Extreme Environments on Earth: Analogs for Exobiology

Extreme Environments on Earth: Analogs for Exobiology

Faculty: Baross, Delaney, Deming, Frederickson, Gammon, Leigh, Rodrigo, Staley

Background Our basic premise is that life on Earth is the best model we have for life elsewhere. Life here is extraordinarily adaptable: organisms thrive in extremes of temperature, pressure, pH, solute concentrations, dryness, ionizing radiation, and other conditions. Study of life in extreme environments on Earth guides the search for extraterrestrial life. We study Earth's "extremophiles", organisms (especially microbes) that live in the most extreme conditions, both to identify the extreme limits of Earth-like life, and to learn to identify the types and activities of those life forms.

Starting Assumptions and General Hypotheses:

Liquid Water Assuming water is required for organisms' activity and growth, limits of microbial life can largely be defined by the boundary conditions for liquid water. Salinity and hydrostatic pressure determine phase changes: different combinations can expand the "liquid" boundaries substantially, especially at high temperatures.
Temperature, Pressure, and Minerals Hydrothermal fluids venting from Earth's subsurface crust onto the deep seafloor meet the carbon, nitrogen, energy and other mineral requirements for life. They also remain liquid at > 400 C at deep-sea pressures (~200 atm), far hotter than both the known upper limit for pure-culture lab growth of microbes (113 C at <3 atm) and the suspected absolute upper limit (120-150 C). The hypothesis that the upper temperature limit for life is extended significantly by fluid mineral content and hydrostatic pressure remains largely untested. Vent systems are the perfect laboratory.
Survival Envelope > Activity Envelope "Domains" (envelopes) for microbial survival are larger than those for activity and growth. This means we must examine the extremes. Some microbial survival envelopes include freezing, dessication, high levels of ionizing radiation, high and low pH, very low nutrients (e.g., oligotrophic oceans and lakes, subsurface ground waters), and regions without fresh nutrients (e.g., brine pockets in wintertime sea-ice, deeply buried sediments, fluid inclusions in rock).
Survival Strategies We know little about survival strategies of extremophiles. In deep terrestrial basalt formations, microorganisms persist indefinitely without organic carbon from the surface. For thermophiles in deep sub-sea-floor formations, neither responses to initial starvation nor strategies for surviving long-term starvation have been examined. Likewise for microorganisms that survive the dark polar winters (at temperatures down to -26 C) in brine pockets of sea-ice, or when trapped in permafrost or glacial ice on geological time scales.

Hypothesis-Testing in Extreme Environments on Earth: Some of the most likely current or past habitats for microbial life elsewhere are on Mars (polar ice and ice deposits from bygone floods or protected subterranean pockets), and an ocean (possibly liquid) overlying the volcanically active crust of Jupiter's moon Europa. Many of these environments' features coincide with those of extreme habitats in the greater Pacific Northwest region (all are easily reached from here). Prime examples include ice cover over the Arctic Ocean, sub-sea-floor environments at hydrothermal vents along the Juan de Fuca Ridge (only 200 miles offshore from Seattle), and land-based subterranean basalt formations (study sites of colleagues at PNNL in Richland, WA).

