Los arranques

My   work   showed   that   production   of   oxygen   in   close   proximity   to   the   cathode   under   illumination   increased   the   efficiency   of   the   cathode   compared   to   the   performance   achievable   with   aeration.   However,   the   photosynthetic   biofilms   grown   in   this   study   did   not   appear   to   facilitate   the   oxygen   reduction   reaction   (ORR)   efficiently,   even   though   non-­‐turnover   voltammetry   (absence   of   oxygen)   revealed   a   reversible   redox   compound   confined   in   the   biofilm.   The   enhancement   of   the   current   output  was  then  mostly  due  to  high  production  of  dissolved  oxygen  by  photosynthesis,  but  not  to   biocatalysis  of  the  ORR.  

Oxygen   diffusion   is   one   of   the   main   factors   limiting   cathode   performance.   The   study   of   sediment-­‐ type   photosynthetic   MFCs   presented   here   showed   that   production   of   oxygen   right   at   the   cathode   surface   by   photosynthesis   offers   a   potential   solution   to   oxygen   mass   transfer   limitation.   In   contradiction,   recent   works   found   no   advantage   of   algal   photosynthesis   in   the   cathode   chamber   compared  with  mechanical  aeration  (Gil  et  al.  2003,  Juang  et  al.  2012,  Kang  et  al.  2003,  Pham  et  al.   2004,   Rodrigo  et  al.  2010).  However,   their   findings   could   be  explained   by  (i)   low   dissolved   oxygen   concentrations  reached  in  the  cathode  chamber  compared  to  the  concentrations  normally  obtained   under   illumination   (>22  mg/L)   (Rodrigo   et   al.   2010)  ;   (ii)   poor   oxygen   reduction   at   the   graphite   cathode   (Gil   et   al.   2003)  ;   and   (iii)   most   of   all,   diffusion   of   oxygen   from   the   cathode   to   the   anode   compartment  interfering  with  electron  transfer  at  the  anode  (Kang  et  al.  2003,  Pham  et  al.  2004).  

To  determine  whether  or  not  the  redox  species  confined  in  the  cathodic  biofilm  was  produced  by  a   particular   community   that   benefits   from   the   flow   of   electrons   from   the   anode,   photosynthetic   biocathodes  were  selected  in  open  and  closed  circuit  pMFCs.  Evidence  of  differences  was  found  for   the  diatom  and  possibly  the  bacterial  communities,  but  the  small  proportion  of  diatom  DNA  within   the  eukaryote  community  did  not  provide  sufficient  DNA  to  be  obtained  to  allow  the  identification  of   the   diatoms   species   present.   No   biocatalysis   of   oxygen   reduction   was   observed,   but   non-­‐turnover   voltammetry  revealed  a  reduction  peak  at  -­‐0.3  V  vs  Ag/AgCl  only  in  biocathodes  selected  in  closed-­‐ circuit  pMFC.  This  reduction  peak  could  be  related  to  the  production  of  hydrogen  by  the  species  most   similar   to   Desulfovibrio   sp.,   but   a   more   complete   analysis   of   the   bacterial   community   of   each   biocathode-­‐type  would  be  necessary.    

Photosynthetic  biocathodes  could  offer  a  good  alternative  to  rare-­‐earth  catalysts  and  so  suppress  a   major   obstacle   to   the   scale-­‐up   of   MFC   for   commercial   production   of   electricity.   Coupling   the   production   of   electricity   with   the   production   of   biofuel   could   also   be   considered.   Additionally,  

  177   organic   matter   excreted   by   the   photosynthetic   biomass   could   be   used   as   a   feedstock   for   the   heterotrophic  electrode-­‐respiring  bacteria  at  the  anodes,  making  energy-­‐efficient,  reliable  MFCs  able   to  operate  in  remote  areas.  

Finally,   this   PhD   project   examining   bioelectrochemical   systems   gave   me   the   opportunity   to   bring   together   several   scientific   disciplines,   including   microbiology,   electrochemistry,   ecology   and   molecular   biology.   Through   this   thesis,   I   contributed   to   the   current   knowledge   of   how   exoelectrogenic  bacteria  form  anodic  biofilms  and  how  they  retain  their  operational  and  structural   stability.   This   work   showed   that   it   is   possible   to   engineer   anodic   or   cathodic   biofilms   for   better   electricity  production.  By  varying  parameters  such  as  the  anode  potential  and  the  carbon  source,  I   managed   to   engineer   Geobacter-­‐dominated   biofilms   with   an   increased   stability   and   a   broad   substrate   usage   for   BOD   sensor   applications.   I   also   improved   the   power   output   of   MFC   by   incorporating  photosynthesizers  at  the  cathode.  

The  power  outputs  of  today’s  MFCs  are  still  too  low  to  consider  the  scale-­‐up  of  these  systems  for  a   commercial  production  of  electricity.  However,  other  applications  such  as  using  engineered  anode-­‐ respiring  biofilms  as  the  biocomponent  of  biosensors  or  using  MFCs  to  power  low-­‐energy  devices  in   remote   areas   where   solar   energy   cannot   be   harvested,   could   be   considered.   The   unceasing   development   of   new   bioelectrochemical   systems   makes   this   field   of   research   very   dynamic   and   promises   technological   applications   and   fundamental   advances   on   the   understanding   of   cellular   physiology  and  the  biology  of  exoelectrogenic  bacteria.    

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