The Chemistry category reflects chemical properties of substances known to be produced or modified by life. Chemical synthesis in biological systems seeks to maximize fitness and adaptive utility. In contrast, abiotic reactions are dictated by kinetics and thermodynamics. This difference can result in diagnostic chemical properties of substances (elements, molecules, compounds, minerals) that serve as proxy for biological or abiotic sources, including the structure of organic molecules, the relative abundance of choice of certain enantiomers, or the isotopic makeup of organic and inorganic compounds. In addition, biological processes can modify the chemical composition of an environment from the cellular to the planetary scale in ways that are distinguishable from abiotic processes — including the composition and relative abundances of elements and molecules within living things (e.g., atmospheric gases, chemical composition of crustal minerals and rocks).
Molecular structure refers to the three-dimensional arrangement of atoms in a substance. Living systems can produce complex molecules that are not observed in abiotic systems. For example, the presence in a sample of a complex polymer with repeating units (monomers), or with a repeating charge (i.e., polyelectrolyte) could be indicative of biological synthesis. Some monomers are also sufficiently complex that their presence alone could point to a biological source, such as the proteinogenic amino acids tryptophan and phenylalanine, or sugar monomers with five or more carbons in their structure.
Living systems utilize only a small subset of monomers (e.g., amino acids, sugars, fatty acids) to synthesize more complex polymers. The relative abundance of each monomer within that subset depends on the structure and functionality of polymers, which are the result of evolutionary adaptations. In contrast, when the same monomers are produced abiotically their relative abundance is a function of their energy of formation, so smaller molecules tend to dominate. The different patterns in the relative abundance of molecules within a certain class of compounds can therefore be used to discriminate biotic sources from abiotic ones (Dorn et al., 2011).
Enantiomers are pairs of the same molecule that are mirror images of each other. Life on Earth often displays a preference toward specific enantiomers to build functional polymers, which can result in high levels of enantiomeric excesses in a sample. Therefore, relative enantiomer abundances might serve as signatures of biological activity. Abiotic organic synthesis tends to form equal mixtures of enantiomers in the absence of an asymmetric chiral influence (i.e., a physical or chemical process that favors a specific enantiomer), although abiotic organic mixtures, such as those found in carbonaceous chondrites, sometimes display enantiomeric imbalances, which point to non-biotic mechanisms of enantioselectivity.
The main elements composing life as we know it are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS). Enrichments of these compounds in a sample could be indicative of biological activity, and some of these elements can display remarkably stable ratios both in living systems and the environments they occupy, such as the well-known Redfield ratio of C:N:P=106:16:1. In addition, biological activity can affect the concentrations of trace metals in the environment, and organisms can generate patterns of trace metal abundances that reflect specific aspects of their biochemistry, such as the metallome requirements of methanogenesis (Cameron et al., 2012).
Patterns of isotopic abundances within and between molecules can serve as ‘fingerprints’ of biological activity – hence isotopic biosignatures. Several biologically important elements (e.g., H, C, N, S, O) have two or more stable isotopes. Multiple isotopes of an element differ in the number of their neutrons. Differences in the nuclear masses of isotopes have minimal effects upon the electron orbitals of atoms and molecules, but they do alter the vibrational energy states of polyatomic molecules. Chemical bonds having heavier isotopes have lower vibrational energy states than those with lighter isotopes; accordingly, bonds between lighter isotopes are somewhat weaker than bonds having a heavier isotope. Molecules having bonds with higher force constants exhibit greater differences in zero-point energies between the molecules’ lighter and heavier isotopes. Therefore, compounds having bonds with higher force constants tend to acquire greater abundances of heavier [read more] isotopes.
Physiochemical and biological processes transport and transform the biologically important elements between their planetary rock, water, and atmospheric reservoirs, and can create patterns of isotopic abundances (isotopic ratio patterns) in their abiotic and biotic products. These patterns are recorded in biomass, water bodies and rocks. Interpreting isotope patterns in the geologic rock record helps us interpret our biosphere’s history.
Fractionation at equilibrium. Coexisting molecules can achieve mutual chemical and isotopic equilibrium when the forward and backward reactions between them occur at equal rates. Among molecules at chemical equilibrium, molecules having bonds with greater force constants will exhibit greater abundances of heavier isotopes. Differences in isotopic abundances between equilibrated molecules generally decrease at higher temperatures. Where isotopic differences between equilibrated molecules can be achieved due purely to chemical and physical phenomena, they do not necessarily require biological processes and therefore are not definitive biosignatures. But in some circumstances, enzymatic processes can achieve equilibrium between molecules under conditions where equilibration would otherwise be extremely slow in the absence of life. In those cases, evidence of isotopic equilibrium could constitute a definitive biosignature. Examples of these are given in subsequent sections.
