Aeonophiles

by Karen G. Lloyd

The sed­i­ments un­der­ly­ing the world's oceans con­tain about a third of the to­tal num­ber of mi­cro­bial cells on the planet. This is a vast bios­phere, mostly com­prised of deep branches on the tree of life that no one has yet fig­ured out how to grow in a lab­o­ra­tory. Since grow­ing mi­crobes in a lab­o­ra­tory is the best way we can learn about them, this means that there's a lot of mys­tery about what makes this strange ecosys­tem tick. Prob­a­bly the strangest thing about these ecosys­tems is that they are es­sen­tially cut off from both the sur­face world and sub­sur­face sources of en­ergy for chemosyn­the­sis, like in hy­drother­mal vents. Yes, there are plenty of vents scat­tered around the seafloor, and other good sources of en­ergy like methane cold seeps and vol­canic erup­tions, but I'm not talk­ing about those places. I'm talk­ing about the great, bor­ing ex­panses of seden­tary muck whose most no­table fea­ture is that noth­ing ever hap­pens. This means that one of the largest mi­cro­bial ecosys­tems on the planet gets no fresh food de­liv­er­ies for mil­lions of years. Every liv­ing be­ing that we com­monly in­ter­act with, in­clud­ing pretty much all our lab­o­ra­tory cul­tures, would die if you didn't feed them for that long. 

So, why is there a highly di­verse, meta­bol­i­cally ac­tive bios­phere hang­ing out where no one can eat? Roberto Kolter, and his for­mer lab mem­bers Steve Finkel, Erik Zinser and oth­ers, showed us that even com­mon bac­te­ria ac­tu­ally have a sur­pris­ing tol­er­ance to star­va­tion. Even E. coli, which can't shut down into meta­bol­i­cally in­ac­tive spores con­tin­ues with sub­pop­u­la­tions grow­ing, dy­ing, and com­pet­ing with each other long af­ter you re­move the food. Af­ter starv­ing for months or even years, they be­come griz­zled old tough guys who ac­tu­ally out­com­pete younger cells un­der star­va­tion con­di­tions. Roberto and his folks called this the Growth Ad­van­tage in Sta­tion­ary Phase, which has about the most adorable acronym ever for a strain des­per­ately fight­ing for life: GASP. 

It's much harder to run an ex­per­i­ment for a thou­sand years (al­though I re­cently had the plea­sure of see­ing an ex­per­i­ment set up by Charles Cock­ell, at the Uni­ver­sity of Ed­in­burgh, that will run for 500 years – some­one else is go­ing to have to write the blog post about that one). How­ever, if we ex­trap­o­late from the ex­per­i­ments run by na­ture un­der­neath our oceans all over the world, the most rea­son­able con­clu­sion is that the bac­te­ria, ar­chaea, and pos­si­bly eu­kary­otes in these ma­rine sed­i­ment ecosys­tems have some­how adapted to have cells that sur­vive the whole lengthy process of bur­ial. This means these mi­crobes might be evo­lu­tion­ar­ily adapted to live for hun­dreds of thou­sands of years. I'm not talk­ing about the age of their lin­eage. I'm talk­ing about THE ACTUAL CELLS be­ing that old.

The ul­tra-long-lived aeonophiles! Source.

It re­cently oc­curred to my col­league Drew Steen and I that we need a word to de­scribe or­gan­isms that have adapted to live for a mil­lion (I mean, maybe?) years, es­pe­cially since the fact that their lifes­pans are this long has to re­main a hy­poth­e­sis, so we need to be able to talk about them as we test it. We came up with aeonophiles. It means that they love long time pe­ri­ods. It seems hard to pro­nounce, but it's ee-ohn-oh-files. Like some­one who loves wine, but with an ex­tra syl­la­ble. I don't know for cer­tain that cells can ac­tu­ally achieve this feat of liv­ing for a mil­lion years, while slowly re­plac­ing bro­ken bits but never di­vid­ing. But cur­rently, all ev­i­dence sug­gests this is a rea­son­able hy­poth­e­sis. Now that they have a name, I'm hop­ing peo­ple will go forth and study these lit­tle weirdos!

Karen G. Lloyd is the Wrigley Pro­fes­sor of Earth Sci­ences, and Ma­rine and En­vi­ron­men­tal Bi­ol­ogy at the Uni­ver­sity of South­ern Cal­i­for­nia. She dis­cov­ers strange types of mi­cro­bial life deeply-buried in­side Earth's crust, and wrote In­trater­res­tri­als: Dis­cov­er­ing the Strangest Life on Earth, long-listed for the PEN EO Wil­son Lit­er­ary Award.