News From the Miss­ing Methanogenic Ar­chaea Front

by Elio

Best I can tell, methano­gen­e­sis re­mains a purview of the Ar­chaea. Bac­te­ria are still not known to do it and plants emit methane but ap­par­ently by re­cy­cling it from the at­mosphere. We asked why this is in an early Tal­mu­dic Ques­tion (see here), which elicited some truly in­ter­est­ing com­ments. Some of which led Bill Mar­tin into ask­ing a whole raft of re­lated ques­tions, for ex­am­ple "How is it pos­si­ble that methanogens re­quire help from chemi­os­mosis in or­der to gen­er­ate a pro­ton gra­di­ent? Re­lated: why don't they make ac­etate from H2 and CO₂ ?". Per­haps some of these ques­tions have been an­swered by now and have grad­u­ated from the Tal­mu­dic class.

Fig­ure 1. Whole cells of one of the hy­per­saline lake strains. Source. Fron­tispiece: A twig from a hy­per­saline lake. Source

But let me stop di­gress­ing. The news is that an in­ter­na­tio­nal group of in­ves­ti­ga­tors have iso­lated novel me­tha­no­ge­nic lin­eages of halophilic ar­chaea. To find them, the re­searchers sam­pled hy­per­saline soda lakes and neu­tral lakes in south­east­ern Siberia and south­ern Rus­sia. They found two kinds of methanogenic strains that are phy­lo­ge­ne­ti­cal­ly close to the halophilic ar­chaea. One rea­son for the ex­cite­ment is that halophilic ar­chaea have been thought to come from the methanogens. The phy­logeny of these new or­gan­isms adds to the plau­si­bil­ity of the no­tion.

Fig­ure 2. Thin sec­tion of a cell of one of the hy­per­saline lake strains. Source

To iso­late these strains, the work­ers grew sam­ples in the pres­ence of methane pre­cur­sors such as a duo of for­ma­te and ei­ther methanol or trimethy­lamine, which are used in the methyl-re­duc­ing path­way, one of the three known for methano­gen­e­sis. They tried to im­i­tate the prove­nance site by adding 4 M salt, us­ing pH 7.0 for sam­ples from neu­tral lakes and pH 9.5 – 10 for soda lake sam­ples, and tem­per­a­tures of 48 – 55°C. The cells are small motile cocci sur­rounded by a sin­gle layer cell wall. One un­usual char­ac­ter­is­tic of these or­gan­isms is that they do not ap­pear to pro­tect them­selves from the high os­motic pres­sure by stor­ing or­ganic os­molytes, but, rather, by ac­cu­mu­lat­ing a high in­tra­cel­lu­lar potas­sium con­cen­tra­tion.

Fig­ure 3. Growth dy­nam­ics of strain AMET1 with MeOH + for­mate at 4 M to­tal Na⁺, pH 9.5 and 50°C Source

The two iso­lates seem to have arisen early in the his­tory of the ar­chaea, as they be­long to deep branches of the Eu­r­yarchaeota. They have been ten­ta­tively named the 'Methanona­troar­chaea'. De­tails of a good many eco­log­i­cal (for ex­am­ple, salt adap­ta­tion), phys­i­o­log­i­cal, and com­par­a­tive ge­nomic facts are pro­vided. The pa­per is un­usual in that its 12 au­thors rep­re­sent 6 dif­fer­ent coun­tries, with Eu­gene Koonin at the rear.