The Vacant Niche Revisited: Using Negative Results to Refine the Limits of Habitability

To define the boundaries of habitability, biologists often search for highly specialized organisms in extreme environments. However, negative life detections--when a method is unable to detect microorganisms in a given setting--are just as important to constrain the environmental limits of life on Earth. In turn, these limits inform the selection of targets for life detection on other worlds. We performed a comprehensive, though non-exhaustive, literature search for negative life detections in polyextreme environments. We then catalogued the physicochemical conditions at these sites to further understand the habitability limits for life on Earth and the effects of multiple stressors on habitability. Using multivariate statistical techniques, our study searched for combinations of environmental parameters where extremes support or inhibit life. Our search raised several methodological and analytical considerations relevant to life detection studies in extreme environments. Incomplete documentation of environmental factors and experimental protocol limitations in the extreme environment literature complicated our analyses. This demonstrates the need to report negative results, particularly in life detection experiments, and the potential value for standardized reporting protocols. Exploring the range of results possible from life-detection methodologies is key to constrain the limits of life on Earth and informs our search for life elsewhere.