Love At First Zap

by Gemma Reguera

Al­though both co­op­er­a­tion and con­flict are de­ci­sive forces in evo­lu­tion, some of the most suc­cess­ful mi­cro­bial strate­gies for sur­vival have arisen from co­op­er­a­tion.  At times, two or more mi­croor­gan­isms can even come to­gether to breathe as one. Breath­ing, or res­pi­ra­tion, ac­com­plishes a most chal­leng­ing fête: the dis­posal of elec­trons gen­er­ated in meta­bolic cel­lu­lar re­ac­tions to gain en­ergy for growth. The elec­tron donors and ac­cep­tors that mi­croor­gan­isms can use for res­pi­ra­tion can be aligned hi­er­ar­chi­cally on a tower based on their re­dox or re­duc­tion po­ten­tial (to wit, their affin­ity for the elec­trons). The more pos­i­tive the po­ten­tial, the eas­i­est it is for the mol­e­cule to cap­ture the elec­trons (i. e., to be re­duced) and the more en­ergy that is gained from the re­ac­tion. The re­dox tower pro­vides a vi­sual ref­er­ence of the en­ergy that mi­croor­gan­isms may gen­er­ate from the trans­fer of elec­trons to each chem­i­cal species. Oxy­gen, for ex­am­ple, has a higher re­dox po­ten­tial (it is more elec­tropos­i­tive) than ni­trate [NO3-)] or iron ([Fe(III], of­ten present in na­ture as hy­drated iron ox­ide ‑FeOOH-). As a re­sult, more en­ergy is gen­er­ated when oxy­gen is used as the ter­mi­nal elec­tron ac­cep­tor.

Re­dox tower show­ing the re­duc­tion po­ten­tial (E'₀, ex­pressed in volt ‑V- units) of re­dox re­ac­tions cat­alyzed by some micro­or­ganisms (ox­i­dized species on the left and re­duced species on the right). The more elec­tropos­i­tive the po­ten­tial, the higher the en­ergy yield from the re­duc­tion one chem­i­cal species (on the left) to an­other (on the right). The re­duc­tion of O₂ to H₂O and iron ox­ides ‑FeOOH- to Fe²⁺ is high­lighted within red boxes. Mod­i­fied from source

Be­cause there is high com­pe­ti­tion for the most en­er­getic avail­able elec­tron ac­cep­tor, mi­croor­gan­isms have also evolved mech­a­nisms to co­op­er­ate dur­ing their res­pi­ra­tion. The process, known as in­ter­species elec­tron trans­fer, in­volves the ex­change of elec­trons be­tween two mi­cro­bial part­ners to en­able a re­ac­tion that each could not ac­com­plish in­di­vid­u­ally. These or­gan­isms are said to be grow­ing syn­troph­i­cally ("feed­ing to­gether"). The elec­trons are ex­changed us­ing me­di­a­tors, most of­ten metabo­lites. The canon­i­cal ex­am­ple of in­ter­species elec­tron trans­fer con­sists of the ex­change of elec­trons as mol­e­c­u­lar hy­dro­gen (H2). Hy­dro­gen be­ing a gas, the mi­cro­bial part­ners can max­i­mize its rate of ex­change by grow­ing in close prox­im­ity to each other, of­ten form­ing cell ag­gre­gates or gran­ules. The fer­men­ta­tion of or­ganic sub­strates by some bac­te­ria is, for ex­am­ple, greatly stim­u­lated in co­cul­tures of hy­dro­gen-form­ing fer­men­ta­tive bac­te­ria and hy­dro­gen-con­sum­ing or­gan­isms such as methanogenic ar­chaea. Both or­gan­isms ben­e­fit from the ex­change: un­less re­moved, hy­dro­gen ac­cu­mu­lates and slows down or even in­hibits the bac­te­r­ial fer­men­ta­tion; with­out the hy­dro­gen, the methanogen can­not grow au­totroph­i­cally and re­duce CO2.

