De Pro­fundis…

by Elio

We are fond of post­ing "Tal­mu­dic Ques­tions," ques­tions we con­sider to be more in­ter­est­ing than their an­swers. The very first of them, dated De­cem­ber 1, 2006, reads as fol­lows:

Where on Earth would one ex­pect to find a sin­gle species of mi­crobes (a pure cul­ture) for sus­tained pe­ri­ods of time? Sym­bioses (mu­tu­al­is­tic or par­a­sitic) do not count, nei­ther do Petri dishes.

Scan­ning elec­tron mi­cro­graph of a sam­ple from a dif­fer­ent mine, a mere 648 me­ters down, where D. au­daxvi­a­tor is also the pre­dom­i­nant or­gan­ism. The tiny glob­u­lar ob­jects are pre­sum­ably sil­i­cates, which have been found to pre­cip­i­tate upon cool­ing af­ter sam­ple col­lec­tion. Source

It turns out there is now a can­di­date (ac­tu­ally, a Candi­da­tus) nom­i­nated for this lone­some role. It is a bac­terium, named Can­di­da­tus 'Desul­forudis au­daxvi­a­tor' , found in the ground wa­ter of a South African gold mine, 2.8 km be­low the sur­face. Here, in this chal­leng­ing lo­cale, D. au­daxviator ac­counts for at least 99.9% of all liv­ing things (with the re­main­ing frac­tion thought to be trace conta­minants from the mine or lab). A fair amount is known about it, even though it has yet to be cul­tured. It is a Gram-pos­i­tive ob­lig­ate anaer­obe, a mem­ber of the Fir­micutes and some­what re­lated to the Clostridia. Home is warm and al­ka­line – around 60°C and pH 9.3. Its fully se­quenced genome tells us that it is prob­a­bly motile and a spore­former, and that it makes its liv­ing from ox­i­diz­ing hy­dro­gen us­ing sul­fate as the elec­tron ac­cep­tor. Both of those sub­stances are rel­a­tively abun­dant in its habi­tat. Its hy­dro­gen "fuel" is prob­a­bly formed as a by-prod­uct of the ra­dioac­tive de­cay of ura­nium within the earth. Thus D. au­daxvi­a­tor car­ries the ban­ner for Life With­out Sun­light to new depths.

How­ever, this is not life at it's fullest.

The bio­mass of D. au­daxvi­a­tor is not over­whelm­ing. By flow cy­tom­e­try, the au­thors de­ter­mined a mod­est pop­u­la­tion of about 3.3x10⁴ cells/ml in the wa­ter of this aquifer. In ad­di­tion, their best guess is that food­stuff is so lim­it­ing that the gen­er­a­tion time of these or­gan­isms may be reck­oned in the hun­dreds, if not thou­sands, of years. This is not life in the fast lane. But not hav­ing compe­titors al­lows for a leisurely ex­is­tence.

Deep gold mines are great places to set up a ge­omi­cro­bi­o­log­i­cal lab­o­ra­tory – if you can stand the heat. But, think of the ad­van­tages of not hav­ing to drill down those thou­sands of me­ters from the sur­face. It bears not­ing that this re­port is based on a lot of work, in­volv­ing 20 re­searchers, and with it comes a colos­sal amount of sup­ple­men­tary data.

The name of the or­gan­ism tells its story. It stems from Jules Verne's Jour­ney to the Cen­ter of the Earth, where the ex­plor­ers find a se­cret in­scrip­tion: De­scende, au­dax vi­a­tor, et ter­restre cen­trum at­tinges (De­scend, bold trav­eler, and you will reach the cen­ter of the earth).