Kinetic isotope fractionation. A kinetically controlled reaction can arise under conditions when its rate is at least somewhat inhibited and the rate from reactant to product is greater than the rate of the reverse reaction. In this case, a reactant equilibrates with a higher energy (less stable) ‘reaction intermediate’ species that ultimately leads to the product. Because the reacting bonds in the ‘reaction intermediate’ are typically weaker (they have lower force constants) than the bonds in the reactant, the ‘reaction intermediate’ becomes enriched in lighter isotopes, relative to the reactant. Also, for a reactant having lighter isotopes, the differences between its vibrational energy states and the states of the ‘reaction intermediate’ are smaller than the corresponding differences for a reactant having heavier isotopes. This also favors the ‘reaction intermediate’ to become enriched in lighter isotopes relative to the reactant. Thus, the ultimate reaction product is enriched in lighter isotopes relative to the reactant.
Enzymes can alter reaction pathways and rates of reactions in ways that create biosignatures. The capacity of an enzyme to modulate the rate of a biochemical reaction arises from the molecular structure of its ‘reaction intermediate’ and the environment within the enzyme’s reaction center. These attributes create the isotopic fractionation expressed in the overall reaction. Differences in the reaction mechanisms employed by biological versus nonbiological processes can create differences in the isotopic compositions of their respective products. Such differences can form the basis for distinguishing between isotopic patterns of biosignatures versus patterns that arise from nonbiological processes.
Isotopic ratios expressed in ‘del’ notation. In the natural sciences, the stable isotopic composition of a sample is typically expressed as the ratio of the abundance of the heavier isotope over that of the lighter isotope, relative to a standard. Carbon isotopic abundances are represented as follows:
δ13CVPDB = ((13C/12C)sample/(13C/12C)VPDB - 1)1000,
where δ13CVPDB is the difference in permil (parts per thousand) between a sample and the carbon isotopic standard (the Vienna Pee Dee Belemnite carbonate -VPDB).
Sulfur isotopic abundances are represented as follows:
δ34SVCDT = ((34S/32S)sample/(34S/32S)VCDT - 1)1000,
where δ34SVCDT is the difference in permil (parts per thousand) between a sample and the sulfur isotopic standard (Vienna Cañon Diablo Troilite - VCDT)
Features of crystalline substances such as crystal structure, mineral impurities, and grain size distributions.
This taxonomic category is an outlier in that it does not conform to the tier 1 (chemical parameters) and tier 2 (substances) structure. However, it was included in the taxonomic scheme to capture physical/chemical phenomena indicative of compounds that can serve important biochemical or physiological roles (e.g., light absorption, electrochemical properties…). Measurements of those chemical properties can point to the presence of life even if the compounds are unknown (the physical/chemical phenomena can sometimes be measured remotely without the need for compositional analyses). For example, absorption of light within specific regions of the solar spectrum could be indicative of a substance that can capture and convert light energy into chemical energy, akin to biological pigments such as chlorophyll.
This category includes visual observable characteristics, arrangements, and spatial relationships of a visual feature that aid in assessing biogenic / abiogenic origin, irrespective of geochemistry or scale. This content was initially developed from geologic phenomena expected on a rocky planet, though measurement parameters (MPs) should apply to any visually observable target.
Measurement parameters (MPs) include fundamental attributes of biogenic features and abiotic mimics, plus key emergent features of structures (i.e., stromatolites, microbially induced sedimentary structures) that result from microbe-mineral interactions. Emergent features have attributes that individual components of those features do not have. MPs considered here are size distribution, spatial distribution, shape/form, orientation, and texture / fabric of features of interest. These measurable parameters were adapted from classic geological terminologies and designed to be useful and “agnostic” for a visual [read more] unknown feature.
The description of structural (visual) attributes included here avoids the use of terms based on (bio)genetic classifications.
Often, companion LDKB entries are included in the “Chemistry” category with a link provided in the relevant “Structure” argument to describe their full value as potential biosignatures.
The distribution of a feature’s dimensions / magnitude. For example, for laminations, this can refer to laminae thickness distribution, measured in cross-section parallel to its growth direction. For tunnels, this can refer to tunnel diameter distributions and length ranges.
Spatial distribution is the proportion of a 2D surface covered by a feature within an area of interest. Of interest are patterns on surfaces and the spatial frequency (including repetitions) of distinctive features that intersect the surface or its component grains (allochthonous, autochthonous), minerals (detrital, chemical), mineraloids (non-X-ray crystalline), and other components (e.g., dispersed organics, fossils, etc.). For example, in basalt, tunnels can be distributed along fractures, starting at a void, and providing an opening for water and / or microorganisms to enter the rock.
Shape / form is the geometric shape, morphology, 2D or 3D structure, habit / crystal habit, surface relief, or surface topography. For example, spherical cells, also called cocci, represent a morphological variety of unicellular organisms and are distinguished from other cellular morphologies, such as rods and filaments. Upon burial and compaction, coccoidal microfossils may undergo deformation to resemble ruptured or deflated spheres (e.g. raisin-like), or may become flattened compressions. Abiotic processes commonly create spherical morphologies, too (i.e. "blueberries" on Mars) and other botryoidal mineral habits.
