Rabaey, Korneel

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Ghent University Academic Bibliography

however, not sure whether it is mainly a ‘I believe’ or ‘Ihope’ – but perhaps it is both as described below.The future of microbial ecology will provide many excit-ing things to the engineers and – what many tend to forget– also visa versa: New microbial processes have in thepast been discovered in engineered systems, e.g. theprocess for removal of the nitrogen by anaerobic ammo-nium oxidation (anammox) and enhanced biologicalremoval of phosphorus. In case of the anammox organ-isms, they proved not just to be important in engineeredsystems, but central to the whole global N-cycle. The nextdecade will undoubtedly reveal several new organisms ormicrobial consortia that can be developed into useful full-scale processes in water and wastewater treatment or inthe recovery and reuse of non-renewable recourses, suchas phosphorus and production of bioplastic.In order to manage the microbial ecosystems, microbialecology needs to provide engineers with better modelsand theories about how these complex ecosystems work.Many wastewater treatment plants are still run as a ‘blackbox’ with a minimum of knowledge about the functioningecosystem that lies behind. In the not too distant future Isee the development of representative ‘model ecosys-tems’ where a detailed understanding of the underlyingorganization of the ecosystem has been obtained. BillSloan and Tom Curtis have initiated to incorporate thehuge microbial diversity into relatively simple models thattake the first steps at elucidating the general rules for theassembly of microbial ecosystems (Ofitery et al., 2010). Isee a further development of such ‘top down’ models anda meeting of these with the deterministic ‘bottom up’ meta-bolic models and mass transfer models, e.g. by advanced3-D biofilm models as developed by Cristian Picioreanuand Mark van Loosdrecht (Graf von der Schulenburget al., 2009). It will be interesting to see these approachesmerge in the future.Metagenomics and other ‘–omics’ methods areextremely useful to obtain lots of information about spe-cific ecosystems. The number of studies of engineeredsystems – although is still few – have given invaluablenew knowledge, but have mainly been limited to thesequencing of genomes of uncultured organisms such asthe anammox bacteria, polyphosphate-accumulatingorganisms and nitrifiers obtained from highly enrichedcultures. Metagenomes of entire model ecosystemswill characterize the diversity and potential function ofthe entire community and are now becoming practicallypossible because of rapidly improving sequencing andbioinformatics capacity. A deeper understanding of eco-systems must combine these approaches with transfor-mation rates to characterize the overall function of thesystem. These advances will be combined with advancesin single cell techniques that link the genomic informa-tion with species-related morphology, surface properties,3-D organization in the aggregates and other func-tional details. The combination of ‘-omics’ with single-cellstudies is extremely important – just ask a plant operatorwhere the entire treatment plant is covered by a thickfoam layer caused by a certain filamentous bacteriumexcreting hydrophobic surface components! My crystalball sees these studies eventually developing an easy andfast community fingerprint of structure and function thatcontains both a diagnosis of problems and suggestionsfor corrective actions of relevance for the operation ofthe system.Is the microbial diversity in engineered systems so highthat it is beyond our reach to cope with? Fortunately, notso! We have been studying in a large number of Danishwastewater treatment plants with enhanced biologicalphosphorus removal and discovered a surprising similarcomposition of the microbial communities (Nielsen et al.,2010). However, indications are strong that the microdi-versity among the different species or genera is very largein the individual plants and perhaps partly decisive forplant function and stability – which should be resolved innear future.The exciting new microbial ecology and ‘SystemsBiology’ will go hand in hand with the next generationof ecosystem theories and modelling and – not to forget –the developing of new technologies in water engineering.This will require more co-ordination and cross-disciplinarycollaboration – already at the university and during post-graduate training. Strangely, this aspect of the future is –in my crystal ball – still unclear. I certainly hope that thiswish will also come true.ReferencesGraf von der Schulenburg, D.A., Pintelon, T.R.R., Picioreanu,C., Van Loosdrecht, M.C.M., and Johns, M.L. (2009)Three-dimensional simulations of biofilm growth in porousmedia. AIChE J 55: 494–504.Nielsen, P.H., Mielczarek, A.T., Kragelund, C., Nielsen, J.L.,Saunders, A.M., Kong, Y., et al. (2010) A conceptual eco-system model of microbial communities in enhanced bio-logical phosphorus removal plants. Wat Res 44: 5070–5088.Ofitery, I., Lunn, M., Curtis, T.P., Criddle, C., Francis, C.A.,and Sloan, W.T. (2010) Combined niche and neutral effectsin a microbial wastewater treatment community. Proc NatlAcad Sci USA 107: 15345–15350.Switch on the bugs! (Where EnvironmentalEngineering meets Synthetic Biology)Korneel Rabaey, Advanced Water Management Centre,The University of Queensland, Brisbane, QLD, AustraliaMicrobial respiration is a form of internal electricity gen-eration. In 1962 Davis and Yarbrough elegantly exploitedthis in the first true MFC: electrical power was produced126 Crystal ball© 2011 The AuthorsJournal compilation © 2011 Society for Applied Microbiology and Blackwell Publishing Ltd, Microbial