Water activity: Difference between revisions - Wikipedia

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[[Water]] is necessary for [[life]] under all its forms presently known on [[Earth]]. Without water, microbial activity is not possible. Even if some [[micro-organism]]s can be preserved in the dry state (e.g., after [[freeze-drying]]), their growth is not possible without water. Micro-organisms also require sufficient space to develop. In highly compacted bentonite and deep clay formations, microbial activity is limited by the lack of space and the transport of [[nutrient]]s towards [[bacteria]] and the elimination of [[toxin]]s produced by their [[metabolism]] is controlled by [[diffusion]] in the pore water. So, "space and water restrictions" are two limiting factors of the microbial activity in deep sediments. Early [[diagenesis]] of sediments just below the [[ocean floor]] driven by microbial activity (e.g., of [[sulfate reducing bacteria]]) end up when the degree of compaction becomes too important.

[[Extremophile]] micro-organisms also require water to be active. The threshold of water activity for their development is around of 0.6. The same rule should also apply for other planets than Earth. After the detection of [[phosphine]] (PH₃) in the atmosphere of the [[Venus (planet)|Venus]] planet, in the absence of known and plausible chemical mechanism to explain the formation of this molecule, the presence of micro-organisms in suspension in the Venus atmosphere at a certain altitude has been suspected and the hypothesis of the microbial formation of phosphine has been formulated.<ref name="Greaves_2020">{{cite journal|last1=Greaves|first1=Jane S.|last2=Richards|first2=Anita M. S.|last3=Bains|first3=William|last4=Rimmer|first4=Paul B.|last5=Sagawa|first5=Hideo|last6=Clements|first6=David L.|last7=Seager|first7=Sara|last8=Petkowski|first8=Janusz J.|last9=Sousa-Silva|first9=Clara|last10=Ranjan|first10=Sukrit|last11=Drabek-Maunder|first11=Emily|last12=Fraser|first12=Helen J.|last13=Cartwright|first13=Annabel|last14=Mueller-Wodarg|first14=Ingo|last15=Zhan|first15=Zhuchang|last16=Friberg|first16=Per|last17=Coulson|first17=Iain|last18=Lee|first18=E’lisa|last19=Hoge|first19=Jim|title=Phosphine gas in the cloud decks of Venus|journal=Nature Astronomy|year=2020|issn=2397-3366|doi=10.1038/s41550-020-1174-4}}</ref>