Cryp­tic Life in the Antarc­tic Dry Val­leys

by Merry Youle

On first in­spec­tion the habi­tat seems as ster­ile as the sur­face of au­to­claved glass­ware... but the trained eye, aided by a mi­cro­scope, sees oth­er­wise.
 —E.O. Wil­son, The Fu­ture of Life

For the mi­cro­bi­ol­o­gist, the next best thing to a trip to Mars might well be an ex­pe­di­tion to the Mc­Murdo Dry Val­leys of Antarc­tica. Here, in the Earth's cold­est and dri­est deserts, the con­di­tions ap­proach those on our neigh­bor­ing planet and are thought to also ap­proach the cold-arid limit for life. Not all of the Dry Val­leys are equally dry. Some have peren­ni­ally frozen, glac­ier-fed lakes and ephemeral streams, and thus have some soil mois­ture. Here one finds more of life, even three taxa of mul­ti­cel­lu­lar an­i­mals – tardi­grades, ro­tifers, and, most nu­mer­ous, bac­te­r­ial-feed­ing ne­ma­todes. (Click here for our ear­lier post about the mi­crobes found in a sub­glacial lake in this re­gion.) Lack­ing such a source of wa­ter, thus be­ing one of the truly dry Dry Val­leys, is McK­elvey Val­ley. What lit­tle snow falls here or blows in sub­li­mates in the cold, hy­per-arid con­di­tions. How cold? The av­er­age air tem­per­a­ture is around –20°C. Win­ter brings pro­longed pe­ri­ods of −55°C cold; in sum­mer, air tem­per­a­ture can rise to a balmy 0°C. Hu­mid­ity is low (<10% RH in win­ter). And then there are the cease­less kata­batic winds (from the Greek word kata­batikos mean­ing "go­ing down­hill"). Cold, dense air falls down­hill off the East Antarc­tic Ice Sheet and races through the val­ley at speeds com­monly ex­ceed­ing 50 km/h, some­times reach­ing 320 km/h (200 mph). These winds evap­o­rate all mois­ture and carry sand grains that scour the bar­ren land­scape. All in all, these con­di­tions bear some re­sem­blance to those used to freeze-dry bi­o­log­i­cal sam­ples.

A ven­tifact (a stone sculpted by wind-dri­ven sand) in the "Mar­t­ian" land­scape of the Mc­Murdo Dry Val­leys. Source

The val­ley floor is an un­sta­ble, grav­elly, des­ic­cated mine­ral soil with high salin­ity, lit­tle or­ganic ma­te­r­ial, and the low­est ni­trate con­cen­tra­tions known for any ter­res­trial soil. Sur­face tem­per­a­tures fluc­tu­ate wildly un­der the in­tense sum­mer sun, see­saw­ing be­tween –15° and +27.5°C in a few hours. UV is in­tense. And, of course, the winds scour the land and carry off any hint of mois­ture. Never­theless, life car­ries on. Some in­tre­pid re­searchers left the com­forts of Hong Kong or New Zealand or even rel­a­tively balmy Min­nesota to look for or­gan­isms liv­ing un­der such ex­treme con­di­tions. As re­ported in their re­cent pa­per, there are some bac­te­ria liv­ing in these dry sur­face soils, mostly Aci­dobac­te­ria and Acti­nobac­te­ria. As you might ex­pect, at the top of the list are des­ic­ca­tion tol­er­ant taxa such as Deinococ­cus and Rubrobac­ter. There are also nu­mer­ous ni­tro­gen fix­ers. With nei­ther pho­toau­totrophs or chemoau­totrophs present, or­ganic car­bon is at a pre­mium, and its lack is thought to pre­clude fur­ther com­mu­nity de­vel­op­ment.

