Are Large Sulfur Isotope Variations Biosignatures in an Ancient, Impact-Induced Hydrothermal Mars Analog?

Discrepancies have emerged concerning the application of sulfur stable isotope ratios as a biosignature in impact crater paleolakes. The first δS data from Mars at Gale crater display a ∼75‰ range that has been attributed to an abiotic mechanism. Yet biogeochemical studies of ancient environments on Earth generally interpret δS fractionations >21‰ as indicative of a biological origin, and studies of δS at analog impact crater lakes on Earth have followed the same approach.

DNA- and RNA-based bacterial communities and geochemical zonation under changing sediment porewater dynamics on the Aldabra Atoll.

The remote Aldabra Atoll, Seychelles, provides the rare opportunity to study bacterial communities in pristine carbonate sediments across an entire biome. The four sampled sites cover sand with high porewater exchange, bioturbated silt and mud with intermediate exchange, as well as a seasonally and episodically desiccated landlocked pool. As sediments harbour dead cells and environmental DNA alongside live cells, we used bacterial 16S rRNA gene and transcript analysis to distinguish between past and present inhabitants.

Morphogenesis of digitate structures in hot spring silica sinters of the El Tatio geothermal field, Chile.

In silica-rich hot spring environments, internally laminated, digitate sinter deposits are often interpreted as bio-mediated structures. The organic components of microbial communities (cell surfaces, sheaths and extracellular polymeric substances) can act as templates for silica precipitation, therefore influencing digitate sinter morphogenesis. In addition to biologic surface-templating effects, various microenvironmental factors (hydrodynamics, local pH and fluctuating wind patterns) can also influence silica precipitation, and therefore the morphology of resulting digitate sinters.

Nitrogen isotope ratios trace high-pH conditions in a terrestrial Mars analog site.

High-pH alkaline lakes are among the most productive ecosystems on Earth and prime targets in the search for life on Mars; however, a robust proxy for such settings does not yet exist. Nitrogen isotope fractionation resulting from NH volatilization at high pH has the potential to fill this gap. To validate this idea, we analyzed samples from the Nördlinger Ries, a Miocene impact crater lake that displayed pH values up to 9.

Bacterial succession along a sediment porewater gradient at Lake Neusiedl in Austria.

We provide bacterial 16S rRNA community and hydrochemical data from water and sediments of Lake Neusiedl, Austria. The sediments were retrieved at 5 cm intervals from 30-40 cm push cores. The lake water community was recovered by filtration through a 3.

Phylogenetic analysis of a microbialite-forming microbial mat from a hypersaline lake of the Kiritimati atoll, Central Pacific.

On the Kiritimati atoll, several lakes exhibit microbial mat-formation under different hydrochemical conditions. Some of these lakes trigger microbialite formation such as Lake 21, which is an evaporitic, hypersaline lake (salinity of approximately 170‰). Lake 21 is completely covered with a thick multilayered microbial mat.

A cryptoendolithic community in volcanic glass.

Fluorescent in situ hybridization (FISH) and 16S rDNA analysis were used to characterize the endolithic colonization of silica-rich rhyolitic glass (obsidian) in a barren terrestrial volcanic environment in Iceland. The rocks were inhabited by a diverse eubacterial assemblage. In the interior of the rock, we identified cyanobacterial and algal 16S (plastid) sequences and visualized phototrophs by FISH, which demonstrates that molecular methods can be used to characterize phototrophs at the limits of photosynthetically active radiation (PAR).

Metabolic microenvironmental control by photosynthetic biofilms under changing macroenvironmental temperature and pH conditions.

Ex situ microelectrode experiments, using cyanobacterial biofilms from karst water creeks, were conducted under various pH, temperature, and constant-alkalinity conditions to investigate the effects of changing environmental parameters on cyanobacterial photosynthesis-induced calcification. Microenvironmental chemical conditions around calcifying sites were controlled by metabolic activity over a wide range of photosynthesis and respiration rates, with little influence from overlying water conditions. Regardless of overlying water pH levels (from 7.

In situ detection of bacteria in calcified biofilms using FISH and CARD-FISH.

Modified protocols of fluorescence in situ hybridization (FISH) and catalyze reporter deposition fluorescence in situ hybridization (CARD-FISH) were developed in order to detect bacteria in situ in calcified stromatolite biofilms. Smooth, well-preserved thin sections of calcified biofilms (approximately 5 microm thin, vertical sectioning of approximately 1 cm deep) were obtained by cryo-sectioning using the adhesive tape-stabilization technique. A modified hybridization buffer was applied during hybridization to prevent calcite dissolution as well as false binding of oligonucleotide probes to the charged mineral surfaces.