An article published 14 years ago (Science 2011 Jun 3; 332: 1163-1166) reported that a microbe found in the inhospitable waters of Mono Lake in California could, unlike any other known life forms, use the heavy metal arsenic (As) to grow. This was the Abstract:
Life is mostly composed of the elements carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Although these six elements make up nucleic acids, proteins, and lipids and thus the bulk of living matter, it is theoretically possible that some other elements in the periodic table could serve the same functions. Here, we describe a bacterium, strain GFAJ-1 of the Halomonadaceae, isolated from Mono Lake, California, that is able to substitute arsenic for phosphorus to sustain its growth. Our data show evidence for arsenate in macromolecules that normally contain phosphate, most notably nucleic acids and proteins. Exchange of one of the major bio-elements may have profound evolutionary and geochemical importance.
GEITP distinctly remembers highlighting this report in a GEITP email blog (but this was before we began posting our articles on genewhisperer.com in 2016). Contrary to what we all (or most of us) know — life cannot be sustained on Arsenic; life on Earth is always sustained on Carbon. WHY does GEITP not believe that Arsenic can replace Carbon??
Let’s review atomic structural chemistry. The key properties of the Carbon atom include: an atomic number of 6, four valence electrons that allow it to form four covalent bonds, and a stable atomic structure consisting of 6 protons and 6 neutrons, giving it an atomic number of 6. This gives Carbon the property of “tetravalency” — which means it is able to form the long chains and complex structures that form the basis of all organic molecules. And life itself. Recall that atoms have a first electron orbital (shell) that can hold a maximum of 2 electrons, the second orbital can hold a maximum of (2 + 6 =) 8 electrons; the third orbital can hold a maximum of (2 + 6 + 10 =) 18 electrons. This maximum capacity for each electron shell is determined by the quantum mechanical principles of electron configuration. Carbon’s electron configuration is .
Arsenic’s atomic structure is defined by its 33 protons and 42 neutrons, giving it an atomic number of 33. Its electron configuration is , which means it has 33 electrons. It has five valence electrons in its outermost shell, which are in the 4s and 4p orbitals. Arsenic, therefore, has five valence electrons and does NOT exhibit the property of “tetravalency.”
In the 2011 Science article, authors suggested that “the bacterium they discovered is able to substitute arsenic for phosphorus to sustain its growth,” and their data show evidence for arsenate in macromolecules that normally contain phosphate, most notably nucleic acids and proteins. A Phosphorus atom has 15 protons and 16 neutrons in its nucleus, with 15 electrons arranged in shells: 2 in the first shell, 8 in the second, and 5 in the outermost (valence) shell. Its ground state electron configuration is 3, meaning that the 5 valence electrons determine its chemical behavior, making it a nonmetal that typically forms covalent bonds, but it does not exhibit the property of “tetravalency”.
Looking at Dmitri Mendeleev’s 1869 Periodic Chart (which most of us likely do — at least once a week), we can see that Silicon is directly below Carbon. A Silicon atom has 14 protons and 14 neutrons in its nucleus, with 14 electrons in a configuration of The outermost shell, or valence shell, contains 4 electrons, which are involved in chemical bonding. In a crystalline solid, silicon atoms form a diamond cubic structure with each atom bonded to four neighbors in a tetrahedral arrangement. Yes, Si exhibits the property of tetravalency. GEITP recalls, back in the 1950s, reading a Sci-Fi novel in which an alien population’s life cycle was based on tetravalent Silicon; they were heat-resistant and, therefore, (this is theoretically plausible) they were able to live on their sun…!!
Directly below Silicon, on the Periodic Chart is Germanium, which has 32 protons and 32 electrons, with an electronic configuration of , meaning that its electron shell structure is 2, 8, 18, 4, and its outermost shell contains 4 valence electrons. In its solid state, germanium (like silicon) forms a diamond cubic crystal structure. Although this is a metal, it does exhibit tetravalency.
Now, the journal Science has finally decided to retract this controversial work [see attached pdf file]. However, instead of closing the books on one of its most infamous papers, the decision has stirred up the old controversy — and sparked new debate on “when the scientific record should formally be corrected.” Several long-standing critics of the research (including the GEITP) applaud the retraction. But the study’s authors have protested the decision and the journal editors’ justification — which included an accusation of flawed data.
In 2012, Science published two papers suggesting GFAJ-1’s growth did require phosphate (like other known life) and that — although the microbe could tolerate arsenic — it didn’t incorporate the element into DNA at significant levels. To many, the matter seemed finally settled, although some still wanted a retraction and the authors have repeatedly defended their findings. They argued that the 2012 papers did not faithfully reproduce the original study.
Others question the retraction’s timing, 15 years after the paper’s publication but just months after The New York Times profiled the first author’s return to research following the scandal. Science’s current Editor-in-Chief Holden Thorp, and Valda Vinson, executive editor of the Science family of journals, emphasize there is no suggestion of foul play in the GFAJ-1 paper; instead, pointing to subsequent commentary and research that suggest some of the paper’s findings stem from contamination — not arsenic use by bacteria, they write: “Science believes that the key conclusion of the paper is based on flawed data.” ☹
DwN
Science 24 Jul; 389: 329-329 [2-page news story]