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Molecular dynamics simulations were performed to characterize reaction products, resulting from solar wind irradiation, namely, H + , of methane and methane-water ices. In our approach, we used seven 0.829 keV H + (total energy of 5.8 keV), with a velocity of 400 km/s, to hit the icy surface simultaneously, and we repeated this process multiple times to simulate continuous irradiation while quenching the ice to 15 K after each irradiation to prevent excessive heating and sublimation. Our simulations produced complex organic molecules previously obtained in laboratory experiments. For methane ice, molecules containing two carbons were predominant, with ethane and ethyl radicals being the most abundant, followed by ethylene, vinyl radical, and acetylene. Hydrocarbons containing three carbons (e.g., propane, propene, and propyl) were minor products, and only a few molecules containing four carbon atoms (e.g., iso-butene, 1-methylpropylidene, and 2-buten-2-yl) formed. Products that can be formed from the reaction of 1-3 impact fragmentation events, ethane, ethyl radical, and ethylene, monotonically increased over time, while products of 3 or more impact fragmentation events, vinyl, propane, and acetylene, formed over longer time scales. The number of methane complexes decreased over time. For a methane/water (1:1) ice mixture, most of the products consisted of methyl-water complexes, and their number increased with time. All the other oxygenated and nonoxygenated products formed in small amounts due to the water solvation of radicals. For a methane/water (4:1) ice mixture, the methyl-water complexes constituted 45% of the total products, with oxygenated and nonoxygenated products being formed in almost equal amounts. For methane-water ices, the proportions of alkanes, alkenes, and alkynes were very similar to those of pure methane. Dimethyl ether and ethanol formed for both 1:1 and 4:1 methane-water ices., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Giant exoplanets orbiting close to their host stars are unlikely to have formed in their present configurations 1 . These 'hot Jupiter' planets are instead thought to have migrated inward from beyond the ice line and several viable migration channels have been proposed, including eccentricity excitation through angular-momentum exchange with a third body followed by tidally driven orbital circularization 2,3 . The discovery of the extremely eccentric (e = 0.93) giant exoplanet HD 80606 b (ref.  4 ) provided observational evidence that hot Jupiters may have formed through this high-eccentricity tidal-migration pathway 5 . However, no similar hot-Jupiter progenitors have been found and simulations predict that one factor affecting the efficacy of this mechanism is exoplanet mass, as low-mass planets are more likely to be tidally disrupted during periastron passage 6-8 . Here we present spectroscopic and photometric observations of TIC 241249530 b, a high-mass, transiting warm Jupiter with an extreme orbital eccentricity of e = 0.94. The orbit of TIC 241249530 b is consistent with a history of eccentricity oscillations and a future tidal circularization trajectory. Our analysis of the mass and eccentricity distributions of the transiting-warm-Jupiter population further reveals a correlation between high mass and high eccentricity., (© 2024. The Author(s).)

Lake Untersee, a lake in Antarctica that is perennially covered with ice, is home to unique microbial structures that are not lithified. We have evaluated the structure of the community and its metabolic potential across the pigmented upper layers and the sediment-enriched deeper layers in these pinnacle and cone-shaped microbial structures using metagenomics. These microbial structures are inhabited by distinct communities. The upper layers of the cone-shaped structures have a higher abundance of the cyanobacterial MAG Microcoleus, while the pinnacle-shaped structures have a higher abundance of Elainellacea MAG. This suggests that cyanobacteria influence the morphologies of the mats. We identified stark contrasts in the composition of the community and its metabolic potential between the upper and lower layers of the mat. The upper layers of the mat, which receive light, have an increased abundance of photosynthetic pathways. In contrast, the lower layer has an increased abundance of heterotrophic pathways. Our results also showed that Lake Untersee is the first Antarctic lake with a substantial presence of ammonia-oxidizing Nitrospiracea and amoA genes. The genomic capacity for recycling biological molecules was prevalent across metagenome-assembled genomes (MAGs) that cover 19 phyla. This highlights the importance of nutrient scavenging in ultra-oligotrophic environments. Overall, our study provides new insights into the formation of microbial structures and the potential metabolic complexity of Antarctic laminated microbial mats. These mats are important environments for biodiversity that drives biogeochemical cycling in polar deserts., (© 2024 The Author(s). Environmental Microbiology published by John Wiley & Sons Ltd.)

Here we present the current state of knowledge on the long-term evolution of Saturn's moon system due to tides within Saturn. First we provide some background on tidal evolution, orbital resonances and satellite tides. Then we address in detail some of the present and past orbital resonances between Saturn's moons (including the Enceladus-Dione and Titan-Hyperion resonances) and what they can tell us about the evolution of the system. We also present the current state of knowledge on the spin-axis dynamics of Saturn: we discuss arguments for a (past or current) secular resonance of Saturn's spin precession with planetary orbits, and explain the links of this resonance to the tidal evolution of Titan and a possible recent cataclysm in the Saturnian system. We also address how the moons' orbital evolution, including resonances, affects the evolution of their interiors. Finally, we summarize the state of knowledge about the Saturnian system's long-term evolution and discuss prospects for future progress., Competing Interests: Competing InterestsThe authors have no relevant financial or non-financial interests to disclose., (© The Author(s) 2024.)

The low organic matter content in the hyperarid core of the Atacama Desert, together with abrupt temperature shifts and high ultraviolet radiation at its surface, makes this region one of the best terrestrial analogs of Mars and one of the best scenarios for testing instrumentation devoted to in situ planetary exploration. We have operated remotely and autonomously the SOLID-LDChip (Signs of Life Detector-Life Detector Chip), an antibody microarray-based sensor instrument, as part of a rover payload during the 2019 NASA Atacama Rover Astrobiology Drilling Studies (ARADS) Mars drilling simulation campaign. A robotic arm collected drilled cuttings down to 80 cm depth and loaded SOLID to process and assay them with LDChip for searching for molecular biomarkers. A remote science team received and analyzed telemetry data and LDChip results. The data revealed the presence of microbial markers from Proteobacteria, Acidobacteria, Bacteroidetes, Actinobacteria, Firmicutes, and Cyanobacteria to be relatively more abundant in the middle layer (40-50 cm). In addition, the detection of several proteins from nitrogen metabolism indicates a pivotal role in the system. These findings were corroborated and complemented on "returned samples" to the lab by a comprehensive analysis that included DNA sequencing, metaproteomics, and a metabolic reconstruction of the sampled area. Altogether, the results describe a relatively complex microbial community with members capable of nitrogen fixation and denitrification, sulfur oxidation and reduction, or triggering oxidative stress responses, among other traits. This remote operation demonstrated the high maturity of SOLID-LDChip as a powerful tool for remote in situ life detection for future missions in the Solar System.

