Mi­cro­bial 'Starstuff'

by Gemma Reguera

Famed as­tronomer and writer Carl Sagan said in his book Cos­mos: "The ni­tro­gen in our DNA, the cal­cium in our teeth, the iron in our blood, the car­bon in our ap­ple pies were made in the in­te­ri­ors of col­laps­ing stars. We are made of starstuff." And so are mi­crobes, we must add. To un­der­stand the sig­nif­i­cance of this state­ment, it is worth first learn­ing about stars and their ex­plo­sive death, the su­per­nova.

Fig­ure 1. Us­ing in­frared and X‑ray data col­lected from sev­eral ob­ser­va­to­ries, sci­en­tist gen­er­ated this im­age show­ing the rem­nant of the Ty­cho su­per­nova, which was gen­er­ated by the ex­plo­sion of a star four cen­turies ago. Source

The life and death of a star

Most stars grow and die qui­etly, cool­ing off slowly un­til they are ex­tin­guished. How­ever, a small per­cent­age of them grow to mas­sive pro­por­tions un­til they can no longer sus­tain their mass and col­lapse. This is a spec­tac­u­lar way to die: a pow­er­ful yet breath­tak­ingly beau­ti­ful ex­plo­sion of en­ergy and light called a su­per­nova. The road to a su­per­nova is an alchemist's dream. The core of the star is like a mas­sive nu­clear re­ac­tor, where atoms col­lide and fuse to make new el­e­ments in a process known as stel­lar nu­cle­osyn­the­sis. In young, small stars, the core is filled with hy­dro­gen fuel, which fuses to gen­er­ate he­lium. As he­lium ac­cu­mu­lates in the core, hy­dro­gen is pushed out­side as an ex­ter­nal con­cen­tric shell. The he­lium atoms in the star's core then col­lide and gen­er­ate car­bon, which pushes the he­lium out­side. And the se­quence of re­ac­tions con­tin­ues with car­bon form­ing neon, neon gen­er­at­ing oxy­gen, oxy­gen fus­ing to pro­duce sil­i­con, and fi­nally, sil­i­con be­com­ing nickel, which de­cays rapidly to form iron. This dy­namic process of nu­cle­osyn­the­sis pushes the older el­e­ments out­side the core and strat­i­fies them as con­cen­tric lay­ers that tell the age of the star, pretty much like the rings of a tree trunk, with hy­dro­gen in the ex­ter­nal shell, then he­lium, car­bon, neon, oxy­gen, sil­i­con and, fi­nally, iron in the core. Each layer is ac­tive, as if burn­ing, and it feeds the prod­uct of its nu­clear re­ac­tion to the layer un­der­neath. Ex­cept for iron, which can­not burn, so it ac­cu­mu­lates in the core un­til it reaches an un­sus­tain­able mass. It then col­lapses, pro­vok­ing a mas­sive nu­clear ex­plo­sion … the su­per­nova. The ex­plo­sion gen­er­ates so much en­ergy that new el­e­ments (those heav­ier than oxy­gen and iron) are formed and spewed into space along with all the other star ma­te­ri­als form­ing what is known as su­per­nova rem­nants.

Fig­ure 2. A star grows form­ing con­cen­tric shells or lay­ers of el­e­ments, from hy­dro­gen (H) on the out­side to iron (Fe) in the core. Each layer tells the age of the star but also the se­quence of nu­clear re­ac­tions tak­ing place in the star's core dur­ing the life­time of the stel­lar body, with hy­dro­gen be­ing the first fuel, then he­lium (He), car­bon ©, neon (Ne), etc. In the end, an iron core is formed that grows in size un­til it col­lapses and causes a ex­plo­sion or su­per­nova. Source