Orientation is the position of a feature relative to its surroundings or another parameter (e.g., “Orientation/lower border of view area”). Orientation includes linear arrangements of features, minerals, cell-sized objects, and 3D morphological spatial arrangements across scales. One example is tunnels in igneous and metamorphic rock, which can be positioned relative to fractures, vesicles (bubbles), minerals, or other tunnels. An example argument for a biogenic prevalence of the tunnels is that cells reside at the margin of the vesicle and mine the glass for energy and material, since tunnel orientation allows the cells to continually encounter fresh glass. An argument for abiotic prevalence of the tunnels is that tunnels around olivine could follow lines of stress created by the differential thermal shrinkage of olivine and glass. An additional example includes filaments in a mat, which can have a vertical or horizontal orientation relative to the defined surface.
Texture describes the relationships of a rock’s components to one another at the microstructural scale. A rock’s texture records its origin and post-depositional (i.e., taphonomic) histories. For a sedimentary rock deposited in the presence of a microbial community, these components include grains, mineral crystals, aqueous precipitates, fossils, organic remains, and other rock fragments. Fabric refers to the spatial arrangement and geometric patterns of the visual components, such as grains, filaments, laminae, of a sedimentary rock that display some level of directionality. A rock fabric can be described by the orientation and pattern produced by single or multiple components of a sediment. Fabric is the relative orientation of multiple features (i.e., grains, filaments, laminae), including the appearance of the components of features (e.g., spatial and geometric pattern formed from all materials present).
For example, stromatolites (laminated geobiological structures) can display [read more] distinctive fabrics and textures (such as laminated fabrics oriented parallel to the growth surface of a sedimentary deposit). The combination of those component parts may reveal biogenic attributes and visual patterns that can be diagnostic of various types of microbial input.
The Activity category reflects time-dependent processes that are the result of life’s exchange of matter and energy with the environment. This is manifested in measurable, time-dependent chemical and physical phenomena (e.g., movement of an object). In addition, biological systems can efficiently catalyze specific chemical reactions at rates significantly higher than abiotic processes, and the activity of biological systems can be modulated by environmental factors in ways that are predictable (e.g., seasonal changes), or within well-defined margins (e.g., temperature ranges). The boundaries of the Activity category may appear ambiguous since biological activity can be manifested in the form of chemical compounds or structural features. This category encompasses changes that a substance or structure undergoes over time, but not the substance (Chemistry category) or structure (Structure category) itself.
Reproduction describes the rate of change in numbers of a physical object. The most immediate biological example is cell division, which can occur at rates that are predictable based on physical and chemical conditions (e.g., temperature). Sometimes, biological reproduction results in distinct morphologies such as that observed during different reproductive stages and modes, like binary fission or budding. Measurements of those morphologies would fall under the Structure category, despite their link to reproduction. This highlights the fact that the boundaries between LDMs are not always sharp, and that combinations of LDMs (i.e., change in numbers plus morphology) often increase their individual diagnostic power.
Growth describes the rate of change in size of a physical object. Organisms, or their structural components, typically change in size during a natural life cycle. Populations of organisms (e.g., a biofilm, a forest…) can also change in size at rates that can be predicted based on physical and chemical conditions. Again, the boundaries between growth and reproduction can be fuzzy (in microbiology they are often interchangeable), and for certain organisms, changes in size can be significant during certain stages of the life cycle, also leading to changes in morphology, like in the case of reproduction.
Seasonality describes changes in the properties of an environment in response to cyclic or periodic changes in environmental conditions. For example, seasonal changes in the wavelength-dependent reflectance of a planetary surface or in its atmospheric composition as a function of the periodic variation in the distance to its star or the latitude of its subsolar point (e.g., Olson et al., 2018). Measurements of seasonality (and growth) are often applied to the search for evidence of life at planetary scales (e.g., exoplanets) by means of remote sensing methods.
Motion describes the change in position of a physical object. The changes in direction of motile organisms appear purposeful and are inconsistent with drift or Brownian motion (Nadeau et al., 2018). Biological motion can be interrupted by changes in temperature, radiation or addition of chemical compounds, and certain organisms navigate in response to various physicochemical parameters (taxis) including light, magnetic fields, or chemical gradients, which are testable means to differentiate biological activity from abiotic phenomena.
Catalysis describes a hastening of chemical reactions leading to a lesser amount of time needed to reach chemical equilibrium between reactants and products at given environmental conditions. For example, reduction of CO2 to CH4, even if the latter is the thermodynamically stable species, can take billions of years in the absence of life (e.g., Etiope and Sherwood Lollar, 2013), but is much faster if living organisms deriving energy from this reaction use enzymes to hasten its completion.