Fine Read­ing: Adap­ta­tions to en­ergy stress dic­tate the ecol­ogy...

...and evo­lu­tion of the Ar­chaea, by D. L. Valen­tine

by Elio

It's been over thirty years since Carl Woese dis­cov­ered the Ar­chaea, yet they still qual­ify as the most puz­zling of the do­mains of life. Many of them are ex­tremophiles, but not all, and they dif­fer from the Bac­te­ria in sig­nif­i­cant ways, such as their rRNAs and tR­NAs, the lipids in their mem­brane, and how they syn­the­size macro­mol­e­cules. Yet typ­i­cal mem­bers of the two do­mains tend to be sim­i­lar in size and body plan, and, when com­pared with the eu­kary­otes, they have a more con­fin­ing de­vel­op­men­tal reper­toire. In dis­tin­guish­ing be­tween Ar­chaea and Bac­te­ria, we are helped by an in­sight­ful pa­per by David Valen­tine, who pro­poses that the dif­fer­ence be­tween them re­sides in the Ar­chaea be­ing bet­ter adapted to en­ergy stress, some­thing that sooner of later all free-liv­ing or­gan­isms ex­pe­ri­ence. In­cluded in the ar­gu­ment are not only the ex­tremophilic Ar­chaea, but also those liv­ing in a more 'reg­u­lar' en­vi­ron­ment, such as ni­tri­fiers and methanogens.

Uni­ver­sal phy­lo­ge­netic tree based on com­par­a­tive se­quence data from 16S or 18S ri­bo­so­mal RNA. Red = hy­per­ther­mophilic species; laven­der = anoxy­genic pho­to­syn­thetic species; green = oxy­genic pho­to­syn­thetic species. Source

Ev­i­dence? Valen­tine points to the dis­tinc­tive lipid com­po­si­tion of their mem­branes, which ren­ders them less per­me­able to ions than are their bac­te­r­ial counterparts—a prop­erty that fa­vors the main­te­nance of en­ergy un­der stress­ful con­di­tions. In ad­di­tion, the Ar­chaea have evolved en­ergy-con­serv­ing cata­bolic abil­i­ties. He writes: …the ar­chaea have evolved to thrive with en­ergy stress, whereas bac­te­ria can adapt to max­i­mize the avail­abil­ity of en­ergy and other re­sources.

This way of think­ing leads to pos­si­ble ex­pla­na­tions for a num­ber of in­ter­est­ing at­trib­utes of the Ar­chaea. For in­stance, Valen­tine writes: Path­o­genic ar­chaea are rare or nonex­is­tent be­cause the in­her­ent ten­dency of ar­chaea to thrive with chronic en­ergy stress is in­com­pat­i­ble with the dy­namic na­ture of patho­gen­e­sis. Bac­te­ria are in­her­ently bet­ter suited to patho­gen­e­sis as it is con­sis­tent with their ca­pac­ity to seek new re­sources and adapt to chang­ing en­vi­ron­ments.

This be­ing an ex­haus­tive pa­per, there is lots more of this sort of ex­cit­ing stuff. If you're ea­ger to keep the Ar­chaea and Bac­te­ria straight, this is a good place to start. Nei­ther of these en­ergy strategies—living with chronic en­ergy stress or seek­ing out new resources—is the best op­tion un­der all cir­cum­stances. Whichever one adopts sets the stage for the fu­ture, for where you will be able to thrive and to win against the com­pe­ti­tion.