Editing Hot spring (section)

=== Limitations ===
A problem with the hot spring hypothesis for an origin of life is that phosphate has low solubility in water.<ref name=":4">{{Cite journal |last1=Longo |first1=Alex |last2=Damer |first2=Bruce |date=2020-04-27 |title=Factoring Origin of Life Hypotheses into the Search for Life in the Solar System and Beyond |journal=Life |volume=10 |issue=5 |pages=52 |doi=10.3390/life10050052 |issn=2075-1729 |pmc=7281141 |pmid=32349245|bibcode=2020Life...10...52L |doi-access=free }}</ref> Pyrophosphite could have been present within protocells, however all modern life forms use pyrophosphate for energy storage. Kee suggests that pyrophosphate could have been utilized after the emergence of enzymes.<ref name=":3" /> Dehydrated conditions would favor phosphorylation of organic compounds and condensation of phosphate to polyphosphate.<ref>{{Cite journal |last1=Kitadai |first1=Norio |last2=Maruyama |first2=Shigenori |date=2018-07-01 |title=Origins of building blocks of life: A review |journal=Geoscience Frontiers |language=en |volume=9 |issue=4 |pages=1117–1153 |doi=10.1016/j.gsf.2017.07.007 |bibcode=2018GeoFr...9.1117K |s2cid=102659869 |issn=1674-9871|doi-access=free }}</ref> Another problem is that solar ultraviolet radiation and frequent impacts would have inhibited habitability of early cellular life at hot springs,<ref name=":4" /> although biological macromolecules might have undergone selection during exposure to solar ultraviolet radiation<ref name=":2" /> and would have been catalyzed by photocatalytic silica minerals and metal sulfides.<ref name=":1" /> Carbonaceous meteors during the Late Heavy Bombardment would not have caused cratering on Earth as they would produce fragments upon atmospheric entry. The meteors are estimated to have been 40 to 80 meters in diameter however larger impactors would produce larger craters.<ref>{{Cite journal |last1=Pearce |first1=Ben K. D. |last2=Pudritz |first2=Ralph E. |last3=Semenov |first3=Dmitry A. |last4=Henning |first4=Thomas K. |date=2017-10-24 |title=Origin of the RNA world: The fate of nucleobases in warm little ponds |journal=Proceedings of the National Academy of Sciences |language=en |volume=114 |issue=43 |pages=11327–11332 |doi=10.1073/pnas.1710339114 |issn=0027-8424 |pmc=5664528 |pmid=28973920|arxiv=1710.00434 |bibcode=2017PNAS..11411327P |doi-access=free }}</ref> Metabolic pathways have not yet been demonstrated at these environments,<ref name=":4" /> but the development of proton gradients might have been generated by redox reactions coupled to meteoric quinones or protocell growth.<ref>{{Cite journal |last1=Chen |first1=Irene A. |last2=Szostak |first2=Jack W. |date=2004-05-25 |title=Membrane growth can generate a transmembrane pH gradient in fatty acid vesicles |journal=Proceedings of the National Academy of Sciences |language=en |volume=101 |issue=21 |pages=7965–7970 |doi=10.1073/pnas.0308045101 |issn=0027-8424 |pmc=419540 |pmid=15148394|bibcode=2004PNAS..101.7965C |doi-access=free }}</ref><ref name=":2" /><ref>{{Cite journal |last1=Milshteyn |first1=Daniel |last2=Cooper |first2=George |last3=Deamer |first3=David |date=2019-08-28 |title=Chemiosmotic energy for primitive cellular life: Proton gradients are generated across lipid membranes by redox reactions coupled to meteoritic quinones |journal=Scientific Reports |language=en |volume=9 |issue=1 |pages=12447 |doi=10.1038/s41598-019-48328-5 |pmid=31462644 |pmc=6713726 |bibcode=2019NatSR...912447M |issn=2045-2322}}</ref> Metabolic reactions in the Wood-Ljungdahl pathway and reverse Krebs cycle have been produced in acidic conditions and thermophilic temperatures in the presence of metals which is consistent with observations of RNA mostly stable at acidic pH.<ref>{{Cite journal |last1=Varma |first1=Sreejith J. |last2=Muchowska |first2=Kamila B. |last3=Chatelain |first3=Paul |last4=Moran |first4=Joseph |date=April 23, 2018 |title=Native iron reduces CO2 to intermediates and end-products of the acetyl-CoA pathway |journal=Nature Ecology & Evolution |language=en |volume=2 |issue=6 |pages=1019–1024 |doi=10.1038/s41559-018-0542-2 |pmid=29686234 |pmc=5969571 |bibcode=2018NatEE...2.1019V |issn=2397-334X}}</ref><ref>{{Cite journal |last1=Muchowska |first1=Kamila B. |last2=Varma |first2=Sreejith J. |last3=Chevallot-Beroux |first3=Elodie |last4=Lethuillier-Karl |first4=Lucas |last5=Li |first5=Guang |last6=Moran |first6=Joseph |date=October 2, 2017 |title=Metals promote sequences of the reverse Krebs cycle |journal=Nature Ecology & Evolution |language=en |volume=1 |issue=11 |pages=1716–1721 |doi=10.1038/s41559-017-0311-7 |pmid=28970480 |pmc=5659384 |bibcode=2017NatEE...1.1716M |issn=2397-334X}}</ref>