Expanding Life's Boundaries: A Grand Challenge of Estimating Where and How Life Survives in Earth and Other Planets

In Earth's deep subsurface the presence of life and the ecological and evolutionary aspects of life, are dictated by a host of physical and chemical constraints. These factors include temperature, pressure, redox and other chemical potential gradients, overlying productivity, hydrologic recharge, water availability, sediment or rock age, and microbial power requirements. We have historically viewed these parameters through a lens that conforms to the convenience of our analytical methods and that occurs over human time scales. But as we learn that microbes respond to phenomena beyond these limited spatial and temporal extents, new questions arise: How are subsurface microbes responsive to rhythmically or acutely changing Earth processes like tectonism, planetary precession, or eustatic sea level change? How do various processes interact with each other to enable or challenge the possibility of life in a given place? How can we conceptualize the inextricably coupled, multiple large-scale geological and biological processes even though they are hidden from observation? Geological processes occur on spatial and temporal scales (molecular to planetary and instantaneous to multi-millennial) that are familiar to geologists and overlap with the range of projected microbial survival conditions in Earth's subsurface. We hypothesize that microbiologists who study the subsurface are still acquainting themselves with meaningful geological and planetary processes that define life in the subsurface. A more complete understanding of deep life - and its capacity for survival, ability to exploit unique niches, and issues related to dispersal, mutation, and biotic interactions - should emerge as we scale habitats spatially from mineral grains to continents and ocean basins and temporally from the short lifetime of some RNA molecules to long, dynamic Earth cycles. Such a reckoning will also help us to conceive of where life might be sustained in places other than Earth, where light from the Sun, or another star, may be too distant or weak to serve as a substantial source of energy, or in contrast, where life in the subsurface is the only possible option because the surface is too energetically illuminated to be inhabited.