The Sea-Ice Environment On the microscale, sea-ice is a complex material composed of solid (ice crystals and mineral or salt precipitates), liquid (brine), and gases (air pockets). Physically, sea-ice varies from fine ice particles suspended in water to thick blocks of ice hundreds of meters across. Initial salinity in newly formed ice depends on rate of freezing; the amount of brine held in the ice decreases with time (e.g., seasonally).
As a microbial habitat, sea ice is rich in temporal and spatial gradients in salt (0 to > 100 ppt), temperature (0 to -26 C) and solid phase surfaces suitable for microbial attachment. Sea ice is colonized internally by many microorganisms. Collectively they support krill and a complex and diverse marine community. Survival strategies of the microorganisms, which are trapped below freezing in sea-ice brine during the long, dark, winter, have not yet been studied well.
We hypothesize that fluid mineral content and sub-zero temperature drove evolution of novel survival strategies. To test this we use extremely psychrophilic (cold-loving) microorganisms, from polar sea ice and the permanently cold deep sea. Culturable microbes from these environments have growth optima at 3 to 5 C. By studying physiological attributes and cellular strategies for survival, growth, and activity at extremes beyond sea-ice, we expect to develop models and predictions of how life, if it exists on Europa, may be persisting in (or beneath) an ice cover accessible to future sampling missions.
Sub-Seafloor Hydrothermal Environment Active ocean-bottom volcano-hydrothermal systems are a probable locus for initiation and early evolution of life on Earth. This could occur on other volcanically active planetary bodies. By studying the physiological attributes and cellular strategies for survival at temperature extremes beyond those currently believed to support life, we expect to develop models and predictions of the boundaries of life in a tectonically active subsurface regime.
In the 1990s, an unexplored sub-seafloor microbial biosphere was discovered; submarine volcanic activity on Earth supports large populations of primitive, heat-loving Archaea. Microbial life thrives in water-saturated pores and cracks within deep, volcanically-active portions of Earth. Chemo-litho-autotrophic (non-photosynthetic, CO2-fixing) microorganisms at vents support a large food chain, including diverse macroscopic communities. Seafloor communities at Earth's numerous submarine hydrothermal vents are sustained from below by flow of hot fluid carrying nutrients dissolved from the underlying rocks.
Seafloor crust at mid-ocean spreading centers harbors a substantial microbial infauna, supported in part by nutrient fluxes released during crustal accretionary episodes. Evidence for this subsurface microbial community is compelling. All eight submarine diking-eruptive events examined recently resulted in massive effusions of microbial floc, including very high densities of live thermophilic bacteria. The precise source of this material is not certain. Perturbations in established hydrothermal flows (triggered by volcanism) may have flushed existing microbial products from pores and cracks in the sub-seafloor. Alternatively, fracturing and magmatic intrusion may have enhanced nutrient fluxes, resulting in microbial blooms carried upwards to the seafloor by upwelling fluid. A significant, but unexplored, fraction of the oceanic crust seems to be a volcanically-supported sub-seafloor biosphere.
All these facts about Earth have raised the hypothesis that water-saturated, outer crustal rocks of any volcanically active planet can harbor an extensive microbial biomass. We know there exists extraterrestrial active volcanism, including beneath Europa's ocean. Europa is a prime candidate for existence of thermophilic extraterrestrial life. The astrobiology program therefore has a strong focus on hyper-thermophilic (extreme heat-loving) microorganisms.
Life in Deep Subterranean Basalt Diverse, active microbial communities also live deep beneath Earth's surface (e.g., in water flowing through fractures in basalt and granite). These environments were believed too nutrient-poor to support life. Based on geological constraints, isotopic dating of groundwater, and hydrological modeling, some subsurface "wet" microorganisms seem to have persisted for millions of years without any surface supply of organic carbon. In Earth's arid subsurface crustal environments, fluxes of nutrients and water are low and microorganisms persist on geological time scales. Many of these organisms are chemo-litho-autotrophs: they use abiotically-produced hydrogen gas for energy. They fix inorganic carbon and release simple organic compounds, which are in turn consumed by chemo-organotrophs.
Due to the increase of temperature with depth, the maximum depth of their biosphere (i.e., down to ~120 C) averages 4 km. Total global subsurface biomass may be immense, a substantial portion of Earth's overall total. Subsurface microorganisms influence ground water chemical properties, may have applications in biotechnology (high-temperature processes), and may alter current concepts of global ecology. These environments (and their flora) can serve as analogs for subsurface refugia on other planetary bodies (perhaps Mars) where life forms from wetter times may persist.
The Extremophile Laboratory Studies of Earth's extremophiles benefit from our new, dedicated extremophile Laboratory (ExLab). The School of Oceanography has committed ~1000 sq ft of new lab space for ExLab. It concentrates on experimental research on extremophiles, particularly at extremes of temperature (-1 to +300 C) and pressure (up to 600 atm (hot) and 1100 atm (cold)). ExLab assembles in one place scattered custom-built equipment and instrumentation. ExLab has a main laboratory (for sharable analytical facilities) and two additional laboratories, one for heavy temperature-pressure equipment, one to host students on rotation from other departments.
Experiments in the ExLab are critical in our educational and research programs. ExLab provides a unique and unifying training experience for all astrobiology participants. Students can work on hypotheses about microbial and enzymatic functions within and beyond the environmental boundaries of Earth's life. They might experiment on microbial activity, growth, and survival across and beyond temperature and salinity gradients that mimic those of Earth's winter sea-ice formations. They can work along adjustable thermal gradients, with and without elevated hydrostatic pressure, to better define environmental optima and boundaries for microbial activity, growth and survival on Earth and, by analogy, elsewhere. Survival strategies and rates of evolution and death on long (geological) time scales are critical research issues for students interested in the terrestrial subsurface.
Classwork All astrobiology students learn about microbial life through the required course work, laboratory rotations, workshops and research seminars and presentations. Special emphasis is placed on "extremophiles", especially psychrophiles, thermophiles and chemo-autotrophs. Students learn how these organisms (1) are classified evolutionarily, (2) obtain energy, and (3) carry out activities that may leave signatures (past or present). All astrobiology students participate in hands-on work in the ExLab.
Research Opportunities Ongoing UW programs in vent and polar-ice biology provide unparalleled training in the testing of specific hypotheses about thermophiles and psychrophiles, respectively.
Faculty can provide opportunities, in the Arctic, for astrobiology students to get samples or conduct in-situ experiments, e.g. aboard the US Coast Guard's program allowing "not-to-interfere" research aboard its icebreakers. Work in the Arctic by UW faculty often yields other opportunities (e.g., extra berths and ship time). We have cultures of about 500 strains of sea-ice bacteria and about 30 from the abyssal Arctic. Scientists at PNNL have unique facilities and expertise to contribute to research on subterranean (land) microbial communities. An extensive and diverse collection of hyper-thermophiles is available in Baross' lab. These are all ideal research materials for astrobiology students.
Faculty also have ongoing programs to sample and study hydrothermal habitats on Juan de Fuca Ridge. A program with NOAA (Baross) provides additional opportunities at submarine hydrothermal vents, using state of the art manned submersibles and remotely operated vehicles (ROVs). Deming has designed and built a novel sampler for deployment at the hottest vent sites. Long-term plans are afoot (Delaney) to develop and install a permanent seafloor observatory along the Juan de Fuca Ridge. This provides design problems for astrobiology's engineering students, plus extraordinary bio-research opportunities. The School of Oceanography provides state-funded ship time for at-sea education and training of astrobiology students, to provide unparalleled research opportunities.