Life, as we don’t know it

Weekend Argus (Saturday Edition) - - WORLD -

WASHINGTON: All life on Earth – from mi­crobes to ele­phants to us – is based on a sin­gle ge­netic model that re­quires the el­e­ment phos­pho­rus as one of its six es­sen­tial com­po­nents.

But now re­searchers have un­cov­ered a bac­terium that has five of those es­sen­tial el­e­ments but has, in ef­fect, re­placed phos­pho­rus with its look-alike but toxic cousin, ar­senic.

News of the dis­cov­ery caused a sci­en­tific com­mo­tion, in­clud­ing calls to Nasa from the White House and Congress ask­ing if a sec­ond line of Earthly life had been found.

A Nasa press con­fer­ence on Thurs­day and an ac­com­pa­ny­ing ar­ti­cle in the jour­nal Sci­ence, gave the an­swer – no, the dis­cov­ery does not prove the ex­is­tence of a so-called “sec­ond ge­n­e­sis” on Earth. But the dis­cov­ery very much opens the door to that pos­si­bil­ity, and to the re­lated ex­is­tence of a the­o­rised “shadow bio­sphere” on Earth – life evolved from a dif­fer­ent com­mon an­ces­tor from all that we’ve known so far.

“Our find­ings are a re­minder that life as we know it could be much more flex­i­ble than we gen­er­ally as­sume or can imag­ine,” said Felisa WolfeSi­mon, the young bio­chemist who led the ef­fort af­ter be­ing se­lected as a Nasa Astro­bi­ol­ogy Re­search Fel­low and as a mem­ber of the Na­tional Astro­bi­ol­ogy In­sti­tute team at Ari­zona State Uni­ver­sity (ASU).

“If some­thing here on Earth can do some­thing so un­ex­pected – that breaks the unity of bio­chem­istry – what else can life do that we haven’t seen yet?” she said.

The re­search, funded through Nasa and con­ducted with sam­ples from Cal­i­for­nia’s Mono Lake, found some of the bac­te­ria not only used ar­senic to live, but had ar­senic embed­ded into their DNA, RNA and other ba­sic un­der­pin­nings.

“This is dif­fer­ent from any­thing we’ve seen be­fore,” said Mary Voytek, se­nior sci­en­tist for Nasa’s pro­gramme in astro­bi­ol­ogy , the arm of the agency in­volved specif­i­cally in the search for life be­yond Earth and for how life be­gan here.

“These bugs haven’t just re­placed one use­ful el­e­ment with an­other, they have the ar­senic in the ba­sic build­ing blocks of their make-up,” she said. “We don’t know if the ar­senic re­placed phos­pho­rus or if it was there from the very be­gin­ning – in which case it would strongly sug­gest the ex­is­tence of a shadow bio­sphere.”

The­o­ret­i­cal physi­cist and cos­mol­o­gist Paul Davies, di­rec­tor of the Be­yond Cen­tre at Ari­zona State Uni­ver­sity and a pro­lific writer, is a co-author. He had been think­ing about the idea for a decade, and had writ­ten a paper in 2005. So had Uni­ver­sity of Colorado, Boul­der philoso­pher and as­tro­bi­ol­o­gist Carol Cle­land. Both asked why no­body was look­ing for life with dif­fer­ent ori­gins on Earth, and Cle­land coined the phrase “shadow bio­sphere”.

At a Be­yond Cen­tre con­fer­ence four years ago, WolfeSi­mon, then in her late 20s, pro­posed a way to search for a pos­si­ble shadow bio­sphere, and it in­volved Mono Lake and its ar­senic.

“We were kick­ing vague ideas around, but she had a very spe­cific pro­posal and went out and ex­e­cuted it,” Davies said. “It de­fies logic to think she found the only ex­am­ple of this kind of un­usual life. Quite clearly, this is the tip of a huge ice­berg.”

All life as we know it con­tains six es­sen­tial el­e­ments – car­bon, oxy­gen, hy­dro­gen, ni­tro­gen, sul­fur and phos­pho­rus – that have qual­i­ties that make them seem­ingly ideal for their tasks. A form of phos­pho­rus, for in­stance, is near per­fect for build­ing the frame­work for the DNA mol­e­cule, and an­other form is cru­cial to the trans­fer of en­ergy within cells.

These forms of phos­pho­rus are well suited for their job be­cause they are es­pe­cially sta­ble in the pres­ence of wa­ter. Ar­senic is not, and that fact is one that raises con­cerns for some re­searchers fa­mil­iar with the Mono Lake bugs.

Chemist Steven Ben­ner of the Foun­da­tion for Ap­plied Molec­u­lar Evo­lu­tion in Florida has been in­volved in “shadow bio­sphere” re­search for sev­eral years, and will speak at the Nasa un­veil­ing of Wolfe-Simon’s work.

He says the Mono Lake re­sults are in­trigu­ing.

“I do not see any sim­ple ex­pla­na­tion for the re­ported re­sults that is broadly con­sis­tent with other in­for­ma­tion well known to chem­istry” but he says they are not yet proven. And a pri­mary rea­son why, is that ar­senic com­pounds break down quickly in wa­ter, while phos­pho­rus com­pounds do not.

His con­clu­sion: “It re­mains to be es­tab­lished that this bac­terium uses ar­se­n­ate as a re­place­ment for phos­phate in its DNA or in any other biomolecule.”

The Mono Lake dis­cov­ery high­lights one of the cen­tral chal­lenges of astro­bi­ol­ogy – know­ing what to look for in terms of ex­trater­res­trial life. While it re­mains un­cer­tain whether the lake’s mi­crobes rep­re­sent an­other line of life, they show or­gan­isms can have a chem­i­cal ar­chi­tec­ture dif­fer­ent from what is cur­rently un­der­stood to be pos­si­ble.

“One of the guid­ing prin­ci­ples in the search for life on other plan­ets, and of our astro­bi­ol­ogy pro­gramme, is that we should ‘fol­low the el­e­ments’,” said Ariel An­bar, an ASU pro­fes­sor and bio­geo­chemist. “Felisa’s study teaches us that we ought to think harder about which el­e­ments to fol­low.”

Mono Lake was se­lected as a work site by Wolfe-Simon be­cause it is highly un­usual and al­ready had been well stud­ied by sci­en­tists try­ing to an­swer other ques­tions.

The lake re­ceives run-off from the Sierra Ne­vada moun­tains, which have rel­a­tively high con­cen­tra­tions of ar­senic. When the wa­ter ar­rives at Mono Lake, it has nowhere to go be­cause there are no rivers car­ry­ing wa­ter fur­ther down­stream.

That means the ar­senic, and other el­e­ments and com­pounds, can con­cen­trate to un­usu­ally high lev­els.

Wolfe-Simon said she hoped to fur­ther test her find­ings in norther n Ar­gentina where, she’s been told, some mi­crobes can not only tol­er­ate ar­senic, but ab­so­lutely need ar­senic to sur­vive. – Washington Post

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