In­trater­res­tri­als

by Manuel Sánchez

Among Jules Verne's fa­mous works is Jour­ney to the Cen­ter of the Earth. Have you read the book or seen one of the movie ver­sions? If so, you will re­mem­ber that when the fear­less ex­plor­ers de­scended into the depths of a vol­cano, they en­coun­tered pre­his­toric life forms such as ple­siosaurs. It turns out that Verne wasn't far off. When we de­scend into the Earth's depths, we may not find gi­gan­tic liv­ing forms, but we do en­counter mi­crobes aplenty. In ad­di­tion, vol­ca­noes have a lot to do with this story. The deep ocean has "ridges" where new crust is be­ing made. Here, vol­ca­noes are con­stantly spew­ing forth molten rock that cools rapidly upon con­tact with sea wa­ter, form­ing "basaltic pil­lows." Be­cause these struc­tures cool rapidly, they be­come sur­rounded by a layer of crys­talline basalt. These rocks are rich in re­duced min­er­als, a pos­si­ble source of en­ergy for chemolitho­troph­ic mi­crobes. Such meta­bolic ac­tiv­i­ties can be ver­i­fied in the lab, but how to mea­sure this ac­tiv­ity deep in the oceans? Keep in mind the prob­lems of work­ing at depths of 5000 me­ters and at pres­sures 600 times greater than those at sea level.

Deep sea basaltic pil­lows. Source

Sure enough, we can only study such mat­ters in­di­rectly. A re­port re­cently pub­lished in Na­ture claims that the ba­saltic rocks of the ocean ridges con­tain an abun­dance of mi­crobes. The re­searchers com­bined three tech­niques: quan­ti­ta­tive PCR, mi­croscopy, and in situ nu­cleic acid hy­bridization. Guess what, these rocks con­tain 1,000 to 10,000 times more mi­crobes than the sur­round­ing wa­ters. Not only that, their bio­di­ver­sity is quite unique. In deep wa­ters near the sea bot­tom, there are be­tween 8 x 10³ and 9 x 10⁴ microbes/ml, half of them bac­te­ria, half ar­chaea. In con­trast, in the sed­i­ments the num­bers are be­tween 3 x 10⁶ and 1 x 10⁹ per gram of rock. More than 90% are bac­te­ria, most γ‑proteobacteria.

Tubes in the crys­talline sur­face layer of a basaltic pil­low. The holes are thought to be made by chemolithotrophic mi­crobes that "eat" the crys­tal. Source

Is this a lot or lit­tle? De­pends on your frame of ref­er­ence. In agri­cul­tural soils, the num­ber of bac­te­ria is higher than 10⁷ per gram. Thus, sea bot­tom basalt is per­haps not a bac­te­r­ial gar­den of Eden, but it is far from a bar­ren and in­hos­pitable site.

Flu­o­res­cence mi­croscopy of a sam­ple of en­dolithic bac­te­ria. Green is flu­o­res­cence of the rock, red of bac­te­ria. Source

What do these or­gan­isms eat? Good ques­tion, with no good an­swer as yet. It is be­lieved that at the base of the trophic pyra­mid (read "food chain") are chemo­lithotrophs or mixotrophs that ox­i­dize the sul­fur, iron, and man­ganese present in the basaltic crys­tals. This has been shown ex­per­i­men­tally in the lab, but in na­ture…? Let's face it, it hasn't been shown there yet. No mat­ter. These in­trater­res­trial or­gan­isms im­pact the biogeoche­mical cy­cles, be­ing re­spon­si­ble for the fix­a­tion of as much as 500,000 tons of car­bon a year.

Not bad for or­gan­isms that we didn't even know ex­isted un­til re­cently.

Orig­i­nally pub­lished in Span­ish in Cu­riosi­dades de la Mi­cro­bi­ología.