The Se­cret Un­der the Ice

Trans­lated by per­mis­sion from his ar­ti­cle posted April 17, 2009, on his blog Cu­riosi­dades de la Mi­cro­bi­ología.

by Manuel Sánchez

There is an old sci­ence fic­tion story called Who Goes There that was twice made into a movie. It dealt with a sci­en­tific ex­pe­di­tion to the North Pole which, un­der the ice, found an ex­trater­res­trial space­ship car­ry­ing alien forms of life. Well, now a real sci­en­tific ex­pe­di­tion to the Antarc­tic found a com­plete ecosys­tem that was trapped un­der ice some 1.5 mil­lion years ago. The or­gan­isms of this ecosys­tem are of earthly ori­gin, but they may open up the pos­si­bil­ity that life ex­ists on other plan­ets.

Shot from the '51 movie The Thing from An­other World. Source

In the Antarc­tic, there is a site called Blood Falls, near the end of the Tay­lor Glac­ier, near the Mc­Murdo Dry Val­leys. This for­ma­tion comes from a deep sub­glacial lake about 400 m be­neath the ice at its high­est point. The strik­ing color is due to the wa­ter emerg­ing from the lake be­ing en­riched for fer­rous iron, which be­comes rapidly ox­i­dized to the in­sol­u­ble bright red fer­ric ion. The sub­glacial lake is part of a sys­tem of fjords that be­came trapped when the Tay­lor Glac­ier was formed some two mil­lion years ago.

Blood Falls. In the left cor­ner, see a cam­p­ing tent. Credit: P. Re­jcek, NSF. Source

Ge­omi­cro­bi­ol­o­gist Jill Mikucki and col­leagues stud­ied a num­ber of sam­ples col­lected over a pe­riod of six years and found that they con­tained 17 types of mi­crobes. The re­sults are pub­lished in Sci­ence, with a com­men­tary. Ge­nomic analy­sis re­vealed that these or­gan­isms are re­lated to sul­fate-re­duc­ing bac­te­ria (SRB) that ox­i­dize the scarce or­ganic ma­te­r­ial in the wa­ter, re­duc­ing sul­fate in the process.

Cu­ri­ously, these don't use sul­fate di­rectly in res­pi­ra­tion like their SRB cousins, nor do they con­vert it all the way to sul­fide. In­stead, these mi­crobes re­duce the in­sol­u­ble fer­ric ion to the sol­u­ble fer­rous ion, which is then ox­i­dized by con­tact with the air. Sul­fate here ap­pears to be used as a cat­a­lyst, its re­duced form be­com­ing re­ox­i­dized by cou­pling to the fer­ric to fer­rous (Fe³⁺ → Fe²⁺) re­ac­tion. These mi­crobes ap­pear to con­sti­tute a con­sor­tium that "breathes" fer­ric ions.

Phy­lo­ge­netic tree of se­quences of the dissimi­latory adeno­sine 5'-phospho­sul­fate–reductase in mi­crobes from the Blood Falls. Most are re­lated to the genus Desul­fo­capsa. Some (group 2) have a closer re­la­tion­ship to Thermodesul­fovi­brio. Source

The wa­ter of the sub­glacial lake con­tains no dis­solved oxy­gen and has a pH of 6.2. Its tem­per­a­ture is −5º C, but the wa­ter does not freeze be­cause its salt con­cen­tra­tion is four times that of sea­wa­ter. The con­cen­tra­tion of mi­crobes is about 10,000 per ml. Their gen­er­a­tion time was es­ti­mated to be 300 days; thus, there have been around one mil­lion gen­er­a­tions since the lake be­came en­cased in ice.

This find­ing helps ex­plain how life might sur­vive the cold pe­ri­ods pro­posed by the "Snow­ball Earth" the­ory, dur­ing which the planet is pos­tu­lated to have been en­tirely cov­ered by ice. More­over, this mi­cro­bial strat­egy prompts spec­u­la­tion about life ex­ist­ing in such in­hos­pitable and cold places as Mars or Jupiter's moon Eu­ropa – the dream of every exo­bi­ol­o­gist.

For a com­men­tary in Na­ture, click here.

Scheme for how the sub­glacial mi­crobiota ox­i­dize or­ganic com­pounds us­ing sul­fate as an elec­tron ac­cep­tor. Re­duced sul­fur is re­ox­i­dized by re­duc­tion of fer­ric to fer­rous ions. Fer­rous is sol­u­ble and is ox­i­dized to fer­ric when in con­tact with oxy­gen, be­com­ing in­sol­u­ble and form­ing red­dish pre­cip­i­tates. Source