En­dolithic com­mu­nity layer in frac­tured sand­stone. Source

Both qual­i­ta­tively and quan­ti­ta­tively, most of the life in this val­ley is as­so­ci­ated not with the soil, but with the rocks. Here and there the flat-ly­ing sed­i­men­tary bedrock is ex­posed. The rock sur­faces, and even the shal­low cracks, are likely ster­ile due to tem­per­a­ture fluc­tu­a­tions and abra­sion from wind­blown sand. But any life that bur­rows in even just a few mil­lime­ters finds more sta­ble tem­per­a­tures and shel­ter from the in­ces­sant wind, even in the in­tense cold. Given the es­ti­mate that –6ºC or –8ºC is the lower limit for meta­bolic processes, some ac­tiv­ity would be pos­si­ble in these rock niches for 1000 hours per year at the most. How­ever, that is enough to sus­tain mi­cro­bial com­mu­ni­ties, both the en­doliths, or­gan­isms that live within the porous struc­ture of the rock it­self and the chas­moliths that live within the cracks and crevices.

Chas­moliths: a lich­enized mi­cro­bial com­munity pro­truding from a gran­ite rock frac­ture. Source

Due to their struc­ture and min­er­al­ogy, sed­i­men­tary rocks such as the lo­cal sand­stone are particu­larly well-suited for hous­ing en­doliths a few mil­lime­ters be­neath their sur­face. Enough light pe­netrates for pho­to­syn­the­sis (at least dur­ing the months when there is sun), while dam­ag­ing UV is re­duced. In con­trast to the soil com­mu­nity, these rock dwellers are mostly pho­toau­totrophs. Two dis­tinct com­mu­ni­ties of bac­te­ria and eu­kary­otes are de­tectable, both vis­i­bly and ex­per­i­men­tally. The up­per 2 mm, called the lichen zone, is dom­i­nated by a lich­enized fun­gus, Tex­ospo­rium sancti-ja­cobi, as­so­ci­ated with the green alga Tre­bouxia jame­sii. Be­low 2 mm, the pho­to­syn­thetic cyano­bacteria rule, mostly Chroococ­cid­iop­sis, a genus noted for hav­ing ra­diore­sis­tance com­pa­ra­ble to that of Deinococ­cus ra­dio­du­rans. Low light lev­els not with­stand­ing, light may not be the fac­tor lim­it­ing these en­dolithic com­mu­ni­ties, but rather lack of avail­able CO₂. The chas­molith commu­nities in the crevices are made up pri­mar­ily of var­i­ous lichens and cyanobac­te­ria, com­bined with a dis­tinc­tive sprin­kling of other bac­te­r­ial groups.

A typ­i­cal quartz hy­polith show­ing no exter­nal ev­i­dence of the un­der­ly­ing mi­crobial com­mu­nity. Source

There is yet a fourth cryp­tic com­mu­nity here, the hypo­liths that live un­der­neath light-col­ored, translu­cent stones. These are al­most ex­clu­sively cyanobac­te­ria, mak­ing do in an en­vi­ron­ment that re­ceives less than 0.1% of the in­ci­dent light. There are no fungi here, and only a very few al­gae.

There is a ten­dency to think that the more ex­treme the en­vi­ron­ment, the less the bi­o­log­i­cal di­ver­sity. The com­munities in the Dry Val­leys don't con­form to that pat­tern. The en­doliths and chas­moliths com­bined com­prise more than 50 bac­te­r­ial species (based on 98% iden­tity of their 16S rDNA se­quences). Al­though eu­kary­otes rep­re­sent only 5% of these com­mu­ni­ties, they in­clude four gen­era of fungi (both As­comy­cota and Basidio­mycota) as well as three groups of al­gae. Add in the hy­poliths, and these three lithic com­mu­ni­ties span 16 phyla in two do­mains.

What about the miss­ing ar­chaea, those mas­ters of ex­treme en­vi­ron­ments? Not a one to be found so far, not even one lurk­ing un­der a rock. Surely there are yet even more sur­prises to be found by the cryophilic re­searcher.

Ref­er­ence

Point­ing SB, Chan Y, La­cap DC, Lau MC, Ju­r­gens JA, Far­rell RL (2009). Highly spe­cial­ized mi­cro­bial di­ver­sity in hy­per-arid po­lar desert. Pro­ceed­ings of the Na­tional Acad­emy of Sci­ences of the Uni­ted States of Amer­ica, 106 (47), 19964–19969. PMID 19850879