Preface

Bioelectrosynthesis: Here to Stay!

Many things in science move in waves until they finally land. In the 1960s, with the advent of the fuel cell, it was found that microbial decomposition of organic matter could also be directly linked to power production in microbial fuel cells (MFC) [1]. The power production was modest, and the top ic lead a latent existence until the petroleum crisis of the late 1970s when novel routes for chemical production were explored. Visionary scientists then coupled this microbial electron transfer for the production of organic molecules such as glutamic acid and butanol [2-4]. At that time, technology did not follow and titers and rates were limited, not competitive to existing bioproduction approaches. Then, in the late 1990s, the old topic of the MFC resurfaced, with new discoveries on electron flow [5-7]. This time, technology had evolved as well, enabling higher production rates [8, 9] and also enabling different process outcomes such as hydrogen [10] or caustic soda [11] production. Unfortunately, for MFCs, even by then also, an alternative technology generating power from biomass had matured: anaerobic digestion. In this process, biogas is produced. Nowadays, anaerobic digesters can deal from small to large scale with complex waste streams, they are very robust, and most importantly they can deal with high loading rates. Top systems now convert over 50?kg organics/m3 reactor per day to methane. In terms of electron flow, this implies a current of almost 7000?A, going to methane. To my opinion, it will be extremely difficult for MFCs to become an alternative to this, certainly considering the higher complexity of the systems and the presently lower rate.

No, besides the niches for MFC in sensing, the major promise lies in the return of the second wave: bioelectrosynthesis. Although there were some isolated reports on production of methane at cathodes in the late 1990s as well [6], around 2010, the topic truly resurfaced in the context of production of modest amounts of acetate from CO2 and electricity [12] in so-called microbial electrosynthesis (MES). Since then, titers and rates rapidly increased because of the availability of better technology, to now reach gram per liter levels [13]. Simultaneously, it is possible to also extract the product and thus obtain concentrates [14]. The product portfolio has expanded, from acetate to butyrate, caproate, and caprylate [15], toward alcohols such as ethanol [16] and isopropanol [17], even toward esters such as ethylacetate [18]. It appears that electricity-driven CO2 reduction is here to stay, and there are multiple good reasons for this: society is electrifying, which means that new applications are shifting to the use of electricity as energy source. The electricity is ubiquitously available, can be produced from renewable sources, and when used in the context of production leaves no traces such as salts in the water or the product. The coupling of electricity to CO2 conversion in the so-called carbon capture and utilization is rapidly emerging and MES will find its place within this portfolio.

Already in 2010, we made the point that MES is more than reducing CO2 [19]. Many existing production processes are imbalanced in terms of electrons, requiring, e.g., the supply of very well-controlled amounts of oxygen, which complicates many production processes. Flynn et al. [20] showed elegantly that an anode could solve the electron imbalance, enabling production of ethanol from glycerol with an engineered Shewanella?oneidensis strain. Later, Lai et al. [21] showed anode-associated conversion of glucose to a-ketogluconic acid at efficiencies over 90% by coupling the metabolism of Pseudomonas?putida, remarkably a strict aerobe, to an anode. The use of this organism opens up an enormous array of novel production routes, multiple of the most attractive routes have recently been identified by Kracke and Krömer [22].

MES thus encompasses a broad range of production processes, both anodic and cathodic, both starting from CO2 and from substrate organics [23]. Similar processes emerge to produce methane or upgrade biogas and to produce inorganic products such as hydrogen peroxide or ammonia, many of which are discussed in detail in the following book and which all have the potential to evolve into mature technologies and processes.

The challenges toward this are considerable and are both technological and microbial. When reading this book, grasp the excitement of this great field of science and engineering on the verge of breakthrough. This interface between biology and electrochemistry has already taught us many things about how microorganisms and microbial communities work, and they will continue to amaze us. Think about new, creative uses of bugs and electricity or how electron flow could affect our natural environment.

Enjoy,

Korneel Rabaey
January 2018
Ghent University, Centre for Microbial Ecology and Technology (CMET), Faculty of Bioscience Engineering, Coupure Links 653, 9000 Gent, Belgium

References

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