Forty years ago, it was proposed that gas-phase organic chemistry in the interstellar medium can be initiated by the methyl cation CH 3 + (refs. 1-3 ), but so far it has not been observed outside the Solar System 4,5 . Alternative routes involving processes on grain surfaces have been invoked 6,7 . Here we report James Webb Space Telescope observations of CH 3 + in a protoplanetary disk in the Orion star-forming region. We find that gas-phase organic chemistry is activated by ultraviolet irradiation., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

We investigate the water sources for a perennial spring, "Little Black Pond," located at Expedition Fiord, Axel Heiberg Island in the Canadian High Arctic based on dissolved gases. We measured the dissolved O2 in the likely sources Phantom Lake and Astro Lake and the composition of noble gases (3He/4He, 4He, Ne,36Ar, 40Ar, Kr, Xe), N2, O2, CO2, H2S, CH4, and tritium dissolved in the outflow water and bubbles emanating from the spring. The spring is associated with gypsum-anhydrite piercement structures and occurs in a region of thick, continuous permafrost (400-600 m). The water columns in Phantom and Astro lakes are uniform and saturated with O2. The high salinity of the water emanating from the spring, about twice sea water, affects the gas solubility. Oxygen in the water and bubbles is below the detection limit. The N2/Ar ratio in the bubbles and the salty water is 89.9 and 40, respectively, and the relative ratios of the noble gases, with the exception of Neon, are consistent with air dissolved in lake water mixed with air trapped in glacier bubbles as the source of the gases. The Ne/Ar ratio is ~62% of the air value. Our results indicate that about half (0.47±0.1) of the spring water derives from the lakes and the other half from subglacial melt. The tritium and helium results indicate that the groundwater residence time is over 70 years and could be thousands of years., Competing Interests: The authors have declared that no competing interests exist., (Copyright: This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.)

Sand mobilized by wind forms decimeter-scale impact ripples and decameter-scale or larger dunes on Earth and Mars. In addition to those two bedform scales, orbital and in situ images revealed a third distinct class of larger meter-scale ripples on Mars. Since their discovery, two main hypotheses have been proposed to explain the formation of large martian ripples-that they originate from the growth in wavelength and height of decimeter-scale ripples or that they arise from the same hydrodynamic instability as windblown dunes or subaqueous bedforms instead. Here we provide evidence that large martian ripples form from the same hydrodynamic instability as windblown dunes and subaqueous ripples. Using an artificial neural network, we characterize the morphometrics of over a million isolated barchan dunes on Mars and analyze how their size and shape vary across Mars' surface. We find that the size of Mars' smallest dunes decreases with increasing atmospheric density with a power-law exponent predicted by hydrodynamic theory, similarly to meter-size ripples, tightly bounding a forbidden range in bedform sizes. Our results provide key evidence for a unifying model for the formation of subaqueous and windblown bedforms on planetary surfaces, offering a new quantitative tool to decipher Mars' atmospheric evolution., (© 2022. The Author(s).)

The Martian highlands contain Noachian-aged areally-extensive (>225 km 2 ) bedrock exposures that have been mapped using thermal and visible imaging datasets. Given their age, crater density and impact gardening should have led to the formation of decameter scale layers of regolith that would overlie and bury these outcrops if composed of competent materials like basaltic lavas. However, many of these regions lack thick regolith layers and show clear exposures of bedrock materials with elevated thermal inertia values compared to the global average. Hypothesized reasons for the lack of regolith include: (a) relatively weaker material properties than lavas, where friable materials are comminuted and deflated during wind erosion, (b) long-term protection from regolith development through burial and later exhumation through one or more surface processes, and (c) spatially concentrated aeolian erosion and wind energetics on well-lithified basaltic substrates. To test the third hypothesis, we used the Mars Regional Atmospheric Modeling System to calculate wind erosive strength at 10 regions throughout the Martian highlands and compared it to their thermophysical properties by using thermal infrared data derived from the Thermal Emission Spectrometer to understand the effect that Amazonian mesoscale wind patterns may have on the exposure of bedrock. We also investigated the effect of planet obliquity, Ls of perihelion, and atmospheric mean pressure on wind erosion potential. We found no evidence for increased aeolian activity over bedrock-containing regions relative to surrounding terrains, including at the mafic floor unit at Jezero crater (Máaz formation), supporting the first or second hypotheses for these regions., (© 2022. The Authors.)

We present a laboratory study of the polycyclic aromatic hydrocarbon (PAH) anthracoronene (AntCor, C 36 H 18 ) in simulated interstellar ices in order to determine its possible contribution to the broad infrared absorption bands in the 5-8 μm wavelength interval. The Fourier transform infrared (FTIR) spectrum of AntCor, codeposited with water ice, was collected. The FTIR spectrum of the sample irradiated with ultraviolet photons was also collected. Unirradiated and UV-irradiated AntCor embedded in water ice have not been studied before; therefore, the molecule's band positions and intensities were compared to published data on AntCor in an argon matrix and theoretical calculations (DFT), as well as the published results of its parent molecules, coronene and anthracene, in water ice. The experimental band strengths for unirradiated AntCor exhibit variability as a function of PAH:H 2 O concentration, with two distinct groupings of band intensities. AntCor clustering occurs for all concentrations and has a significant effect on PAH degradation rates and photoproduct variability. Near-IR spectra of irradiated AntCor samples show that AntCor + production increases as the concentration of AntCor in water ice decreases. Photoproduct bands are assigned to AntCor + , cationic alcohols, protonated AntCor, and ketones. We report the rate constants of the photoproduct production for the 1:1280 AntCor:H 2 O concentration. CO 2 production from AntCor is much less than what was previously reported for Ant and Cor and exhibits two distinct regimes as a function of AntCor:H 2 O concentration. The contribution of AntCor photoproducts to astronomical spectra can be estimated by comparison with the observed intensities in the 7.4-8.0 μm range., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