Biotechnology, 4, 109–137from organic conversion and sunlight (Davis andYarbrough, 1962). In 1979 the process was reversed,electricity was given to a fermenting organism and itsproduct spectrum changed (Hongo and Iwahara, 1979).While MFCs resurfaced strongly in the past years, we nowknow that effective power production at scale is challeng-ing, and that the economic and environmental drivers forthis bioelectricity are limited. Aside from developingadvanced bioremediation approaches, many researchersare now refocusing towards the production of chemicalsand fuels with this technology (‘bioelectrochemicalsystems’). Electrical current can indeed be used to drivemicrobial metabolism for bioproduction, by changing theoutcome of fermentation, or by driving respiration (‘micro-bial electrosynthesis’) (Nevin et al., 2010; Rabaey andRozendal, 2010).Producing (bio)chemicals starting from electricity andCO2 or substrate organics could have many advantages,such as high-production density, CO2 capture, facilitatedstorage and ‘transport’ of electricity. Particularly attractiveis the combination with wastewater. Although its supply islimited and localized, wastewater organics can provide anenergy-efficient source of the electrical current and CO2(anode), while the wastewater nutrients can support thegrowth of producing organisms (present at the cathode).Principally, microbial electrosynthesis uses microorgan-isms to convert mixed organics into electrical current, andconverts electrical current into specific organics of inter-est. Biorefining in one reactor!What is needed to make this work is a good technol-ogy platform (including electrode materials), microbialpopulations to degrade the wastewater organics andeffective production strains to convert the electricalcurrent into the desired product. We have none of thesetoday. This is hence where environmental engineeringmeets synthetic biology. Breakthroughs are needed tocreate biocompatible, low-cost electrode materials, com-bining surface area with surface charge and functional-ity. We need to engineer microorganisms that effectivelyproduce biochemicals using electrical current as electrondonor. And, we need to develop a technology that bringsthis process to scale, in a wastewater context or in astraight bioproduction context. Quite some wallsbetween microbiologists and engineers need to be torndown, and the challenging creation of a common lan-guage will be part of this.If we succeed, we will extract sufficient value fromwastewater to turn it from a waste into a resource, witha positive value. Wastewater treatment can thus be pro-fessionalized to the level of true biorefining, wherewater, organics/energy and nutrients are extracted andbrought back to the market. Outside the wastewatercontext, the sun can (via photovoltaics) drive microbialelectrosynthesis, and thus enable bioproduction in areasthat are not suitable for agriculture. Areas such as theAustralian outback and the Arizona desert suddenlyprovide a tremendous opportunity for large-scale biore-fining. So if all goes well, you may find that somedayyour wastewater will go to the highest bidder, and thatyou can fuel your car not only in the Saudi-Arabiandesert . . .ReferencesDavis, J.B., and Yarbrough, H.F. (1962) Preliminary experi-ments on a microbial fuel cell. Science 137: 615–616.Hongo, M., and Iwahara, M. (1979) Electrochemical studieson fermentation. 1. Application of electro-energizingmethod to L-glutamic acid fermentation. Agric Biol Chem43: 2075–2081.Nevin, K.P., Woodard, T.L., Franks, A.E., Summers, Z.M.,and Lovley, D.R. (2010) Microbial electrosynthesis: feedingmicrobes electricity to convert carbon dioxide and water tomulticarbon extracellular organic compounds. mBio 1:e00103–00110.Rabaey, K., and Rozendal, R.A. (2010) Microbial electrosyn-thesis – revisiting the electrical route for microbial produc-tion. Nat Rev Microbiol 8: 706–716.Giving a little help to our prokaryote friendsBruce E. Rittmann, Center for Environmental Biotechnol-ogy, Biodesign Institute at Arizona State University,Tempe, AZ, USABecause of their small size and simple physiology,prokaryotes are especially capable of exploiting an amaz-ingly wide range of metabolic niches that also providewonderful services to society. I illustrate the amazingvariety by giving a few examples of prokaryotic physiolo-gies that are relatively recent discoveries and that holdpromise for providing important services:• Certain bacteria are able to carry out respiration viaextracellular electron transport through a conductivebiofilm matrix (Torres et al., 2010). These anode-respiring bacteria offer promise for producing renew-able electricity, hydrogen gas and other materials frombiomass organics.• Species of Dehalococcoides are able to reductivelydechlorinate organic solvents, such as trichloroethene,to harmless end-products in another unique form ofrespiration (Löffler et al., 2005). Dehalococcoidesalready are being used commercially for bioremediationof solvent-contaminated sites.• The ANAMMOX planktomycete is able to oxidizeammonium while respiring nitrite, creating a novel formof anaerobic ammonium oxidation (Strous et al., 1999).This startling discovery is being pursued for the treat-ment of high-ammonium waste streams (Van Loos-drecht et al., 2004).Crystal ball 127© 2011 The AuthorsJournal compilation © 2011 Society for Applied Microbiology and Blackwell Publishing Ltd, Microbial Biotechnology, 4, 109–137

Switch on the bugs! (Where environmental engineering meets synthetic biology)

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Switch on the bugs! (Where environmental engineering meets synthetic biology)

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