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. These microbial edge cases have been used to define astrobiological targets for life detection on other worlds. However, null-life detection eventswhen researchers are simply unable to detect microorganisms in a given environmentare just as important for understanding the environmental limits of life on Earth. We performed a comprehensive though inexhaustive literature search for null-life detections in polyextreme environments, and catalogued the physicochemical conditions of uninhabited environments to better describe the combinations of conditions that inhibit growth. We used multivariate statistical techniques (including principal component analysis) to identify subtle or previously unrecognized co-varying environmental extremes that, in combination, may support or inhibit life. Our search raised several methodological and analytical considerations relevant to extremophile detection surveys. We found that null-life results are constrained by physical scale, as some life-detection methods fail to identify microbes in low biomass environments. Timescale also impacts null results, because dormant cells and organisms with slow growth rates are seldom detected by traditional methods. Inadvertent site contamination by researchers can cause a site with a succession of previously reported null-life detections to become inhabited, whereas truly sterile zones with extreme conditions are never impacted by microbial contamination. Finally, the disparate life detection methods scientists have used call into question the universality of life-detection results; in particular, viability assays, cultures, and stains can be used to differentiate between living, dead, and dormant cells, while other techniques may incorrectly identify detritus as an active microbial community. Our study demonstrates the need for guidelines on the publication of negative results and life-detection methodology, so that astrobiologists can effectively differentiate between habitable and uninhabitable zones in the search for life on other worlds.