When su­per­novae rem­nants reached the Earth…

A su­per­nova may be the death of the star but it is also the be­gin­ning of life as we know it. If close enough to the Earth, the ra­di­a­tion and ma­te­ri­als spewed dur­ing the ex­plo­sion pen­e­trate through the ozone layer, af­fect­ing the chem­istry of the at­mos­phere and dis­pers­ing the su­per­nova el­e­ments onto our planet. The in­tense ra­di­a­tion that per­me­ates the at­mos­phere and reaches the bios­phere can also be lethal. Some sci­en­tists spec­u­late, for ex­am­ple, that a near-Earth su­per­nova could have been re­spon­si­ble for the Or­dovi­cian ex­tinc­tion, one of the five ma­jor ex­tinc­tion events in the his­tory of our planet that led to the dis­ap­pear­ance of 60% of all ocean life some 450 mil­lion years ago. To gather ev­i­dence for su­per­nova ex­plo­sions that could have reached the Earth, sci­en­tists be­gan look­ing in our rock record for star el­e­ments (Carl Sagan's starstuff) that could re­main in our planet even af­ter mil­lions of years. One such el­e­ment is ⁶⁰Fe, an iron iso­tope gen­er­ated dur­ing stel­lar nu­cle­osyn­the­sis, which is es­pe­cially abun­dant in su­per­novae de­bris. As ⁶⁰Fe is only scarcely found on Earth and it de­cays slowly (half-life of 2.6 mil­lion years), re­searchers looked for high con­cen­tra­tions of cos­mic ⁶⁰Fe in the rock record as in­di­ca­tors of past su­per­nova events that reached our planet. And this is when things got in­ter­est­ing...

Fig­ure 3. The ⁶⁰Fe-Fe ra­tio of a fer­ro­man­ganese crust of the Pa­cific ocean floor spiked in lay­ers de­posited some 3 mil­lion years ago, sug­gest­ing they are rem­nants of a su­per­nova event. Source

'Starstuff' in the ocean floor

In 2004, Knie and col­lab­o­ra­tors mea­sured high ra­tios of su­per­nova ⁶⁰Fe to to­tal Fe present in fer­ro­man­ganese crust lay­ers de­posited some 2.8 mil­lion years ago on the Pa­cific ocean floor. Based on the de­cay rates of ⁶⁰Fe and its es­ti­mated con­cen­tra­tion and ve­loc­ity in mod­ern su­per­novae, the re­searchers cal­cu­lated that the amount of ⁶⁰Fe de­tected in the ocean crust cor­re­sponded to a su­per­nova lo­cated at a dis­tance of "just" a few 10 pc from the Earth. Keep in mind that a 'pc' or par­sec unit equals 19.2 TRILLION miles! These stel­lar dis­tances sound out­ra­geously large to us mi­cro­bi­ol­o­gists, but they are rel­a­tive short dis­tances in as­tro­nomic terms. The bot­tom line is that the su­per­nova ex­plo­sion was big, re­ally big, and hap­pened very close to our planet. Based on the high lev­els of ⁶⁰Fe de­po­si­tion still re­main­ing in the rocks, the re­searchers pre­dicted that the su­per­nova event ex­posed the Earth to a high level of cos­mic ra­di­a­tion dur­ing ap­prox­i­mately 300,000 years. In­ter­est­ingly, the time and du­ra­tion of the cos­mic ra­di­a­tion flux co­in­cides with the on­set and length of a pe­riod in the his­tory of Earth when the African cli­mate shifted to more arid con­di­tions. This cli­mate shift has been linked to spe­ci­a­tion events crit­i­cal to the evo­lu­tion of life on Earth in­clud­ing the evo­lu­tion of ho­minid species.

Fig­ure 4. (Left) Elec­tron mi­cro­graph of a thin sec­tion of a mag­ne­to­some chain (Source). (Right) Mag­ne­to­somes are the pre­dom­i­nant form of fer­ro­mag­netic min­er­als in the sed­i­ment core that had the ⁶⁰Fe anom­aly around 2.8 Myr ago (mod­i­fied from Source).

Mi­crobes with a taste for 'starstuff'

