Methods

This study utilised 17 samples of hot spring sinter, mostly from New Zealand, representing a range of different environmental conditions. Representative materials from each sample were prepared for optical microscope examination. The sinter rock materials were cut and polished down to only 30 micrometres thick so that light could pass through the wafer-thin rock. This allowed for petrographic observation of textural relationships between the fossilised microbes and plants that were living in the geothermal waters and the silica precipitate that entombed the organisms in the developing sinter deposit. Petrographically, the textures and features observed under the microscope are linked to the growth of thermophilic microorganisms and relate definitively to the sample’s location along the vent-to-marsh thermal gradient. This therefore tells us about the specific environmental conditions under which the rock formed (Figure 1). Such variable paleoenvironmental conditions affected the growth of hot spring-related microbial life, representative trace elemental biosignatures, and the types of patterns/textures entombed and preserved in the sinter. Characterising these textures and growth patterns is important in understanding how these microbes have been able to flourish in harsh conditions. In addition, all samples had previously been chemically mapped at the Institute for Planetary Materials, Okayama University on an electron microprobe, providing two-dimensional images of the chemical variability in each sample. Electron probe microanalysis (EPMA) measures the concentrations of major elements, which in this case are associated with both microbial life and environmental processes. Elemental concentrations around and within silicified microbial filaments are often well preserved, detailing growth patterns. This research project required processing the EPMA images and comparing them to textural observations from the microscopy I undertook.

Figure 1: Diagram of the schematic cross section of a hot spring/geothermal paleoenvironment [4] with temperature gradients [5], highlighting the differing microbial textures developed across the apron.

Figure 2: Comparison of the EPMA and petrographic imagery of a palisade texture sample. Aluminium is observed to be primarily concentrated around “sheaths” of the course filament laminations and terracettes. These filaments fluctuate in length but are largely uniform, indicative of low-temperature conditions (calothrix/cyanobacteria growth).

Conclusion

Textures, both macroscopic and microscopic, suggest sinter samples in this research likely precipitate from fluids with a temperature range of <40-90°C and mostly from alkali-chloride near-neutral pH thermal waters. These types of thermal fluids characteristically construct large sinter aprons, as observed at Orakei Korako or Wai-O-Tapu, for example. I observed variations in the orientation of silicified microbes through individual samples, with some growing parallel to the flow of thermal water down a hot spring discharge channel, while others grew vertically in shallow hot spring pools. The filaments also varied in thickness, ranging from a few micrometres to over 10 micrometres. Silicified plant material was also present, with some samples having abundant reed and marshy material, along with algal balls and diatoms representing a range of different types of “life”.

Textural evidence of microbes varied depending on their proximity to the vent, forming unique microbially mediated textures within hot spring “sub-environments”, related to their distance from the ancient spring-vent discharge point. Within these textures, there were clear variations in density, thickness, and growth habits of the microbial fossils in the silica, reflecting the type and abundance of microbial/organic material within each sample. Microbial filaments, one of the most common and easily identifiable microbial textures, are interpreted to be fossil cyanobacteria (photosynthesising bacteria), typical of medium to low sinter apron discharge temperatures (Figure 2). The orientation of filaments is indicative of changing fluid flow direction and microbial growth, while the size of the filaments relates to the temperature and water depth during silicification.

Geochemically, it is clear that the high-temperature sinter textures have distinctive characteristics. Sinter from the vent region has dense silica with minor instances of calcium and aluminium in the layers and no clearly identifiable microbes at the rather coarse resolution of optical microscopy. However, it is evident from other studies that these textures are constructed of very finely layered microscopic organic biofilms (Figure 3). In contrast, the microbial filaments evaluated in this study, within the lower temperature domains, are characterised by strong chemical enrichments in aluminium, and sometimes calcium, which are isolated in discrete zones around the filaments. These zones are interpreted to represent the outer “sheath” of the microbe which has been silicified. A recently completed MSc thesis from the University of Auckland [2], focusing on a specific morphology of sinter deposit, also highlights the concentrations of metals in relation to cyanobacteria filament morphology. In both Nersezova’s study [2] and this research, there is a clear sentiment that these “sheaths”, with their durable preservation potential and specific elemental chemical enrichments, may be the best biosignatures to focus on when endeavouring to understand early life in hydrothermal paleoenvironments on Earth and possibly Mars. However, results also suggest that these particular biosignatures may be unique to cyanobacteria; will other microbial species show similar traits?

Figure 3: Comparison of the EPMA and petrographic imagery of a geyserite texture sample. High concentration of silicon is observed throughout the spicule structure with instances of aluminium and calcium, although no definitive microbes. The spicule has a fluctuating density of organic layers with lateral linkages to the surrounding fabric.

Takeaways

· The strongest chemical signature of biological activity is observed around filaments in the low-temperature siliceous hot spring deposits, or sinter.

· High-temperature sinter, which forms close to the vent, has less definitive chemical biosignatures due to difficulties in imaging features at a very fine scale using the methods of this study.

· When exposed to acidic conditions, many of the biosignatures are altered and less apparent.

· This research supports the findings of the previous MSc thesis from the University of Auckland [2], which focused on one particular sinter morphology and found distinct chemical biosignatures physically associated with microbial filaments.