The climatology of baroclinic waves in the northern hemisphere of Mars is investigated through analysis of observations by the infrared sounders on Mars Reconnaissance Orbiter (MRO) and Mars Global Surveyor (MGS). We focus on the lowest scale height above the surface, where the waves have a large impact on the Martian dust cycle. Profiles retrieved by the MRO Mars Climate Sounder (MCS) rarely reach the lower atmosphere at the season and location of interest. To fill this gap, we turn to observations in the MCS B1 channel (32 microns) when the instrument is viewing the surface. The signature of baroclinic waves appears in these data because of dust-related emission from the lower atmosphere and wave-induced variations of surface temperature. We supplement the MCS data with measurements of temperature at the 610-Pa pressure level from the MGS Thermal Emission Spectrometer (TES). Both data sets provide systematic coverage in latitude and longitude at two local times. Characteristics of baroclinic waves are derived through analysis of observations with a combined duration of about 8 Mars years. Basic results include least-squares solutions for wave amplitude and period at zonal wavenumber 1-3; the resolution is 4° in latitude and 14 solar days in time of observation. There is a strong similarity between the baroclinic waves observed by MCS and TES, which confirms the sensitivity of the MCS B1 channel to wave activity at pressures near 610 Pa. In all 8 Mars years, the baroclinic waves exhibit periodic transitions among modes with different zonal wavenumbers and a distinctive solstitial pause. Although the weather in each Mars year is unique in some respects, a composite of results from all years reveals a well-defined wave climatology. At each zonal wavenumber, large amplitudes are restricted to a pair of seasonal windows positioned symmetrically about the winter solstice. The wave-2 mode is strongest in early autumn and near the vernal equinox, whereas wave 3 is the dominant mode in mid-autumn and mid-winter, immediately before and after the solstitial pause. The interaction between baroclinic waves and dust storms is investigated through comparisons with spacecraft measurements of dust opacity. A strong wave-3 mode is often present during the initial growth phase of large, seasonal dust storms, which reflects the importance of wave-generated frontal dust storms in triggering these events. The wave-3 amplitude then decreases rapidly as the dust storm evolves; this occurs routinely in all Mars years considered here in connection with both mid-autumn "A" storms and mid-winter "C" storms. In some years A-storm suppression of the wave-3 mode marks the beginning of the solstitial pause. These results provide a basis for testing and development of Mars General Circulation Models as well as context for interpreting contemporaneous observations, such as spacecraft images of frontal and flushing dust storms.

Wind-formed features are abundant in Oxia Planum (Mars), the landing site of the 2022 ExoMars mission, which shows geological evidence for a past wet environment. Studies of aeolian bedforms at the landing site were focused on assessing the risk for rover trafficability, however their potential in recording climatic fluctuations has not been explored. Here we show that the landing site experienced multiple climatic changes in the Amazonian, which are recorded by an intriguing set of ridges that we interpret as Periodic Bedrock Ridges (PBRs). Clues for a PBR origin result from ridge regularity, defect terminations, and the presence of preserved megaripples detaching from the PBRs. PBR orientation differs from superimposed transverse aeolian ridges pointing toward a major change in wind regime. Our results provide constrains on PBR formation mechanisms and offer indications on paleo winds that will be crucial for understanding the landing site geology., (© 2021. The Authors.)

On Mars, seasonal martian flow features known as recurring slope lineae (RSL) are prevalent on sun-facing slopes and are associated with salts. On Earth, subsurface interactions of gypsum with chlorides and oxychlorine salts wreak havoc: instigating sinkholes, cave collapse, debris flows, and upheave. Here, we illustrate (i) the disruptive potential of sulfate-chloride reactions in laboratory soil crust experiments, (ii) the formation of thin films of mixed ice-liquid water "slush" at -40° to -20°C on salty Mars analog grains, (iii) how mixtures of sulfates and chlorine salts affect their solubilities in low-temperature environments, and (iv) how these salt brines could be contributing to RSL formation on Mars. Our results demonstrate that interactions of sulfates and chlorine salts in fine-grained soils on Mars could absorb water, expand, deliquesce, cause subsidence, form crusts, disrupt surfaces, and ultimately produce landslides after dust loading on these unstable surfaces., (Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

The role of minerals in the events that led to the origin of life is discussed with regard to (1) their catalytic role for the formation of RNA-like oligomers from their monomers and (2) their protective role for organic molecules formed in space that were delivered to planetary surfaces. Results obtained in the laboratory demonstrate that minerals do catalyze the oligomerization of ribonucleic acid (RNA) monomers to produce short RNA chains. Furthermore, and more importantly, these synthetic RNA chains formed by mineral catalysis serve as a template for the formation of complementary RNA chains, which is a significant finding that demonstrates the role of minerals in the origin of life. Simulation experiments run under Mars-like conditions have also shown that Mars analog minerals can shield the precursors of RNA and proteins against the harmful effects of UV and gamma radiation at the martian surface and 5 cm below the surface.

Antarctic perennially ice-covered lakes provide a stable low-disturbance environment where complex microbially mediated structures can grow. Lake Untersee, an ultra-oligotrophic lake in East Antarctica, has the lake floor covered in benthic microbial mat communities, where laminated organo-sedimentary structures form with three distinct, sympatric morphologies: small, elongated cuspate pinnacles, large complex cones and flat mats. We examined the diversity of prokaryotes and eukaryotes in pinnacles, cones and flat microbial mats using high-throughput sequencing of 16S and 18S rRNA genes and assessed how microbial composition may underpin the formation of these distinct macroscopic mat morphologies under the same environmental conditions. Our analysis identified distinct clustering of microbial communities according to mat morphology. The prokaryotic communities were dominated by Cyanobacteria, Proteobacteria, Verrucomicrobia, Planctomycetes, and Actinobacteria. While filamentous Tychonema cyanobacteria were common in all mat types, Leptolyngbya showed an increased relative abundance in the pinnacle structures only. Our study provides the first report of the eukaryotic community structure of Lake Untersee benthic mats, which was dominated by Ciliophora, Chlorophyta, Fungi, Cercozoa, and Discicristata. The eukaryote richness was lower than for prokaryote assemblages and no distinct clustering was observed between mat morphologies. These findings suggest that cyanobacterial assemblages and potentially other bacteria and eukaryotes may influence structure morphogenesis, allowing distinct structures to form across a small spatial scale., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2020 Greco, Andersen, Hawes, Bowles, Yallop, Barker and Jungblut.)

