Читать книгу Astrobiology - Charles S. Cockell - Страница 74

That all life on Earth uses six core elements might suggest that whatever the architecture of life, these are the minimum elemental requirements. You might like to question this idea. Could life be constructed with an even smaller subset as a minimal requirement? One study of cell metabolism was motivated by the observation that phosphorus is often geochemically unavailable or poorly available on planetary surfaces. The availability of phosphorus in our biosphere is linked to our oxygenated atmosphere that makes phosphorus readily dissolved in its oxidized form, phosphate, and our extensive biosphere and hydrological cycle that erodes phosphate-containing minerals, such as apatite. The authors suggested that it might be possible to assemble metabolic pathways without phosphorus. Metabolic pathways free of phosphorus can be proposed using known pathways in cells (a so-called systems biology approach) that are rich in enzymes that have iron–sulfur clusters and use thioester (sulfur-containing) couplings rather than phosphate couplings. Is such a pathway a relic or fossil of a pre-P era in life? Even if early life on Earth did use phosphorus, can such theoretical ideas lead to a demonstration of how life could evolve without using phosphorus? Might CHNOS be the minimal elemental requirements for life?

Goldford, J.E., Hartman, H., Smith, T.F. et al. (2017). Remnants of an ancient metabolism without phosphate. Cell 168: 1126–1134.

Of course, any given organism uses a wide variety of other elements. For example, metal ions, such as iron and copper, are involved in energy transfer in life because of their ability to give and accept electrons (discussed in Chapter 6). Calcium is found in calcium phosphate, used to make bones in animals. Vanadium is used by some microbes in the enzyme that takes nitrogen gas from the atmosphere and makes this nitrogen available for biological processes (nitrogen fixation). And so on. At the end of this book (Appendix A.1), you can see an “Astrobiological Periodic Table” where the main biological uses of elements across the Table are shown. These non-CHNOPS elements are used in more specific situations than the CHNOPS elements, where their properties fulfill some function that has been selected by evolution. In an informal way, we can think of the process of evolution as selecting CHNOPS to build the chassis of life and rummaging around in the Periodic Table selecting other elements whose specific chemical characteristics turn out to be useful in particular biochemical roles and for which there is a selective advantage.