On Thursday, December 2, NASA scientist announced the discovery of bacteria in Mono Lake in California that, in an environment low in phosphorus and high in arsenic, have evolved to replace phosphorus in its biology with arsenic, the next element down in the periodic table. Before this discovery, the principal elements necessary for life (absent a few elements required in small quantities) were only carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus. The newly discovered bacteria present new possibilities for the chemistry of life.
This discovery got me thinking about if any of the other usual elements of life might similarly be substituted by the next element down the periodic table. In fact, the only other candidate for such a substitution would be sulfur, which might be substituted by the element below it in the periodic table, selenium. After a certain amoun of thought, I've decided that such substitution is unlikely.
See below the fold to see why...
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In living systems, phosphorus occurs as phosphate (PO43-). (I asked my biochemist colleague if she could think of an exception to this in the biochemistry of phosphorus, and she couldn't.) As such, phosphorus in life is always surrounded by four oxygen atoms in a tetrahedral geometric arrangement. The analogous arsenic species is arsenate (AsO43-), and, presumably, this is the species that would substitute for phosphate. There is a difference in the relative sizes of the species (P-O bond length = 154-156 pm; As-O bond length = 167-170 pm), which will affect the structures of biomolecules that incorporate the arsenate in place of phosphate, and presumably, this is why it's deadly to most life. But it's not hard to imagine that the process natural selection would allow the incorporation of arsenate in place of phosphate in an environment where phosphorus is rare.
In contrast to phosphorus, sulfur in living systems occurs in a variety of different states. Sulfur can be bonded to carbon, to hydrogen, to oxygen, or to another sulfur. The likelihood that selenium could replace sulfur in all those different contexts is very low.
While it's entirely possible that, somewhere in the universe, the chemistry of life incorporates selenium, I think it's highly unlikely that selenium will be found incorporated in living systems to any great extent here on Earth.
Now that that's settled (more or less), let's go on to Top Comments!
From sardonyx:
In bruh1's interesting diary Obama, African-Americans and Economic Policy, TarheelDem gives the perspective of a white progressive Southerner.
From Ed Tracey
I like this snark from Land of Enchantment
:
88% of Democrats believe John Boehner is an alien from a planet of
soulless, self-tanning aliens that can secrete their own hair gel when sexually
aroused.
And this was before Boehner's "60 Minutes" self-pity bout.
[From the kOscars diary.--ed]
From bronte17:
inclusiveheart tells how the tables have been turned in Elizabeth Warren: Washington, We Still Have a Problem by bobswern.
From your humble diarist:
In Dante Atkins' front page story commemorating the Supreme Court's decision in Bush v. Gore, Free Jazz at High Noon makes a comment regarding what it represents in the history of the decline of the U. S.
In BFSkinner's recommended diary Has being on Daily Kos changed your views on people? Dreaming of Better Days gets a good one in.
Top 30 Comments excluding tip jars, first comments, pooties and stuff:
1) From a contemporary hippie to a priceless one... by APA Guy — 100
2) I Am Worried Younger People Will Forget by webranding — 96
3) When Obama really wants it, he doesn't wait by New Deal democrat — 91
4) I love you. by Miep — 91
5) Like Bedford Falls by triv33 — 82
6) Economic questions regarding race by bruh1 — 79
7) You write this diary, this week and you make this by Clytemnestra — 78
8) Why? Republicans lying, MSM covering for them... by pollwatcher — 78
9) comments in response are appreciated by teacherken — 77
10) I'm working on a two-part by brooklynbadboy — 77
11) Noticed a lot of glee at sticking it to liberals by paradox — 74
12) It Better Be a LARGE Chimney by JekyllnHyde — 72
13) Letting our kids by beltane — 70
14) Yeah, because they pay so f*cking by davidseth — 70
15) Hijack. Why don't you write on those by DonkSlayer — 70
16) The Economic Blues by JekyllnHyde — 70
17) Both Eugene Debs and John Lennon by JekyllnHyde — 68
18) I always like your diaries by nancat357 — 67
19) They say in Harlan County by litho — 67
20) Nice diary. by SpecialKinFlag — 67
21) The most discouraging thing about Prez Obama by RainbowGirl — 66
22) Ah, they could get a job if by divineorder — 66
23) I see. Economic issues that impact race by bruh1 — 66
24) there are those times by valadon — 65
25) last winter when i was suffering by ridemybike — 64
26) I Am Embarassed to Admit by shanikka — 63
27) Thank you for this diary. by blue jersey mom — 63
28) Democracy: Majority rules with the rights of the by daliscar — 61
29) We love by crose — 61
30) The Villagers learn only what their paychecks by JimWilson — 61
31) Maher should not have by slinkerwink — 61
Top 30 Comments with no exclusions, aka the Tip Jar list :-):
1) This community means everything to me by noweasels — 469
2) Tip Jar by teacherken — 435
3) All power to the people. by davidseth — 387
4) Tip Jar by Democrats Ramshield — 283
5) Tip Jar by gjohnsit — 262
6) Tip Jar by Kristina40 — 232
7) Tip Jar by bobswern — 232
8) Best wishes from Daily Jerome for a better 2011! by Seneca Doane — 224
9) Tip Jar by mka193 — 196
10) Tip Jar by edrie — 165
11) Tip Jar by bruh1 — 156
12) Tip Jar by SulaymanF — 143
13) Tip Jar for the former middle class by gjohnsit — 126
14) Tip Jar by BFSkinner — 115
15) Tip Jar by Julian Domain — 107
16) From a contemporary hippie to a priceless one... by APA Guy — 100
17) I Am Worried Younger People Will Forget by webranding — 96
18) I love you. by Miep — 91
19) When Obama really wants it, he doesn't wait by New Deal democrat — 91
20) Tips for Zen Master by whoknu — 88
21) Tip Jar by Ojibwa — 87
22) Like Bedford Falls by triv33 — 82
23) Economic questions regarding race by bruh1 — 79
24) I'm working on a two-part by brooklynbadboy — 79
25) You write this diary, this week and you make this by Clytemnestra — 78
26) Why? Republicans lying, MSM covering for them... by pollwatcher — 78
27) comments in response are appreciated by teacherken — 77
28) Noticed a lot of glee at sticking it to liberals by paradox — 74
29) It Better Be a LARGE Chimney by JekyllnHyde — 73
30) Tip Jar by demoKatz — 72