To understand the physiological changes that occur in response to spaceflight, mice are transported to the International Space Station (ISS) and housed for variable periods of time before euthanasia on-orbit or return to Earth. Sample collection under such difficult conditions introduces confounding factors that need to be identified and addressed. We found large changes in the transcriptome of mouse tissues dissected and preserved on-orbit compared with tissues from mice euthanized on-orbit, preserved, and dissected after return to Earth. Changes due to preservation method eclipsed those between flight and ground samples, making it difficult to identify spaceflight-specific changes. Follow-on experiments to interrogate the roles of euthanasia methods, tissue and carcass preservation protocols, and library preparation methods suggested that differences due to preservation protocols are exacerbated when coupled with polyA selection. This has important implications for the interpretation of existing datasets and the design of future experiments., Competing Interests: The authors declare no competing interests.

The origin of diamonds in ureilite meteorites is a timely topic in planetary geology as recent studies have proposed their formation at static pressures >20 GPa in a large planetary body, like diamonds formed deep within Earth's mantle. We investigated fragments of three diamond-bearing ureilites (two from the Almahata Sitta polymict ureilite and one from the NWA 7983 main group ureilite). In NWA 7983 we found an intimate association of large monocrystalline diamonds (up to at least 100 µm), nanodiamonds, nanographite, and nanometric grains of metallic iron, cohenite, troilite, and likely schreibersite. The diamonds show a striking texture pseudomorphing inferred original graphite laths. The silicates in NWA 7983 record a high degree of shock metamorphism. The coexistence of large monocrystalline diamonds and nanodiamonds in a highly shocked ureilite can be explained by catalyzed transformation from graphite during an impact shock event characterized by peak pressures possibly as low as 15 GPa for relatively long duration (on the order of 4 to 5 s). The formation of "large" (as opposed to nano) diamond crystals could have been enhanced by the catalytic effect of metallic Fe-Ni-C liquid coexisting with graphite during this shock event. We found no evidence that formation of micrometer(s)-sized diamonds or associated Fe-S-P phases in ureilites require high static pressures and long growth times, which makes it unlikely that any of the diamonds in ureilites formed in bodies as large as Mars or Mercury., Competing Interests: The authors declare no competing interest., (Copyright © 2020 the Author(s). Published by PNAS.)

Perennially ice-covered lakes that host benthic microbial ecosystems are present in many regions of Antarctica. Lake Untersee is an ultra-oligotrophic lake that is substantially different from any other lakes on the continent as it does not develop a seasonal moat and therefore shares similarities to sub-glacial lakes where they are sealed to the atmosphere. Here, we determine the source of major solutes and carbon to Lake Untersee, evaluate the carbon cycling and assess the metabolic functioning of microbial mats using an isotope geochemistry approach. The findings suggest that the glacial meltwater recharging the closed-basin and well-sealed Lake Untersee largely determines the major solute chemistry of the oxic water column with plagioclase and alumino-silicate weathering contributing < 5% of the Ca 2+ -Na + solutes to the lake. The TIC concentration in the lake is very low and is sourced from melting of glacial ice and direct release of occluded CO 2 gases into the water column. The comparison of δ 13 C TIC of the oxic lake waters with the δ 13 C in the top microbial mat layer show no fractionation due to non-discriminating photosynthetic fixation of HCO 3 - in the high pH and carbon-starved water. The 14 C results indicate that phototrophs are also fixing respired CO 2 from heterotrophic metabolism of the underlying microbial mats layers. The findings provide insights into the development of collaboration in carbon partitioning within the microbial mats to support their growth in a carbon-starved ecosystem.

The outer Solar System object (486958) Arrokoth (provisional designation 2014 MU 69 ) has been largely undisturbed since its formation. We studied its surface composition using data collected by the New Horizons spacecraft. Methanol ice is present along with organic material, which may have formed through irradiation of simple molecules. Water ice was not detected. This composition indicates hydrogenation of carbon monoxide-rich ice and/or energetic processing of methane condensed on water ice grains in the cold, outer edge of the early Solar System. There are only small regional variations in color and spectra across the surface, which suggests that Arrokoth formed from a homogeneous or well-mixed reservoir of solids. Microwave thermal emission from the winter night side is consistent with a mean brightness temperature of 29 ± 5 kelvin., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

The Atacama Desert, the oldest and driest desert on Earth, displays significant rains only once per decade. To investigate how microbial communities take advantage of these sporadic wet events, we carried out a geomicrobiological study a few days after a heavy rain event in 2015. Different physicochemical and microbial community analyses were conducted on samples collected from playas and an alluvial fan from surface, 10, 20, 50, and 80 cm depth. Gravimetric moisture content peaks were measured in 10 and 20 cm depth samples (from 1.65 to 4.1% w/w maximum values) while, in general, main anions such as chloride, nitrate, and sulfate concentrations increased with depth, with maximum values of 13-1,125; 168-10,109; and 9,904-30,952 ppm, respectively. Small organic anions such as formate and acetate had maximum concentrations from 2.61 to 3.44 ppm and 6.73 to 28.75 ppm, respectively. Microbial diversity inferred from DNA analysis showed Actinobacteria and Alphaproteobacteria as the most abundant and widespread bacterial taxa among the samples, followed by Chloroflexi and Firmicutes at specific sites. Archaea were mainly dominated by Nitrososphaerales, Methanobacteria, with the detection of other groups such as Halobacteria. Metaproteomics showed a high and even distribution of proteins involved in primary metabolic processes such as energy production and biosynthetic pathways, and a limited but remarkable presence of proteins related to resistance to environmental stressors such as radiation, oxidation, or desiccation. The results indicated that extra humidity in the system allows the microbial community to repair, and prepare for the upcoming hyperarid period. Additionally, it supplies biomarkers to the medium whose preservation potential could be high under strong desiccation conditions and relevant for planetary exploration.

Here, we report the draft genome sequence for a new putative genus and species in the Methanoregulaceae family, whose members are generally slow-growing rod-shaped or coccoid methanogenic archaea. The information on this sediment-dwelling organism sheds light on the prokaryotes inhabiting isolated, deep, and extremely cold methane-rich environments., (Copyright © 2019 Wagner et al.)