Mi­cro­bial life was also thriv­ing in the oceans at the time of the su­per­nova event. How­ever, it is dif­fi­cult to know how the su­per­nova rem­nants, and the ra­di­a­tion flux that came with it, im­pacted the mi­cro­bial ac­tiv­i­ties in the ocean sed­i­ments. In a pa­per pub­lished a cou­ple of years ago, Bishop and Egli rea­soned that some of the mi­crobes in the sed­i­ment could have min­er­al­ized the cos­mic ⁶⁰Fe, form­ing min­er­als that may still re­tain the dis­tinc­tive su­per­nova el­e­ment till this day. Some of the mi­croor­gan­isms liv­ing in the ocean sed­i­ments at these times in the Earth his­tory, the so-called dis­sim­i­la­tory iron re­duc­ers, could have used the ⁶⁰Fe to sup­port their res­pi­ra­tion. This is an ex­tra­cel­lu­lar process that leads to the for­ma­tion of mag­netite, a mag­netic iron min­eral of mixed Fe(III)/Fe(II) va­lence and dis­tinc­tive struc­tural prop­er­ties. Mag­ne­to­tac­tic mi­crobes could have also chelated and taken up the ⁶⁰Fe from the sed­i­ments to make in­tra­cel­lu­lar mag­netic min­eral in­clu­sions, or mag­ne­to­somes. By align­ing the mag­ne­to­some in­clu­sions as chains in­side the cell, these mi­crobes ori­ent them­selves in the ge­o­mag­netic field. The mag­ne­to­somes are mag­netic crys­tals made up of ei­ther iron ox­ides (mag­netite, Fe₃O₄) or iron sul­phides (greig­ite, Fe₃S₄) and they are formed un­der strictly con­trolled chem­i­cal con­di­tions in­side the cell. As a re­sult, the crys­tals are very pure and have de­fined and unique par­ti­cle size, mor­phol­ogy and mag­netic prop­er­ties. They are also highly sta­ble and can per­sist un­al­tered in the en­vi­ron­ment for long pe­ri­ods of time (mil­lions of years!) af­ter the cells have died. Thus, they serve as fos­sils (mag­neto­fos­sils) and in­di­ca­tors of past mag­ne­to­tac­tic ac­tiv­ity. Bishop and Egli hy­poth­e­sized that mag­neto­fos­sils present in the en­riched ⁶⁰Fe sed­i­ments may also be use­ful in­di­ca­tors of mi­cro­bial ac­tiv­i­ties dur­ing the time that the oceans were ex­posed to the su­per­nova rem­nants and ra­di­a­tion. To test this, they mea­sured the mag­netic mo­men­tum of all the fer­ro­mag­netic min­er­als and of in­tact mag­ne­to­somes across a sed­i­ment core rep­re­sent­ing sed­i­ment ages within the 2.4–3.3 Myr age in­ter­val and cal­cu­lated the con­cen­tra­tion of mag­neto­fos­sils. The mag­ne­ti­za­tion pro­file showed, quite con­sis­tently, that mag­ne­to­somes were the pre­dom­i­nant form of mag­netic iron min­er­als in the sed­i­ment lay­ers pre­ced­ing and span­ning the ⁶⁰Fe de­po­si­tion some 2.8 Myr ago. Thus, mag­ne­to­tac­tic bac­te­ria ap­peared to have con­tin­ued to carry out their busi­ness us­ing the cos­mic iron!

What I would have loved to see are ac­tual mea­sure­ments of the ⁶⁰Fe con­tent of in­tact mag­ne­to­somes iso­lated from these sed­i­ment cores, as Knie and col­lab­o­ra­tors did for the bulk sed­i­ments. Yet even with­out this fi­nal cor­rob­o­ra­tion, the stud­ies strongly sug­gest that the mi­crobes in the ocean floor adapted to the su­per­nova event and used the avail­able cos­mic el­e­ments to sup­port their ac­tiv­i­ties. ⁶⁰Fe serves as a proxy of the time that our planet was ex­posed to su­per­nova de­bris be­cause we can still mea­sure it af­ter al­most 3 mil­lion years. How­ever, the su­per­nova rem­nants en­riched the Earth with many other el­e­ments, po­ten­tially sup­port­ing other mi­cro­bial ac­tiv­i­ties and en­abling mi­cro­bial life to evolve in new ways. In the end, Carl Sagan's words say it all. We are all made of starstuff. Spe­cially mi­crobes, which have been feed­ing on stars for a long, long time. They are in­deed liv­ing be­ings with star-like qual­i­ties, so think about it next time you get a Strep throat!

Ref­er­ences

Knie K, Ko­rschinek G, Faester­mann T, Dorfi E, Rugel,G, Wall­ner A. (2004). F60e Anom­aly in a Deep-Sea Man­ganese Crust and Im­pli­ca­tions for a Nearby Su­per­nova Source Phys­i­cal Re­view Let­ters, 93 (17). DOI 10.1103/PhysRevLett.93.171103

Bishop S, Egli R (2011). Icarus, 212, 960−962. arXiv 1010.5109v3

Gemma is as­sis­tant pro­fes­sor in the De­part­ment of Mi­cro­bi­ol­ogy and Mol­e­c­u­lar Ge­net­ics, Michi­gan State Uni­ver­sity.