TABLE 5.1 Biosignatures: Specificity for Life Detection, and Applicability to Detecting Extant and Extinct Life and Terrestrial Contamination of Spacecraft

Signatures of Life

Application for Life Detection

Specificity

Fossil and Nonterrestrial Life Detection

Morphology (Micro-and Macroscopic)

Shape; size; replication structures (buds, chains of cells, septa, fruiting bodies, and spores); some biominerals; macrostructures such as biofilms and stromatolite-like structures

Detect extant terrestrial life, fossils, indication of active cells; application to spacecraft contamination.

Shape and size are not definitive (terrestrial life is >100 nm in diameter); replication structures are definitive indicators of life; can identify eukaryotes; biofilms and stromatolite-like structures could be definitive.

Replication structures can be definitive; size, shape, and numbers of identical morphotypes such as are seen in biofilms or laminated structures observed in stromatolites may or may not be definitive for life, and additional chemical and isotopic analyses are necessary.

Organic Chemistry and Biochemistry

Cell walls (variety of biopolymers)

Membranes (fatty acids)

Nucleic acids (DNA, RNA)

Proteins

Hydrocarbons, steroids, hopanes

Amino acids

Organic metal and phosphate compounds

Porphyrins, flavins, etc.

Carbohydrates

Nucleic acids with same genetic code as terrestrial life would likely indicate terrestrial contamination; steroids generally indicate eukaryotes; hopanes found in cyanobacteria; chirality and presence of the 20 key amino acids associated with terrestrial life indicate terrestrial and spacecraft contamination.

Bacteria, archaea, and eukaryotes have specific cell-wall chemical structures; chirality, enantiomeric excess, and repeating structural units such as C5, C6 (sugars), C2 (polymethylenic lipids), C5 ( polyisoprenoids), α-substitution of protein amino acids, and L-amino acids and D-sugars are canonical for terrestrial life. Nucleic acids, proteins, and phosphates and organic-phosphate compounds could be indicative of recent or extant life.

Hydrocarbons, steroids, and hopanes have been observed in the fossil record; other macromolecules (nucleic acids, proteins, and carbohydrates) are extremely labile. Nothing is known about the long-term stability of cell-wall polymers of archaea and their chemical transformations during fossilization.

Inorganic Chemistry

Iron minerals (e. g., magnetite)

Sulfur compounds

Carbonates

Silicates

Other biologically important metals (e.g., Cu, Mo, Ni, W, etc.)

Nitrogen compounds

Phosphorus compounds

Ratios of biologically important elements (CHONPS)

Disequilibrium in biologically important oxidation- reduction couples

Best application is as additional information in conjunction with microscopic, isotopic, and organic chemical analyses for fossils and possibly for detecting presence of living extraterrestrial organisms; probably not applicable for detecting spacecraft contamination.

C, N, and S can be highly specific for terrestrial life in conjunction with stable isotope or organic analyses. Some bacteria form iron compounds with highly specific structures (e.g., magnetosomes and the ferruginous ribbons formed by the bacterium Gallionella spp). Other microbes deposit silicates and carbonates and elemental sulfur as metabolites.

Some crystal structures of magnetite are thought to be produced only by organisms. C, S, and N isotopes are essential additional targets of analyses for inferring past life. Oxygen isotope ratios associated with phosphates may be indicative of life. Heterogeneous distribution of biologically important minerals (Cu, Mo, Ni, W) and disequilibrium in the chemistry of rock samples could be supporting evidence for past life.



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