chemistries. Scanning electron microscopy and molecular analyses revealed that TVZ digitate sinters are intimately associated with a diverse array of bacterial, archaeal and eukaryotic micro-organisms, and for most digitate structures the diversity and quantity of prokaryotes was higher than that of eukaryotes. However, microbial community composition was not correlated with morphologic subtypes of digitate sinter, and observations provided limited evidence that pH (acidic versus alkali) affects morphology. Instead, results suggest hydrodynamics may be an important factor influencing variations in morphology, while water chemistry, pH, and temperature are strong drivers of microbial composition and diversity.

Formation and Preservation of Microbial Palisade Fabric in Silica Deposits from El Tatio, Chile

Palisade fabric is a ubiquitous texture of silica sinter found in low temperature (<40°C) regimes of hot spring environments, and it is formed when populations of filamentous microorganisms act as templates for silica polymerization. Although it is known that postdepositional processes such as biological degradation and dewatering can strongly affect preservation of these fabrics, the impact of extreme aridity has so far not been studied in detail. Here, we report a detailed analysis of recently silicified palisade fabrics from a geyser in El Tatio, Chile, tracing the progressive degradation of microorganisms within the silica matrix. This is complemented by heating experiments of natural sinter samples to assess the role of diagenesis. Sheathed cyanobacteria, identified as Leptolyngbya sp., were found to be incorporated into silica sinter by irregular cycles of wetting, evaporation, and mineral precipitation. Transmission electron microscopy analyses revealed that nanometer-sized silica particles are filling the pore space within individual cyanobacterial sheaths, giving rise to their structural rigidity to sustain a palisade fabric framework. Diagenesis experiments further show that the sheaths of the filaments are preferentially preserved relative to the trichomes, and that the amount of water present within the sinter is an important factor for overall preservation during burial. This study confirms that palisade fabrics are efficiently generated in a highly evaporative geothermal field, and that these biosignatures can be most effectively preserved under dry diagenetic conditions.

Terrestrial Hot Spring Systems: Introduction

This report reviews how terrestrial hot spring systems can sustain diverse and abundant microbial communities and preserve their fossil records. Hot springs are dependable water sources, even in arid environments. They deliver reduced chemical species and other solutes to more oxidized surface environments, thereby providing redox energy and nutrients. Spring waters have diverse chemical compositions, and their outflows create thermal gradients and chemical precipitates that sustain diverse microbial communities and entomb their remnants. These environments probably were important habitats for ancient benthic microbial ecosystems, and it has even been postulated that life arose in hydrothermal systems. Thermal spring communities are fossilized in deposits of travertine, siliceous sinter, and iron minerals (among others) that are found throughout the geological record back to the oldest known well-preserved rocks at 3.48 Ga. Very few are known before the Cenozoic, but it is likely that there are many more to be found. They preserve fossils ranging from microbes to trees and macroscopic animals. Features on Mars whose morphological and spectroscopic attributes resemble spring deposits on Earth have been detected in regions where geologic context is consistent with the presence of thermal springs. Such features represent targets in the search for evidence of past life on that planet.

Tracing Biosignature Preservation of Geothermally Silicified Microbial Textures into the Geological Record

New Zealand and Argentine (Late Jurassic-Recent) siliceous hot-spring deposits (sinter) reveal preservation pathways of environmentally controlled, microbe-dominated sedimentary facies over geological time scales. Texturally distinctive, laminated to thinly layered, dense and vertically oriented, microtubular "palisade" fabric is common in low-temperature (<40°C) sinter-apron terraces. In modern hot springs, the dark green to brown, sheathed, photosynthetic cyanobacterium Calothrix spp. (family Rivulariaceae) constructs felted palisade mats in shallow terrace(tte) pools actively accreting opaline silica. The resulting stacked layers of silicified coarse filaments-a stromatolite-are highly porous and readily modified by postdepositional environmental perturbations, secondary silica infill, and diagenetic silica phase mineral transformations (opal-A to quartz). Fossil preservation quality is affected by relative timing of silicification, and later environmental and geological events. A systematic approach was used to characterize palisade fabric in sinters of different ages to refine tools for recognizing biosignatures in extreme environments and to track their long-term preservation pathways into the geological record. Molecular techniques, scanning electron microscopy, Raman spectrometry, X-ray powder diffraction, petrography, and lipid biomarker analyses were applied. Results indicate that microbial communities vary at the micron scale and that early and rapid silicification is paramount to long-term preservation, especially where minimal postdepositional disturbance follows fossilization. Overall, it appears that the most robust biomarkers of fossil microbial activity in hot-spring deposits are their characteristic macro- and microtextures and laser micro-Raman identified carbon. Studies of Phanerozoic geothermal deposits with mineralized microbial components are relevant analogs for Precambrian geobiology because early life is commonly preserved as microbial microfossils and biofilms in silica, some of it hydrothermal in origin. Yet the diagenetic "movie" has already been run. Hence, studying younger sinters of a range of ages provides an opportunity to "play it again" and follow the varied influences on biosignatures into the deep-time geological record.