Gradients generated in hydrothermal systems provide a significant source of free energy for chemosynthetic life and may play a role in present-day habitability on ocean worlds. Electron/proton/ion gradients, particularly in the context of hydrothermal chimney structures, may also be relevant to the origins of life on Earth. Hydrothermal vents are similar in some ways to typical fuel cell devices: redox/pH gradients between seawater and hydrothermal fluid are analogous to the fuel cell oxidant and fuel reservoirs; the porous chimney wall is analogous to a separator or ion-exchange membrane and is also a conductive path for electrons; and the hydrothermal minerals are analogous to electrode catalysts. The modular and scalable characteristics of fuel cell systems make for a convenient planetary geology test bed in which geologically relevant components may be assembled and investigated in a controlled simulation environment. We have performed fuel cell experiments and electrochemical studies to better understand the catalytic potential of seafloor minerals and vent chimneys, using samples from a black smoker vent chimney as an initial demonstration. In a fuel cell with Na + -conducting Nafion ® membranes and liquid fuel/oxidant reservoirs (simulating the vent environment), the black smoker mineral catalyst in the membrane electrode assembly was effective in reducing O 2 and oxidizing sulfide. In a H 2 /O 2 polymer electrolyte membrane (PEM) fuel cell with H + -conducting Nafion membranes, the black smoker catalyst was effective in reducing O 2 but not in oxidizing H 2 . These fuel cell experiments accurately simulated the redox reactions that could occur in a geological setting with this particular catalyst, and also tested whether the minerals are sufficiently active to replace a commercial fuel cell catalyst. Similar experiments with other geocatalysts could be utilized to test which redox reactions could be driven in other hydrothermal systems, including hypothesized vent systems on other worlds.

This study describes microbial community compositions, and various cold-responsive stress genes, encompassing cold-induced proteins (CIPs) and cold-associated general stress-responsive proteins (CASPs) in selected Antarctic lake water, sediment, and soil metagenomes. Overall, Proteobacteria and Bacteroidetes were the major taxa in all metagenomes. Prochlorococcus and Thiomicrospira were highly abundant in waters, while Myxococcus , Anaeromyxobacter , Haliangium, and Gloeobacter were dominant in the soil and lake sediment metagenomes. Among CIPs, genes necessary for DNA replication, translation initiation, and transcription termination were highly abundant in all metagenomes. However, genes for fatty acid desaturase (FAD) and trehalose synthase (TS) were common in the soil and lake sediment metagenomes. Interestingly, the Lake Untersee water and sediment metagenome samples contained histone-like nucleoid structuring protein (H-NS) and all genes for CIPs. As for the CASPs, high abundances of a wide range of genes for cryo- and osmo-protectants (glutamate, glycine, choline, and betaine) were identified in all metagenomes. However, genes for exopolysaccharide biosynthesis were dominant in Lake Untersee water, sediment, and other soil metagenomes. The results from this study indicate that although diverse microbial communities are present in various metagenomes, they share common cold-responsive stress genes necessary for their survival and sustenance in the extreme Antarctic conditions.

Dryness is one of the main environmental challenges to microbial survival. Understanding the threshold of microbial tolerance to extreme dryness is relevant to better constrain the environmental limits of life on Earth and critically evaluate long-term habitability models of Mars. Biomolecular proxies for microbial adaptation and growth were measured in Mars-like hyperarid surface soils in the Atacama Desert that experience only a few millimeters of precipitation per decade, and in biologically active soils a few hundred kilometers away that experience two- to fivefold more precipitation. Diversity and abundance of lipids and other biomolecules decreased with increasing dryness. Cyclopropane fatty acids (CFAs), which are indicative of adaptive response to environmental stress and growth in bacteria, were only detected in the wetter surface soils. The ratio of trans to cis isomers of an unsaturated fatty acid, another bacterial stress indicator, decreased with increasingly dry conditions. Aspartic acid racemization ratios increased from 0.01 in the wetter soils to 0.1 in the driest soils, which is indicative of racemization rates comparable to de novo biosynthesis over long timescales (∼10,000 years). The content and integrity of stress proteins profiled by immunoassays were additional indicators that biomass in the driest soils is not recycled at significant levels. Together, our results point to minimal or no in situ microbial growth in the driest surface soils of the Atacama, and any metabolic activity is likely to be basal for cellular repair and maintenance only. Our data add to a growing body of evidence that the driest Atacama surface soils represent a threshold for long-term habitability (i.e., growth and reproduction). These results place constraints on the potential for extant life on the surface of Mars, which is 100-1000 times drier than the driest regions in the Atacama. Key Words: Atacama Desert-Dryness-Growth-Habitability-Biomarker-Mars. Astrobiology 18, 955-966.

The matrix-isolated mid-IR (MIR) spectrum of neutral and ionized anthracoronene (C 36 H 18 , AnthCor) in argon has been measured experimentally, compared to the spectrum of its parent molecules, coronene and anthracene, and analyzed by comparison to a theoretical spectrum computed using density functional theory (DFT). The experimental and theoretical band positions generally agree within 0-10 cm -1 . Anthracoronene exhibits extremely intense cation and anion bands around 1330 and 1318 cm -1 . The intensity of these two bands approaches what is traditionally observed over the entire 1000-1600 cm -1 range for a typical PAH cation or anion. The matrix-isolated near-IR (NIR) through overlap region (OVR) spectrum of ionized AnthCor in argon has been reported for the first time and compared to the spectrum of its parent molecules, coronene and anthracene. The spectrum of AnthCor contains a very strong electronic transition around 6175 cm -1 , placing it outside the range of the electronic transitions typically observed for PAHs. Anthracoronene is one of the few PAHs studied to date which has exhibited the formation of anions upon UV photolysis.

