In situ Detection of Microbial Life in the Deep Biosphere in Igneous Ocean Crust

Investigating Habitability with an Integrated Rock-Climbing Robot and Astrobiology Instrument Suite

A prototype rover carrying an astrobiology payload was developed and deployed at analog field sites to mature generalized system architectures capable of searching for biosignatures in extreme terrain across the Solar System. Specifically, the four-legged Limbed Excursion Mechanical Utility Robot (LEMUR) 3 climbing robot with microspine grippers carried three instruments: a micro-X-ray fluorescence instrument based on the Mars 2020 mission's Planetary Instrument for X-ray Lithochemistry provided elemental chemistry; a deep-ultraviolet fluorescence instrument based on Mars 2020's Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals mapped organics in bacterial communities on opaque substrates; and a near-infrared acousto-optic tunable filter-based point spectrometer identified minerals and organics in the 1.6-3.6 μm range. The rover also carried a light detection and ranging and a color camera for both science and navigation. Combined, this payload detects astrobiologically important classes of rock components (elements, minerals, and organics) in extreme terrain, which, as demonstrated in this work, can reveal a correlation between textural biosignatures and the organics or elements expected to preserve them in a habitable environment. Across >10 field tests, milestones were achieved in instrument operations, autonomous mobility in extreme terrain, and system integration that can inform future planetary science mission architectures. Contributions include (1) system-level demonstration of mock missions to the vertical exposures of Mars lava tube caves and Mars canyon walls, (2) demonstration of multi-instrument integration into a confocal arrangement with surface scanning capabilities, and (3) demonstration of automated focus stacking algorithms for improved signal-to-noise ratios and reduced operation time.

Indigenous Ammonia-Oxidizing Archaea in Oxic Subseafloor Oceanic Crust

Ridge flanks represent the major avenue of chemical and heat exchange between the Earth’s oceans and the lithosphere and are thought to harbor an enormous and understudied biosphere. However, little is known about the diversity and functionality of the crustal biosphere. Here, we report an indigenous community of archaea specialized in ammonia oxidation (i.e., AOA) in the oxic oceanic crust at North Pond. These AOA are the dominant archaea and are likely responsible for most of the cycling taking place in the first step of nitrification, a feasible nitrogen cycling step in the oxic basement. The crustal AOA community structure significantly differs from that in deep ocean water but is similar to that of the community in the overlying sediments in close proximity. This report links the occurrence of AOA to their metabolic activity in the oxic subseafloor crust and suggests that ecological selection and in situ proliferation may shape the microbial community structure in the rocky subsurface.

Oceanic ridge flank systems represent one of the largest and least-explored microbial habitats on Earth. Fundamental ecological questions regarding community activity, recruitment, and succession in this environment remain unanswered. Here, we investigated ammonia-oxidizing archaea (AOA) in the sediment-buried basalts on the oxic and young ridge flank at North Pond, a sediment-filled pond on the western flank of the Mid-Atlantic Ridge, and compared them with those in the overlying sediments and bottom seawater. Nitrification in the North Pond basement is thermodynamically favorable and is supported by a reaction-transport model simulating the dynamics of nitrate in the crustal fluids. Nitrification rate is estimated to account for 6% to 7% of oxygen consumption, which is similar to the ratios found in marine oxic sediments, suggesting that aerobic mineralization of organic matter is the major ammonium source for crustal nitrifiers. Using the archaeal 16S rRNA and amoA genes as phylogenetic markers, we show that AOA, composed solely of Nitrosopumilaceae, are the major archaeal dwellers at North Pond. Phylogenetic analysis reveals that the crustal AOA communities are distinct from those in the bottom seawater and the upper oxic sediments but are similar to those in the basal part of the overlying sediment column, suggesting either similar environmental selection or the dispersal of microbes across the sediment-basement interface. Additionally, quantitative abundance data suggest enrichment of the dominant Nitrosopumilaceae clade (Eta clade) in the basement compared to the seawater. This study explored AOA and their activity in the upper oceanic crust, and our results have ecological implications for the biogeochemical cycling of nitrogen in the crustal subsurface.

IMPORTANCE Ridge flanks represent the major avenue of chemical and heat exchange between the Earth’s oceans and the lithosphere and are thought to harbor an enormous and understudied biosphere. However, little is known about the diversity and functionality of the crustal biosphere. Here, we report an indigenous community of archaea specialized in ammonia oxidation (i.e., AOA) in the oxic oceanic crust at North Pond. These AOA are the dominant archaea and are likely responsible for most of the cycling taking place in the first step of nitrification, a feasible nitrogen cycling step in the oxic basement. The crustal AOA community structure significantly differs from that in deep ocean water but is similar to that of the community in the overlying sediments in close proximity. This report links the occurrence of AOA to their metabolic activity in the oxic subseafloor crust and suggests that ecological selection and in situ proliferation may shape the microbial community structure in the rocky subsurface.

WATSON: In Situ Organic Detection in Subsurface Ice Using Deep-UV Fluorescence Spectroscopy

Terrestrial icy environments have been found to preserve organic material and contain habitable niches for microbial life. The cryosphere of other planetary bodies may therefore also serve as an accessible location to search for signs of life. The Wireline Analysis Tool for the Subsurface Observation of Northern ice sheets (WATSON) is a compact deep-UV fluorescence spectrometer for nondestructive ice borehole analysis and spatial mapping of organics and microbes, intended to address the heterogeneity and low bulk densities of organics and microbial cells in ice. WATSON can be either operated standalone or integrated into a wireline drilling system. We present an overview of the WATSON instrument and results from laboratory experiments intended to determine (i) the sensitivity of WATSON to organic material in a water ice matrix and (ii) the ability to detect organic material under various thicknesses of ice. The results of these experiments show that in bubbled ice the instrument has a depth of penetration of 10 mm and a detection limit of fewer than 300 cells. WATSON incorporates a scanning system that can map the distribution of organics and microbes over a 75 by 25 mm area. WATSON demonstrates a sensitive fluorescence mapping technique for organic and microbial detection in icy environments including terrestrial glaciers and ice sheets, and planetary surfaces including Europa, Enceladus, or the martian polar caps.