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Strange, underworld microbes raise hopes for alien life

Oct. 19, 2006
Special to World Science  

Re­search­ers have found what they say are iso­lat­ed bac­te­rial co­lo­nies flou­rish­ing deeper in the Earth’s crust than was known to be pos­si­ble.

The strange beings thrive on ra­di­o­ac­tive wa­ter, in a harsh set­ting cut off from sur­face life and its de­pen­d­ence on sun ener­gy, the sci­en­tists claim. That, they add, rais­es hopes that oth­er plan­ets in our so­lar sys­tem could al­so har­bor hardy mi­crobes within them.

Li­sa Pratt of In­di­an­a Uni­ver­si­ty Bloom­ing­ton (left) and Tul­lis On­stott of Prince­ton Uni­ver­si­ty in Prince­ton, N.J. col­lect mi­crobes in the Lu­pin gold mine in Nu­na­vut Ter­ri­to­ry, Can­a­da. (Cred­it: Li­sa Pratt).
“These bac­te­ri­a are tru­ly unique in the pur­est sense of the word,” said Li-Hung Lin of Na­tion­al Tai­wan Uni­ver­si­ty in Tai­pei, lead au­thor of a pa­per on the find in the Oct. 20 is­sue of the re­search jour­nal Sci­ence.

The mi­crobes were re­port­ed to come from 2.8 km (al­most two miles) un­der­ground at a gold mine site.

Most crea­tures in the known food web de­pend on sun­light one way or an­oth­er. They eith­er live off it di­rect­ly, like plants, or they eat those that do. 

Some organisms live so far un­der­ground or be­low the sea, though, that they seem to live apart from this net­work, in­de­pen­dent of sunlight.

But proof that these are to­tal­ly iso­lat­ed from sur­face food webs has been lack­ing, the re­search­ers said. And for many of these creatures, it has been un­clear wheth­er they were re­cent ar­ri­vals bound for ex­t­inc­tion or per­ma­nent re­si­dents. 

That could change with the new find, they said. 

“We know sur­pris­ing­ly lit­tle about the or­i­gin, ev­o­l­u­tion and lim­its for life on Earth,” said bio­geo­chem­ist Li­sa Pratt of In­di­an­a Uni­ver­si­ty Bloo­m­ing­ton, a mem­ber of the re­search team. 

“Sci­en­tists are just be­gin­ning to stu­dy the di­verse or­gan­isms liv­ing in the deep­est parts of the ocean. The rocky crust on Earth is vir­tu­al­ly un­ex­p­lored at depths more than half a ki­l­o­me­ter be­low the sur­face. The or­ga­n­isms we de­s­cribe in this pa­per live in a com­p­lete­ly dif­fer­ent world than the one we know.”

The re­search­ers ar­gued that the bac­te­ri­al com­mu­ni­ties they found are per­ma­nent, ap­par­ent­ly mil­lions of years old, and de­pend on ra­di­a­tion from ura­ni­um ores rath­er than sun­light. This raises the pos­si­bil­i­ty that si­m­i­lar bac­te­ri­a could live be­neath the sur­faces of worlds such as Mars or Jupiter’s moon Eu­ro­pa, the sci­en­t­ists said.

The find­ing came af­ter they learned of a newly cracked rock seeping water in a gold mine near Jo­han­nes­burg, South Af­ri­ca, and saw a chance to study deep rock un­touched by ma­n. They de­scended hot, gas-choked mine­shafts to in­ves­t­i­gate.

Ge­net­ic anal­y­ses re­vealed mul­ti­tudes of bac­te­ri­al spe­cies, the re­search­ers found, dom­i­nat­ed by one type re­lat­ed to bac­te­ri­a from the di­vi­sion Fir­mi­cutes. Oth­er Fir­mi­cutes are fa­mil­iar to sci­en­tists from deep-sea springs called hydroth­ermal vents. The stud­ies sug­gested the un­der­ground Fir­mi­cutes lost con­tact with their sur­face cousins be­tween 3 mil­lion and 25 mil­lion years ago, the re­search­ers added.

“We know how iso­lat­ed the bac­te­ri­a have been be­cause our anal­y­ses show that the wa­ter they live in is very old and has­n’t been di­lut­ed by sur­face wa­ter,” among oth­er rea­sons, Lin said. 

