Where the microbes aren't

Abstract

Although a large fraction of Earth's volume and most places beyond the planet lack life because physical and chemical conditions are too extreme, intriguing scientific questions are raised in many environments within or at the edges of life's niche space in which active life is absent. This review explores the environments in which active microorganisms do not occur. Within the known niche space for life, uninhabited, but habitable physical spaces potentially offer opportunities for hypothesis testing, such as using them as negative control environments to investigate the influence of life on planetary processes. At the physico-chemical limits of life, questions such as whether spaces devoid of actively metabolizing or reproducing life constitute uninhabitable space or space containing vacant niches that could be occupied with appropriate adaptation are raised. We do not know the extent to which evolution has allowed life to occupy all niche space within its biochemical potential. The case of habitable extraterrestrial environments and the scientific and ethical questions that they raise is discussed.

Keywords: astrobiology; biogeography; extremophiles; limits; microbial distribution.

Figure 1.

Types of habitats and niches. Schematic illustrating the main types of physical spaces, described in relation to habitat space and the presence or absence of niches.

Figure 2.

The three primary environment types in the universe with respect to life. Each environment can transform into another by the processes summarized here.

Figure 3.

A schematic illustrating habitability of physical spaces as a function of physical and chemical conditions. The diagram is a simplified depiction of niche space (here three dimensions are shown—in reality, the space is an n-dimensional hypervolume) and the extent to which these niche conditions are occupied in physical spaces. The habitable space for life is enclosed within a much larger space that is uninhabitable (also see Fig. 4). The central object represents habitable space that is inhabited. At the edge of this habitable and inhabited set of physical and chemical conditions is a putative ‘rind’ of habitat space at the edges of life that could be inhabited by life with appropriate evolutionary innovations. Within the habitable and inhabited space, there are shapes depicting localized physical spaces that are habitable, but uninhabited (uninhabited habitats). The purpose of the schematic is to illustrate that uninhabited, but habitable spaces exist both within physical spaces that are known to be permissive for microbial growth and potentially at the limits of life at extremes. Life, especially using technology, can establish itself in nominally uninhabitable space with appropriate expenditure of energy (e.g. a self-sustaining space settlement).

Figure 4.

An illustration of temperature and pressure extremes in the universe and the range that life can occupy according to current knowledge. The space occupied by life at this scale is too close to the origin to be distinguishable (the dot depicted in the centre of the graph is the spheroid object shown in Fig. 3). The blown-up central part of the graph shows the known temperature and pressure range of life. On the axes, the lowest (natural) temperature limit is shown here as the Cosmic Microwave Background Radiation (2.7 K); the upper limit is illustrated here as the highest produced by humans (at the Relativistic Heavy Ion Collider at Brookhaven National Lab), although in theory the highest temperature in the universe is the Planck temperature (10³² K). The lowest pressure limit is shown as a vacuum, the upper pressure limit is taken as that achieved in a Diamond Anvil Cell in the laboratory, although at macroscopic scales in the universe, pressures of ∼10³⁴ Pa probably exist in the core of a neutron star.

Figure 5.

Examples of uninhabited, but habitable spaces at the (A) macroscopic and (B) microscopic scale. See text for discussion.

Figure 6.

At the limits of life. Laboratory investigations of H. hydrothermalis growth at pH 8.0 (adapted from Dickinson et al. 2021). Point ‘X’ marks a combination of extremes that resulted in no growth, yet the two individual parameters of temperature and salinity do not, at the same levels, on their own prevent the growth of the organism when combined with either lower salinity or temperature, respectively. Point X could represent a vacant niche or uninhabitable space.

Figure 7.

At the limits of life in the natural environment. At the limits of life in natural settings, samples devoid of active life (accepting methodological difficulties of showing a lack of actively metabolizing life) cannot easily provide information to determine where those limits are since they could lie anywhere to the right of the limit of life (e.g. X₁ or X₂ or beyond). To find the limit, we are usually forced back into exploring the limits of life using organisms (i.e. finding the location of Y), especially if the space to the right of Y is a vacant niche(s) and not uninhabitable, in which case evolutionary experiments can be used to explore the limit.

Figure 8.

Varieties of uninhabited habitats in the universe. Ways in which habitable environments devoid of active life (uninhabited habitats) can exist on both inhabited and uninhabited planets. The definition of a ‘habitable’ space is necessarily with reference to life known at the time of that assessment.

Figure 9.

Examples of potential uninhabited habitats at the macroscopic scale on Mars or an extreme Mars-like planet. Diagram illustrates the transient melting of water in an impact structure. In the present day, the water would eventually boil or freeze in direct contact with the Martian atmosphere. Even on an inhabited planet, a lack of connectivity between an inhabited region (shown here speculatively as organisms in the subsurface) and the new habitat could maintain it in a sterile state, as could a lack of surface hydrology and an inclement atmosphere. Also illustrated (left side of diagram) is the artificial creation of such an environment by deliberate melting of ice/permafrost. A similar situation might occur by an unintentional spacecraft impact causing localized melting.

Figure 10.

Some key unknowns about habitable space in the universe. The different types of habitable and uninhabited regions in the universe are divided into proportions that are currently unknown. Top left: In the vast spaces that are uninhabitable, what proportion of the universe is habitable? Top right: Where spaces are habitable (at least with respect to known life), what proportion of that habitable space is to be found on uninhabited planets and what proportion on inhabited planets (Fig. 8)? Bottom middle: On planets that are inhabited, what proportion of habitable spaces are inhabited of all habitable spaces? Note that in Cockell (2021a), the questions in the top right and bottom middle were posed in the reverse order, but they remain the same.