Laguna Negra and Lo Encañado are two oligotrophic Andean lakes forming part of the system fed by meltwater from distinct glacial tongues of the Echaurren glacier in central Chile, which is in a recession period. The recent increase in temperature and decline in precipitation have led to an increase of glacial meltwater and sediments entering these lakes. Although the lacustrine systems are also hydrogeologically connected, the limnology of the lakes is strongly controlled by the surface processes related to the respective sub-watersheds and hydrology. Watershed characteristics (area and length, slope, lithology, resistance to erosion, among others) affect the chemical and physical characteristics of both lakes (e.g., nutrient concentration and turbidity). We studied physical and chemical variables and performed 16S rRNA amplicon sequencing to determine the specific microbial signature of the lakes. The transparency, temperature, turbidity and concentrations of chlorophyll-a, dissolved organic matter, nutrients and the total number of cells, revealed the different status of both lakes at the time of sampling. The predominant bacterial groups in both lakes were Proteobacteria, Verrucomicrobia, and Bacteroidetes. Interestingly, the contribution of phototrophs was significantly higher in LN compared to LE (13 and 4% respectively) and the major fraction corresponded to Anoxygenic Phototrophs (AP) represented by Chloroflexi, Alpha, and Betaproteobacteria. Multivariate analyses showed that the nutrient levels and the light availability of both lakes, which finally depend on the hydrological characteristics of the respective watersheds, explain the differential community composition/function. The abundance of a diverse photoheterotrophic bacterioplankton community suggests that the ability to utilize solar energy along with organic and inorganic substrates is a key function in these oligotrophic mountain lakes.

We have investigated the mid-infrared spectral characteristics of a series of polycyclic aromatic hydrocarbons (PAHs) with straight edges and containing an even or odd number of carbons using density functional theory (DFT). For several even and odd-carbon PAHs, the 8.6/6.2 and 7.6/6.2 intensity ratios computed in emission after the absorption of a 8 eV photon match the observed ratios obtained for three reflection nebulae (RNe), namely NGC 1333, NGC 7023, and NGC 2023. Odd-carbon PAHs are favored, particularly for NGC 1333. Both cations and anions are present with the cations being predominant. Relevant PAHs span sizes ranging from 46 to 103-113 carbons for NGC 7023 and NGC 2023 and from 38 to 127 carbons for NGC 1333 and have symmetries ranging from D 2 h to C s . Our work suggests that even and odd-carbon PAHs with straight edges are viable candidates for the PAH emission seen towards irradiated Photo-Dissociation Regions (PDRs).

The Mars Science Laboratory rover Curiosity engaged in a monthlong campaign investigating the Bagnold dune field in Gale crater. What represents the first in situ investigation of a dune field on another planet has resulted in a number of discoveries. Collectively, the Curiosity rover team has compiled the most comprehensive survey of any extraterrestrial aeolian system visited to date with results that yield important insights into a number of processes, including sediment transport, bed form morphology and structure, chemical and physical composition of aeolian sand, and wind regime characteristics. These findings and more are provided in detail by the JGR-Planets Special Issue Curiosity's Bagnold Dunes Campaign, Phase I.

This work presents the photochemistry of ultraviolet (UV) irradiated coronene in water ices at 15 K, studied using mid-infrared Fourier transform (FTIR) spectroscopy for C 24 H 12 :H 2 O at concentrations of (1:50), (1:150), (1:200), (1:300) and (1:400). Previous UV irradiation studies of anthracene:H 2 O, pyrene:H 2 O and benzo[ghi]perylene:H 2 O ices at 15 K have shown that aromatic alcohols and ketones, as well as CO 2 and H 2 CO are formed at very low temperatures. Like-wise, here, in addition to the coronene cation, hydroxy-, keto-, and protonated coronene (coronene-H + ) are formed. The rate constants for the decay of neutral coronene and for the formation of photoproducts have been derived. It is shown that PAHs and their UV-induced PAH:H 2 O photoproducts have mid-infrared spectroscopic signatures in the 5-8 μ m region that can contribute to the interstellar ice components described by Boogert et al. (2008) as C1-C5. Our results suggest that oxygenated and hydrogenated PAHs could be in UV-irradiated regions of the ISM where water-rich ices are important.

In this study, using NextGen sequencing of the collective 16S rRNA genes obtained from two sets of samples collected from Lake Obersee, Antarctica, we compared and contrasted two bioinformatics tools, PICRUSt and Tax4Fun. We then developed an R script to assess the taxonomic and predictive functional profiles of the microbial communities within the samples. Taxa such as Pseudoxanthomonas, Planctomycetaceae, Cyanobacteria Subsection III, Nitrosomonadaceae, Leptothrix, and Rhodobacter were exclusively identified by Tax4Fun that uses SILVA database; whereas PICRUSt that uses Greengenes database uniquely identified Pirellulaceae, Gemmatimonadetes A1-B1, Pseudanabaena, Salinibacterium and Sinobacteraceae. Predictive functional profiling of the microbial communities using Tax4Fun and PICRUSt separately revealed common metabolic capabilities, while also showing specific functional IDs not shared between the two approaches. Combining these functional predictions using a customized R script revealed a more inclusive metabolic profile, such as hydrolases, oxidoreductases, transferases; enzymes involved in carbohydrate and amino acid metabolisms; and membrane transport proteins known for nutrient uptake from the surrounding environment. Our results present the first molecular-phylogenetic characterization and predictive functional profiles of the microbial mat communities in Lake Obersee, while demonstrating the efficacy of combining both the taxonomic assignment information and functional IDs using the R script created in this study for a more streamlined evaluation of predictive functional profiles of microbial communities., (Copyright © 2017 Elsevier B.V. All rights reserved.)

In this study, we report the distribution of microbial taxa and their predicted metabolic functions observed in the top (U1), middle (U2), and inner (U3) decadal growth laminae of a unique large conical microbial mat from perennially ice-covered Lake Untersee of East Antarctica, using NextGen sequencing of the 16S rRNA gene and bioinformatics tools. The results showed that the U1 lamina was dominated by cyanobacteria, specifically Phormidium sp., Leptolyngbya sp., and Pseudanabaena sp. The U2 and U3 laminae had high abundances of Actinobacteria, Verrucomicrobia, Proteobacteria, and Bacteroidetes. Closely related taxa within each abundant bacterial taxon found in each lamina were further differentiated at the highest taxonomic resolution using the oligotyping method. PICRUSt analysis, which determines predicted KEGG functional categories from the gene contents and abundances among microbial communities, revealed a high number of sequences belonging to carbon fixation, energy metabolism, cyanophycin, chlorophyll, and photosynthesis proteins in the U1 lamina. The functional predictions of the microbial communities in U2 and U3 represented signal transduction, membrane transport, zinc transport and amino acid-, carbohydrate-, and arsenic- metabolisms. The Nearest Sequenced Taxon Index (NSTI) values processed through PICRUSt were 0.10, 0.13, and 0.11 for U1, U2, and U3 laminae, respectively. These values indicated a close correspondence with the reference microbial genome database, implying high confidence in the predicted metabolic functions of the microbial communities in each lamina. The distribution of microbial taxa observed in each lamina and their predicted metabolic functions provides additional insight into the complex microbial ecosystem at Lake Untersee, and lays the foundation for studies that will enhance our understanding of the mechanisms responsible for the formation of these unique mat structures and their evolutionary significance.

