Polyextremophiles and the Constraints for Terrestrial Habitability

Abstract

Life on Earth occupies a multidimensional niche space that has yet to be fully described. The niche space of terrestrial life is knowable, and thus, it guides the search for hypothesized life. Since terrestrial life is constrained by its organic carbon foundation and requirement for water as a solvent, extremes in such environmental parameters as temperature, pH, and pressure will determine the geographic range in which life can survive. Most previous studies have focused on individual environmental variables, but in fact, each niche space occupies all parameters. Thus, to not only understand the limits of life on Earth but also use these constraints as a framework for the identification of potential abodes for life elsewhere necessitates an analysis of multiple environmental parameters simultaneously. After searching the published literature, we have compiled data about mechanisms of survival at individual and multiple extremes in order to model the niche space for terrestrial life. Published data are incomplete and inconsistent for a full analysis of all extremes and their polyextremophilic combinations, so after describing the breadth of the field, we focus on the two best-documented parameters, temperature and pH, to create a two-dimensional niche space model for future analysis. We conclude by pointing out that synthetic biology has the ability to expand the limits for life on Earth and thus increase the chances of overlap between terrestrial and potential extraterrestrial biota.