Water, Thermodynamics and Insight


Water, life’s solvent, is the current yardstick by which scientists use to characterize earthly life and is in part the basis for understanding the “Goldilocks zone.” As a result of life’s watermark, it is often assumed that where there is life—water is nearby. That statement, by itself, is more complex than most can appreciate. Early earth did not have “enough” of life’s present necessities—one primary element comes to mind, reduced phosphorous. When one looks for phosphorus—the primary form that is found is in oxidized form. Chemistry, as we know it, needs reduced phosphorus to readily form the phosphate groups in RNA and DNA. The elementary chemistry of nucleotides (that of RNA and DNA) is notoriously difficult in the laboratory—so much so that science at present cannot hope to fully elucidate the origin of life. The oceans of the early earth were far more conducive to forming life’s early molecules and possibly even more so at or around hydrothermal vents (a source of heat energy).

Reduced phosphorus is found in sedimentary layers of the Earth’s crust—and is a predominant mineral found in SNC –meteorites (meteorites that are primarily carbonaceous chondrules that contain iron-nickel-phosphorus minerals). The chemical nature of phosphorus on earth is such that it is in the predominantly oxidized form—as chemical thermodynamics dictates. Thus, examination of phosphorus “thermodynamic phase diagrams” indicates that early earth conditions (specifically during the Archean)—favored the reaction of reduced phosphorus with prebiotic soup of the time period.

In papers by Pasek and others, mounting evidence may point toward the Late Heavy Bombardment of the Archean era as one likely source for reduced phosphorus. Through the sampling of “archean sedimentary rock” it seemed as if prebiotic conditions were conducive to RNA-world life. The reasons for the hypothesis are time of the Late Heavy Bombardment, and presence of schreibersite (nickel-iron-phosphorus containing) meteoritic material in the sedimentary layers. Although further evidence is warranted so as to be conclusive, layers of sediment from Australia are indicative of reduced phosphorus being present.

(At the time of this writing, Dr. Steven Benner at University of Florida announced the intriguing hypothesis that life may owe its origins to Mars due to the relative paucity of water, and readily available metal ions needed stabilize early nucleotides; e.g. RNA and DNA.)—It should be noted that most SNC-meteorites originate from Martian crust.

REFERENCES:

Pasek, Matthew a, Jelte P Harnmeijer, Roger Buick, Maheen Gull, and Zachary Atlas. 2013. “Evidence for Reactive Reduced Phosphorus Species in the Early Archean Ocean.” Proceedings of the National Academy of Sciences of the United States of America 110 (25) (June 18): 10089–94. doi:10.1073/pnas.1303904110. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=3690879&tool=pmcentrez&rendertype=abstract.

Pasek, Matthew a. 2008. “Rethinking Early Earth Phosphorus Geochemistry.” Proceedings of the National Academy of Sciences of the United States of America 105 (3) (January 22): 853–8. doi:10.1073/pnas.0708205105. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2242691&tool=pmcentrez&rendertype=abstract.

Citations regarding Dr. Benner’s announcement:

Pale Blue Blog– Men are from Mars and Women are from Mars, too? By: S. DOMAGAL-GOLDMAN. http://www.astrobio.net/paleblueblog/?p=2067.

The Guardian– Life on earth ‘began on Mars’ Geochemist argues that seeds of life originated on Mars and were blasted to Earth by meteorites or volcanoes. By: Press Association. http://www.theguardian.com/science/2013/aug/29/life-earth-originated-mars

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15 thoughts on “Water, Thermodynamics and Insight

  1. SubOptimist

    Interesting, never really thought about phosphorous this much except that it links nucleotides together. If I can ask a real simple question: what is reduced phosphorus bound to (as phosphorous is bound to oxygens in phosphate)? I would have thought that phosphate would be considered oxidized, but I guess that is all relative. Thanks for stirring my brain up.

    Reply
    1. jaksichja Post author

      The papers by Pasek indicate that schreibersite (tri-nickel phosphide or tri-iron phosphide) would have reacted quickly in the earth’s ocean to form calcium and magnesium phosphites, hypophosphites, and phosphates.

      Actually, you asked a great question—phosphorus chemistry is difficult but answerable.

