Of Microbes and Men

 Earlier published in JFWTC inhouse Journal  

A microbial power plant?   Concept is not new, has been bouncing around for almost 100 years.  Bruce Sterling’s science fiction “Distraction” set in the year 2044 does allude to it.  A series of articles in a recent issue of Nature ( 18^th May 2006)  focuses on microbial capabilities and efforts to harness them for serving mankind.  A team of   electrical engineers, microbiologists, biotechnologists and environmental chemists spread across globally, definitely see  a possibility, albeit  not immediate. There are several hurdles to overcome before  the lab  model becomes a commercial reality.

The focus is on the oxidative metabolic pathway of the microbes.  Chemically oxidation is stripping of electrons and reduction is gaining of electrons.  The essential consequence of the oxidative metabolic pathway is an electron transport chain which begins with  the nutrient  and after several steps  ends at oxygen.  Flow of electrons means passage of current.  So in a microbial soup if you can siphon off the electrons onto a suitable anode instead off to oxygen,  while continuously replenishing the nutrient medium then you have a fuel cell.

Yuri Gorby¹ and his team have put to work photosynthetic bacteria Synechosytis in a microbial fuel cell.  Central to this set is Gorby’s observation that the bacterial surface has thin whiskers of nanometer dimensions which together with cytochrome  facilitates conductivity.

At Penn State University Bruce E. Logan² and his colleagues are using these miniature power plants to clean wastewater and also to generate hydrogen.  By blocking the supply of oxygen and a meagre input of 0.25 volt, the team could achieve four fold increase of hydrogen production. 

The current per se might be infinitesimally small, but the potential?  That is what  Prof. Peter Girguis’³ team at  Harvard  is interested in.  But the problem is to make the electrodes “ bacteriophilic” or coax the bacteria to get closer and adhere to the electrode.

A couple of years earlier Schroder etal⁴ from Institute for Chemistry and Biochemistry, University of  Greifswald, Germany used platinum with a coating of poly (tetrafluor aniline) to  improve electrode/bacterial interface.  They reported Clostridium butyricum or Clostridium beijerinckii with   carbohydrates as nutrients could  generate  current densities between 1 and 1.3mA/sqcm. 

Recently Willy Verstraete and his team⁵ (Laboratory of Microbial Ecology and Technology,   Belgium) demonstrated that when microbial fuel cell units are stacked together the power output could be multifold. They reported a  “Maximum hourly averaged power output of 258 W m^-3 using a hexacyanoferrate”. Their observation that in an MFC, the microorganisms colonise  as a biofilm and live in close contact with the electrode  is a crucial piece of information.   Because biofilms  are the toughest architecture of bacterial colonies and might be the surest way to improve the electrode-microbe interface. 

Kolter and Greenberg⁶ report that when bacteria opt  to settle down as a biofilm it secretes a glue which holds the colony together and also helps the film  cling  firmly onto the substrate surface.   Biofilms of microbial colonies  are tough, mutate quickly and become drug resistant.   Naturally    Kolter’s   interest is in rupturing the  film  so as to  break up the colony and subdue the microbes on a one to one basis.    But from the MFC  perspective   important question to ask is can we facilitate the secretion of that glue  selectively so  that the MFC microbes  adhere more firmly to  the electrode surface ?

1.      Batteries not included : News Feature ,  Lane, Nature 441, p274 (2006)

2.      Increased power and Coulombic efficiency of single-chamber microbial fuel cells through an improved cathode structure, Logan etal.  Electrochem. Comm. 8:489 (2006).

3.      Circuits of slime:   News feature,  Schibert,  Nature 441, p276 (2006)

4.      Electrochemistry Communications, Schroder et al   6, p955 (2004)

5.      Continuous electricity generation at high voltages and currents using stacked microbial cells,  Verstraete et al  Env. Sci. Techn 40, 3388   (2006)

6.      Superficial Life of microbes: Kolter and Greenberg,  Nature 441, p 300 (2006)