For some mi­croor­gan­isms, syn­trophic part­ner­ships are ob­lig­ate. The fer­men­ta­tive and res­pi­ra­tory me­tab­o­lism of the aptly named bac­terium Syn­tro­phus acid­itroph­i­cus, for ex­am­ple, is re­stricted to spe­cial­ized path­ways for the degra­da­tion of low-en­ergy sub­strates such as fatty acids and ben­zoate and for rerout­ing meta­bolic elec­trons to hy­dro­ge­nases, thus gen­er­at­ing hy­dro­gen. The re­moval of hy­dro­gen makes an oth­er­wise en­der­gonic (en­ergy re­quir­ing) re­ac­tion ex­er­gonic (en­ergy re­leas­ing) and al­lows the bac­terium to gen­er­ate en­ergy for growth. As a re­sult, this bac­terium can de­grade com­pounds such as fatty acids and ben­zoate but only when grow­ing syn­troph­i­cally with a hy­dro­gen-con­sum­ing part­ner, typ­i­cally a sul­fate-re­duc­ing bac­terium or an ar­chaeal methanogen.

In­ter­species hy­dro­gen trans­fer en­ables Syn­tro­phus acid­itroph­i­cus to gain en­ergy for growth from the degra­da­tion of ben­zoate. The syn­trophic part­ner is typ­i­cally a hy­dro­gen-con­sum­ing sul­fate-re­duc­ing bac­terium but it can also be a methanogen. Source

A re­cent pa­per by K. Watanabe's team re­ports a new mode of in­ter­species elec­tron trans­fer me­di­ated by con­duc­tive min­er­als. They stud­ied Geobac­ter sul­furre­ducens, a bac­terium that cou­ples ox­i­da­tion of ac­etate to re­duc­tion of iron but that can­not use ni­trate as an elec­tron ac­cep­tor. On the other hand, an­other bac­terium, Thiobacil­lus den­i­tri­f­i­cans can re­duce ni­trate but can­not use ac­etate as an elec­tron donor.  G. sul­furre­ducens is known to pro­duce hy­dro­gen as an elec­tron sink un­der some con­di­tions, i.e., when grown un­der elec­tron-ac­cep­tor lim­it­ing con­di­tions. How­ever, Thiobacil­lus can­not use hy­dro­gen as an elec­tron donor for ni­trate respiration.Thus, these two or­gan­isms can­not grow syn­troph­i­cally with ac­etate and ni­trate by car­ry­ing out in­ter­species hy­dro­gen trans­fer. The au­thors noted how­ever that ad­di­tion of mag­netite par­ti­cles to the co­cul­ture pro­moted their meta­bolic co­op­er­a­tion and en­abled the cou­pling of ac­etate ox­i­da­tion and ni­trate re­duc­tion. Mag­netite is a con­duc­tive iron ox­ide min­eral (Fe3O4) so the team hy­poth­e­sized that the min­eral par­ti­cles were en­abling the trans­fer of elec­trons in the form of elec­tric cur­rent flow­ing be­tween the two mi­cro­bial part­ners. In sup­port of this no­tion, in­su­lat­ing min­eral par­ti­cles such as the iron ox­ide min­eral fer­ri­hy­drite (FeOOH) and alu­mina (Al2O3) par­ti­cles did not work. An­other iron ox­ide min­eral hematite (α‑Fe2O3), which is semi­con­duc­tive, en­abled the trans­fer of some elec­trons be­tween the two mi­cro­bial part­ners but much less ef­fi­ciently than the con­duc­tive mag­netite.