Micro-opto-electro-mechanical systems (MOEMS) Deformable Mirrors (DM) are key components for next generation optical instruments implementing innovative adaptive optics systems, both in existing telescopes and in the future ELTs. Characterizing these components well is critical for next generation instruments. This is done by interferometry, including surface quality measurement in static and dynamical modes, at ambient and in vacuum/cryo. We use a compact cryo-vacuum chamber designed for reaching 10⁻6 mbar and 160 K in front of our custom Michelson interferometer, which is able to measure performance of the DM at actuator/segment level and at the entire mirror level, with a lateral resolution of 2 µm and a sub-nanometer z-resolution. We tested the PTT 111 DM from Iris AO: an array of single crystalline silicon hexagonal mirrors with a pitch of 606 µm, able to move in tip, tilt, and piston (stroke 5⁻7 µm, tilt ±5 mrad). The device could be operated successfully from ambient to 160 K. An additional, mainly focus-like, 500 nm deformation of the entire mirror is measured at 160 K; we were able to recover the best flat in cryo by correcting the focus and local tip-tilts on all segments, reaching 12 nm rms. Finally, the goal of these studies is to test DMs in cryo and vacuum conditions as well as to improve their architecture for stable operation in harsh environments., Competing Interests: The authors declare no conflict of interest.

The matrix-isolated, mid-infrared spectra of seven acridine-based polycyclic aromatic nitrogen heterocycles (PANHs) have been measured and compared to their non-nitrogen containing parent molecule. The acridine species investigated include acridine, benz[a]acridine, benz[c]acridine, dibenz[a,j]acridine, dibenz[c,h]acridine, dibenz[a,h]acridine and dibenz[a,c]acridine. The previously reported results for 1 and 2-azabenz[a]anthracenes are included for comparison. The experimentally determined band frequencies and intensities are compared with their B3LYP/6-31G(d) values. The overall agreement between experimental and theoretical values is good and in line with our previous investigations. Shifts, typically to the blue, are noted for the C-H out-of-plane (CH oop ) motions upon insertion of a nitrogen atom. The formation of a bay region upon addition of additional benzene rings to the anthracene/acridine structure splits the solo hydrogen motions into a bay region solo and an external solo hydrogen, with the bay region solo hydrogen coupling to the quartet hydrogen motions and the external solo hydrogen coupling with the duo hydrogen motions resulting in an extreme decrease in intensity for the CH oop solo hydrogen band when the external hydrogen is replaced by a nitrogen atom. The C-C and C-H in-plane region of this acridine series exhibits the characteristic two fold increase in intensity, noted previously for PANHs. The strong ≈1400cm -1 band, which was identified in the previous PANH study, is noted in several molecular species as well as another strong PANH feature between 1480 and 1515cm -1 for several molecules. The presence of these strong bands appear to be primarily responsible for the two-fold increase in the C-H in-plane region's (1100-1600cm -1 ) intensity. The C-H stretching region can be characterized by contributions from the solo (bay or external), duo and quartet hydrogens, similar to what was observed in the dibenzopolyacene compounds., (Published by Elsevier B.V.)

This paper describes the complete genome sequences and methylome analysis of six psychrotrophic strains isolated from perennially ice-covered Lake Untersee in Antarctica., (Copyright © 2017 Fomenkov et al.)

In the giant-impact hypothesis for lunar origin, the Moon accreted from an equatorial circum-terrestrial disk; however, the current lunar orbital inclination of five degrees requires a subsequent dynamical process that is still unclear. In addition, the giant-impact theory has been challenged by the Moon's unexpectedly Earth-like isotopic composition. Here we show that tidal dissipation due to lunar obliquity was an important effect during the Moon's tidal evolution, and the lunar inclination in the past must have been very large, defying theoretical explanations. We present a tidal evolution model starting with the Moon in an equatorial orbit around an initially fast-spinning, high-obliquity Earth, which is a probable outcome of giant impacts. Using numerical modelling, we show that the solar perturbations on the Moon's orbit naturally induce a large lunar inclination and remove angular momentum from the Earth-Moon system. Our tidal evolution model supports recent high-angular-momentum, giant-impact scenarios to explain the Moon's isotopic composition and provides a new pathway to reach Earth's climatically favourable low obliquity.

A unique feature of Pluto's large satellite Charon is its dark red northern polar cap. Similar colours on Pluto's surface have been attributed to tholin-like organic macromolecules produced by energetic radiation processing of hydrocarbons. The polar location on Charon implicates the temperature extremes that result from Charon's high obliquity and long seasons in the production of this material. The escape of Pluto's atmosphere provides a potential feedstock for a complex chemistry. Gas from Pluto that is transiently cold-trapped and processed at Charon's winter pole was proposed as an explanation for the dark coloration on the basis of an image of Charon's northern hemisphere, but not modelled quantitatively. Here we report images of the southern hemisphere illuminated by Pluto-shine and also images taken during the approach phase that show the northern polar cap over a range of longitudes. We model the surface thermal environment on Charon and the supply and temporary cold-trapping of material escaping from Pluto, as well as the photolytic processing of this material into more complex and less volatile molecules while cold-trapped. The model results are consistent with the proposed mechanism for producing the observed colour pattern on Charon.

Rio Tinto in southern Spain has become of increasing astrobiological significance, in particular for its similarity to environments on early Mars. We present evidence of tubular structures from sampled terraces in the stream bed at the source of the river, as well as ancient, now dry, terraces. This is the first reported finding of tubular structures in this particular environment. We propose that some of these structures could be formed through self-assembly via an abiotic mechanism involving templated precipitation around a fluid jet, a similar mechanism to that commonly found in so-called chemical gardens. Laboratory experiments simulating the formation of self-assembling iron oxyhydroxide tubes via chemical garden/chemobrionic processes form similar structures. Fluid-mechanical scaling analysis demonstrates that the proposed mechanism is plausible. Although the formation of tube structures is not itself a biosignature, the iron mineral oxidation gradients across the tube walls in laboratory and field examples may yield information about energy gradients and potentially habitable environments.

