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.
This study seeks to determine habitability limits by combining traditional and negative life detection approaches. Conventional life detection studies have typically explored the ranges of habitability by moving across a parameter gradient from moderate conditions into more harsh extremes, as exemplified by the blue arrows in (a) which point “outward” along a line representing the parameter space for a given variable. This strategy has identified life’s ability to contend with extremes but rarely are the conditions that truly limit life fully delineated. To complement the approach, negative life detection (b) seeks to determine the conditions under which life may not be found, allowing constraint of the unknown habitability limit for the given parameter. This is shown by the orange arrows pointing “inward,” creating an outer limit and then lowering that threshold. The threshold itself is identified by the gradient band, in which the limit resides but is unknown. With both of these methodologies together, the lower and upper limit can be constrained and potentially found, shown in (c).
Various physicochemical extremes drive negative life detections. Six physicochemical parameters are shown with selected values for negative life detections from the literature. Citation numbers correspond to a negative life detection, reported as wholly or partially caused by that physicochemical factor and identified by the location of the sampling site. In the case of multiple negative life detections in one paper, the value depicted is the least biologically extreme. Brackets beneath each scale mark the widely accepted ranges for non-extremophiles and extremophiles of that physicochemical factor3. Scales are independent, without implied relationships between them. Chaotropicity is difficult to empirically calculate, making numerical determination of ranges challenging.
Heather V Graham, Laura E Ratliff, Avery Fulford, Chad I Pozarycki, Gina M Wimp, Floyd Nichols, Magdalena Osburn doi: https://doi.org/10.1101/2023.11.06.565904
Astrobiology