The mi­crobes use hy­dro­gen for res­pi­ra­tion, an en­er­gy-generating chem­i­cal pro­cess, the re­search­ers added. The hy­dro­gen is av­ai­l­a­ble thanks to ra­di­o­ac­tive de­cay of the el­e­ments ura­ni­um, tho­ri­um, and po­tas­si­um, they said, which emit ra­di­a­tion as they de­cay. The ra­di­a­tion breaks down wa­ter, re­leas­ing its com­po­nent hy­dro­gen.

Hy­dro­gen gas con­tains pent-up en­er­gy that can be re­leased in the pres­ence of cer­tain oth­er sub­stances such as ox­y­gen or sul­fate—as the 1937 Hin­den­burg dis­as­ter dem­on­strat­ed. Fir­mi­cutes can har­vest en­er­gy from the re­ac­tion of hy­dro­gen and sul­fate, the re­search­ers found, and the bac­te­ri­a’s chem­i­cal waste prod­ucts serve as food for oth­er, neigh­bor­ing mi­crobes.

In­di­rect­ly, the fir­mi­cutes cap­ture ra­di­a­tion en­er­gy and pass the ben­e­fits along to eve­ry­one else, Lin and col­leagues ar­gued—­much as or­gan­isms that thrive on sun­light, such as trees and plank­ton, do in our sur­face hab­i­tat. 

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Researchers have found what they say is an isolated bacterial community living deeper in the Earth’s crust than was thought possible. The otherworldly creatures thrive on water tainted with radioactive uranium in a harsh setting disconnected from surface life forms, the scientists claim. That, they add, is raising hopes that other planets in our solar system could also harbor microbes. “These bacteria are truly unique, in the purest sense of the word,” said Li-Hung Lin of National Taiwan University, lead author of a paper in the Oct. 20 issue of the research journal Science detailing the find. The microbes were reported to come from 2.8 km (almost two miles) underground in a South African gold mine. “We know surprisingly little about the origin, evolution and limits for life on Earth,” said biogeochemist Lisa Pratt of Indiana University Bloomington, a member of the research team. “Scientists are just beginning to study the diverse organisms living in the deepest parts of the ocean. The rocky crust on Earth is virtually unexplored at depths more than half a kilometer below the surface. The organisms we describe in this paper live in a completely different world than the one we know.” Underground bacteria aren’t news, but it wasn’t known before now whether they were recent arrivals bound for extinction or permanent fixtures of an unlikely habitat, the scientists said. Also, many scientists have doubted whether such communities could be totally disconnected from surface food webs, the scientists said. These networks ultimately depend on sunlight, which directly nourishes plants and many microorganisms. The researchers argued that the bacterial communities they found are permanent, apparently millions of years old, and depend on radiation from uranium ores rather than sunlight. This raises the possibility that similar bacteria could live beneath the cold surfaces of other worlds, such as Mars or Jupiter’s moon Europa, they researchers said. The finding came after the scientists learned of a new water-filled fracture in a gold mine near Johannesburg, South Africa, and saw a chance to study deep rock untouched by man. They descended the mine’s hot, gas-choked shafts to study water seeping from the crack. Genetic analyses revealed multitudes of bacterial species, dominated by one new species related to bacteria from the division Firmicutes, the researchers noted. Other members are familiar to scientists from deep-sea springs called hydrothermal vents. The studies suggested the underground Firmicutes lost contact with their surface cousins from 3 million to 25 million years ago, the researchers added. “We know how isolated the bacteria have been because our analyses show that the water they live in is very old and hasn’t been diluted by surface water,” among other reasons, Lin said. The microbes depend on hydrogen for respiration, an energy-generating chemical process, the researchers added. This element arises thanks to radioactive decay of the elements uranium, thorium, and potassium, they added, which emit radiation as they decay. The radiation breaks down water, releasing its component hydrogen. Hydrogen gas contains pent-up energy that can be released in the presence of certain other substances such as oxygen or sulfate—as the 1937 Hindenburg disaster demonstrated. Firmicutes can harvest energy from the reaction of hydrogen and sulfate, the researchers found, and the bacteria’s chemical waste products serve as food for other, neighboring microbes.