The part­ner­ship be­tween G. sul­furre­ducens and T. den­i­tri­f­i­cans is a good ex­am­ple of how in­ter­species elec­tron trans­fer al­lows or­gan­isms to ex­tend their meta­bolic ca­pa­bil­i­ties be­yond the con­fines of their own meta­bolic net­work. The me­tab­o­lism of Geobac­ter en­ables the ef­fi­cient ox­i­da­tion of ac­etate, which is of­ten abun­dant in anaer­o­bic en­vi­ron­ments dur­ing the fer­men­ta­tion of or­ganic mat­ter. Thus, by part­ner­ing with Geobac­ter, Thiobacil­lus cells ac­cess an abun­dant elec­tron donor and ben­e­fit from its ef­fi­cient uti­liza­tion. G. sul­furre­ducens, on the other hand, ex­pands the range of elec­tron ac­cep­tors it can use for the ox­i­da­tion of ac­etate to ni­trate. Ni­trate is also more elec­tropos­i­tive than iron (note that it is placed higher than iron in the re­dox tower shown in Fig. 1). Thus, it has more affin­ity for elec­trons and cells gen­er­ate more en­ergy from its re­duc­tion. Every­body wins. It is also im­por­tant to note that mag­netite (a mixed Fe[III]-Fe[II] va­lence min­eral) is also the nat­ural byprod­uct of the re­duc­tion of hy­drated iron ox­ide by Geobac­ter. Hence, once the iron ox­ides are re­duced, Geobac­ter cells can 're­cy­cle' their own waste prod­uct and reuse it to es­tab­lish elec­tronic con­tact with the Thiobacil­lus part­ner. Smart strat­egy.

In­ter­species elec­tron trans­fer be­tween Geobac­ter and Thiobacil­lus via con­duc­tive mag­netite par­ti­cles en­ables the cou­pling of ac­etate ox­i­da­tion to ni­trate re­duc­tion. Source

The idea that mi­crobe­suse con­duc­tive min­er­als for res­pi­ra­tion also has im­por­tant biotech­no­log­i­cal im­pli­ca­tions. It has be­ing known for long that some mi­crobes­can use elec­trodes as elec­tron donors or ac­cep­tors in de­vices known as mi­cro­bial fuel cells. The cells ac­tu­ally stack on the elec­trode sur­face and form an elec­troac­tive biofilm that fa­cil­i­tates elec­tron flow be­tween the cells and the elec­trode. Geobac­ter, for ex­am­ple, can ox­i­dize ac­etate and other or­ganic acids us­ing elec­trodes as elec­tron ac­cep­tors, thus gen­er­at­ing elec­tric cur­rents.  Other mi­crobes like Thiobacil­lus can use the elec­tric cur­rent to re­duce elec­tron ac­cep­tors such as ni­trate. Each or­gan­ism per­forms its re­ac­tion in sep­a­rate cham­bers of the fuel cell: Geobac­ter in the an­ode, where elec­trons are gen­er­ated, and Thiobacil­lus inthe cath­ode, where elec­trons are re­ceived. The work of Watanabe's team sup­ports the no­tion that Na­ture may have se­lected for other or­gan­isms that ex­change elec­tric cur­rents to cou­ple re­dox re­ac­tions. These very same re­ac­tions could be mim­ic­ked in mi­cro­bial fuel cells.

Per­haps the most fas­ci­nat­ing as­pect of in­ter­species elec­tron trans­fer is not so much the mech­a­nism that me­di­ates the trans­fer but the evo­lu­tion­ary gain that mi­croor­gan­isms get when breath­ing as one. The famed Lynn Mar­gulis and her son Do­rion Sagan dis­cussed in their 1997 book What is Sex? "the ba­sic arith­metic of live". "In bi­ol­ogy", they wrote, "1 + 1 does not equal 2, but rather 1 + 1 = 1". Their equa­tion does in­deed de­scribe the na­ture of the chem­istry that brings two species to­gether to func­tion as one. In some cases, all it takes is a 'zap'.

Ref­er­ence

Kato S, Hashimoto K, Watan­abe K (2012). Mi­cro­bial in­ter­species elec­tron trans­fer via elec­tric cur­rents through con­duc­tive min­er­als. Proc Nattl Acad Sci USA, 109 (25), 10042−10046. PMID 22665802

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