Unlabelled: Advances in planetary and space sciences, astrobiology, and life and cognitive sciences, combined with developments in communication theory, bioneural computing, machine learning, and big data analysis, create new opportunities to explore the probabilistic nature of alien life. Brought together in a multidisciplinary approach, they have the potential to support an integrated and expanded Search for Extraterrestrial Intelligence (SETI (1) ), a search that includes looking for life as we do not know it. This approach will augment the odds of detecting a signal by broadening our understanding of the evolutionary and systemic components in the search for extraterrestrial intelligence (ETI), provide more targets for radio and optical SETI, and identify new ways of decoding and coding messages using universal markers., Key Words: SETI-Astrobiology-Coevolution of Earth and life-Planetary habitability and biosignatures. Astrobiology 16, 661-676., Competing Interests: Author Disclosure Statement The author declares no conflict of interest.

Halite endoliths in the Atacama Desert represent one of the most extreme ecosystems on Earth. Cultivation-independent methods were used to examine the functional adaptations of the microbial consortia inhabiting halite nodules. The community was dominated by haloarchaea and functional analysis attributed most of the autotrophic CO2 fixation to one unique cyanobacterium. The assembled 1.1 Mbp genome of a novel nanohaloarchaeon, Candidatus Nanopetramus SG9, revealed a photoheterotrophic life style and a low median isoelectric point (pI) for all predicted proteins, suggesting a 'salt-in' strategy for osmotic balance. Predicted proteins of the algae identified in the community also had pI distributions similar to 'salt-in' strategists. The Nanopetramus genome contained a unique CRISPR/Cas system with a spacer that matched a partial viral genome from the metagenome. A combination of reference-independent methods identified over 30 complete or near complete viral or proviral genomes with diverse genome structure, genome size, gene content and hosts. Putative hosts included Halobacteriaceae, Nanohaloarchaea and Cyanobacteria. Despite the dependence of the halite community on deliquescence for liquid water availability, this study exposed an ecosystem spanning three phylogenetic domains, containing a large diversity of viruses and predominance of a 'salt-in' strategy to balance the high osmotic pressure of the environment., (© 2016 Society for Applied Microbiology and John Wiley & Sons Ltd.)

The New Horizons spacecraft mapped colors and infrared spectra across the encounter hemispheres of Pluto and Charon. The volatile methane, carbon monoxide, and nitrogen ices that dominate Pluto's surface have complicated spatial distributions resulting from sublimation, condensation, and glacial flow acting over seasonal and geological time scales. Pluto's water ice "bedrock" was also mapped, with isolated outcrops occurring in a variety of settings. Pluto's surface exhibits complex regional color diversity associated with its distinct provinces. Charon's color pattern is simpler, dominated by neutral low latitudes and a reddish northern polar region. Charon's near-infrared spectra reveal highly localized areas with strong ammonia absorption tied to small craters with relatively fresh-appearing impact ejecta., (Copyright © 2016, American Association for the Advancement of Science.)

Janthinobacterium sp. Ant5-2-1, isolated from the Schirmacher Oasis of East Antarctica, produces a purple-violet pigment, manifests diverse energy metabolism abilities, and tolerates cold, ultraviolet radiation, and other environmental stressors. We report here the 6.19-Mb draft genome of strain Ant5-2-1, which will help understand its survival mechanisms in extreme Antarctic ecosystems., (Copyright © 2016 Koo et al.)

One of the fundamental structures of a cell is the membrane. Self-assembling lipid bilayer vesicles can form the membrane of an artificial cell and could also have plausibly assembled prebiotically for the origin of life. Such cell-like structures, that encapsulate some basic subset of the functions of living cells, are important for research to infer the minimum chemistry necessary for a cell, to help understand the origin of life, and to allow the production of useful species in microscopic containers. We show that the encapsulation of TiO2 particles has the potential to provide the basis for an energy transduction system inside vesicles which can be used to drive subsequent chemistry. TiO2 encapsulated in vesicles can be used to produce biochemical species such as NADH. The NADH is formed from NAD(+) reduction and is produced in a form that is able to drive further enzymatic chemistry. This allows us to link a mineral-based, nonbiological photosystem to biochemical reactions. This is a fundamental step toward being able to use this mineral photosystem in a protocell/artificial cell.

The Atacama Desert, northern Chile, is one of the driest deserts on Earth and, as such, a natural laboratory to explore the limits of life and the strategies evolved by microorganisms to adapt to extreme environments. Here we report the exceptional adaptation strategies of chlorophototrophic and eukaryotic algae, and chlorophototrophic and prokaryotic cyanobacteria to the hyperarid and extremely high solar radiation conditions occurring in this desert. Our approach combined several microscopy techniques, spectroscopic analytical methods, and molecular analyses. We found that the major adaptation strategy was to avoid the extreme environmental conditions by colonizing cryptoendolithic, as well as, hypoendolithic habitats within gypsum deposits. The cryptoendolithic colonization occurred a few millimeters beneath the gypsum surface and showed a succession of organized horizons of algae and cyanobacteria, which has never been reported for endolithic microbial communities. The presence of cyanobacteria beneath the algal layer, in close contact with sepiolite inclusions, and their hypoendolithic colonization suggest that occasional liquid water might persist within these sub-microhabitats. We also identified the presence of abundant carotenoids in the upper cryptoendolithic algal habitat and scytonemin in the cyanobacteria hypoendolithic habitat. This study illustrates that successful lithobiontic microbial colonization at the limit for microbial life is the result of a combination of adaptive strategies to avoid excess solar irradiance and extreme evapotranspiration rates, taking advantage of the complex structural and mineralogical characteristics of gypsum deposits-conceptually called "rock's habitable architecture." Additionally, self-protection by synthesis and accumulation of secondary metabolites likely produces a shielding effect that prevents photoinhibition and lethal photooxidative damage to the chlorophototrophs, representing another level of adaptation.