      According to the authors of the two cited papers, calcium or magnesium hypo- and (non hypo-) phosphite were the species of interest in the Earth’s oceans. Phosphates “seem” to not be readily available for reasons of solubility and not mixing well in the ocean. (I will add more to this shortly.)

      Reply
      1. SubOptimist

        Good point about solubility. I always dreaded phosphate buffers in the lab because they would fall out of solution at the drop of a hat. I will never forget phosphorous: I had just started teaching, and in front of 100 students that I had doing the subatomic book-keeping I very publicly (re)learned that phosphorous is one of the exceptions to the octet rule. Bright red in my confusion. Great example of forgetting knowledge learned that you don’t actually use.

  2. jaksichja Post author

    I will post the abstracts to the cited papers for anyone who may be interested (in the hopes that you will download the papers). The two papers should be free if you reside in the U.S.

    Reply
    1. Erik Andrulis

      John, Thanks for these papers. Hadn’t seen them. Off to see how their proposals regarding the origin and evolution of phosphoric chemistry relates to the origin of water, organic chem, RNA, protein, and DNA.

      I’m always intrigued by how thoroughly “specific” modern science is in its speculative ideas. What I mean by “specific” is that there is no synthesis of the ontogeny and phylogeny, nor the micro and the macro, nor the ordering of phenomena. Usually a group focuses on one small part of the elephant, blindfolded all along.

      Reply
  3. jaksichja Post author

    Here is the abstract to Pasek’s 2013 paper–

    It has been hypothesized that before the emergence of modern DNA–RNA–protein life, biology evolved from an “RNA world.” However, synthesizing RNA and other organophosphates under plausible early Earth conditions has proved difficult, with the incorporation of phosphorus (P) causing a particular problem because phosphate, where most environmental P resides, is relatively insoluble and unreactive. Recently, it has been proposed that during the Hadean–Archean heavy bombardment by extraterrestrial impactors, meteorites would have provided reactive P in the form of the iron–nickel phosphide mineral schreibersite. This reacts in water, releasing soluble and reactive reduced P species, such as phosphite, that could then be readily incorporated into prebiotic molecules. Here, we report the occurrence of phosphite in early Archean marine carbonates at levels indicating that this was an abundant dissolved species in the ocean before 3.5 Ga. Additionally, we show that schreibersite readily reacts with an aqueous solution of glycerol to generate phosphite and the membrane biomolecule glycerol–phosphate under mild thermal conditions, with this synthesis using a mineral source of P. Phosphite derived from schreibersite was, hence, a plausible reagent in the prebiotic synthesis of phosphorylated biomolecules and was also present on the early Earth in quantities large enough to have affected the redox state of P in the ocean.

    Reply
    1. Pasek's lab.

      Hi Jaksichja!
      Thanks for putting this here. Interesting comments :). We have some more stuff coming up published soon which will probably emphasize more on the significance of reduced P and it will be in series to this work.

      Reply
  4. jaksichja Post author

    And the 2008 paper —

    Phosphorus is a key biologic element, and a prebiotic pathway leading to its incorporation into biomolecules has been difficult to ascertain. Most potentially prebiotic phosphorylation reactions have relied on orthophosphate as the source of phosphorus. It is suggested here that the geochemistry of phosphorus on the early Earth was instead controlled by reduced oxidation state phosphorus compounds such as phosphite (HPO32), which are more soluble and reactive than orthophosphates. This reduced oxidation state phosphorus originated from extraterrestrial material that fell during the heavy bombardment period or was produced during impacts, and persisted in the mildly reducing atmosphere. This alternate view of early Earth phosphorus geochemistry provides an unexplored route to the formation of pertinent prebiotic phosphorus compounds, suggests a facile reaction pathway to condensed phosphates, and is consistent with the biochemical usage of reduced oxidation state phosphorus compounds in life today. Possible studies are suggested that may detect reduced oxidation state phosphorus compounds in ancient Archean rocks

    Reply
  5. jaksichja Post author

    One last note of worth–iron/nickel meteorites also have a great deal of phosphide–so it is “important” that as with all research it is work in progress.

    Reply

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