microbial communities in anaerobic digestion processes for waste and wastewater treatment: a...

Microbial communities in anaerobic digestion processes forwaste and wastewater treatment: a microbiological updateTakashi Narihiro and Yuji Sekiguchi

Anaerobic digestion technology is the biological treatment of

organic waste and wastewater without input of external

electron acceptors (oxygen), offering the potential to reduce

treatment cost and to produce energy as ‘biogas’ (methane)

from organic waste. The technology has become enormously

popular in the past two decades, and knowledge of

microbiological aspects of the technology has also

accumulated significantly. Major advances have been made in

elucidating the diversity of yet-to-be cultured microbes in

anaerobic digestion processes, and the cultivation of

uncultured organisms is of great interest with regard to gaining

insights into the function of these organisms. In addition, recent

advances have been made in the development of microbial fuel

cells as an alternative, direct energy-yielding treatment system.

Addresses

Bio-Measurement Research Group, Institute for Biological Resources

and Functions, National Institute of Advanced Science and

Technology (AIST), AIST Tsukuba Central 6, Ibaraki 305-8566, Japan

Corresponding author: Sekiguchi, Yuji ([email protected])

Current Opinion in Biotechnology 2007, 18:273–278

This review comes from a themed issue on

Environmental biotechnology

Edited by Eliora Z Ron and Philip Hugenholtz

Available online 25th April 2007

0958-1669/$ – see front matter

# 2007 Elsevier Ltd. All rights reserved.

DOI 10.1016/j.copbio.2007.04.003

IntroductionIn theory, anaerobic digestion technology is an ideal bio-

logical means for the removal of organic pollutants in waste

and wastewater. The technology has two significant advan-

tages over the conventional aerobic biological treatment:

firstly, it is cost-effective because aeration is not required

and a small amount of excess sludge is produced, and

secondly, it generally produces gaseous methane as an

energy resource [1,2]. The largest application of this diges-

tion technology is the stabilization of sludge, such as a

sludge digester commonly used at municipal wastewater

treatment plants. The technology has also become popular

in dilute and concentrated wastestream treatment fields;

anaerobic digestion technology for wastewater treatment

can now also be considered a matured biotechnology.

Owing to the development of sophisticated wastewater

treatment technologies (such as upflow anaerobic sludge

blanket [UASB] technology), more than 1500 anaerobic

www.sciencedirect.com

wastestream treatment processes are now installed and

operating worldwide [3,4]. So far, the most practical target

of this technology is high-rate treatment of high-strength

industrial organic wastewater [1,4]. The types of waste-

water that can be treated by such technology have recently

been expanded notably; for example, low-strength organic

wastewater, complex wastewater containing persistent

chemical compounds, and wastewater discharged at tem-

peratures ranging from psychrophilic (4 8C to 20 8C) to

thermophilic (above 45 8C) [2,5]. In addition, the technol-

ogy has also been shown to offer interesting potentials for

metal removal and recovery with sulfate reduction,

removal of nitrates with nitrification, and bioremediation

for breakdown of toxic substances.

One of the most advanced fields associated with the

technology in the past few years is the microbiology of

anaerobic digestion processes. Because knowledge of the

ecology and function of the microbial community in these

processes is required to better control the biological

processes, considerable effort has been made to under-

stand the microbial community structure by using

culture-dependent and culture-independent molecular

approaches [2,6,7]. Through these analyses, particularly

those targeting the 16S rRNA gene, detailed pictures of

the community compositions are being documented. In

addition, several functionally important anaerobes, play-

ing key roles in the treatment process, have been culti-

vated and characterized. Furthermore, the ecophysiology

of yet-to-be cultured organisms in the ecosystems is being

elucidated using a variety of approaches.

In this review, we focus on microbiological aspects of

anaerobic (mainly methanogenic) microbial communities

for high-rate organic waste and wastewater treatment, and

update the recent findings in this field. We highlight a

variety of recent approaches in microbiological fields,

including microbial community analysis, the domesti-

cation of uncultured microorganisms, and ecophysiologi-

cal analysis of yet-to-be cultured anaerobes that are

relevant to the technology. In addition, we raise the topic

of microbial fuel cells, an alternative, direct energy-

yielding wastewater treatment system, and describe

recent findings on the microbiological aspects.

Community analysis of anaerobic digestionprocesses: the uncultured, yet-to-becharacterized lineagesIn anaerobic treatment processes, there has been a rela-

tively limited number of studies conducting 16S rRNA

gene cloning-based analyses of the microbial community


mailto:[email protected]

http://dx.doi.org/10.1016/j.copbio.2007.04.003

274 Environmental biotechnology

(often known as ‘sludge’) [2,6,7]. However, constituents of

more than 20 bacterial phyla have been detected in

anaerobic (mostly methanogenic) waste and wastewater

sludges [2,6,7]. For example, 16S rRNA gene clones that

were frequently and commonly retrieved from these

sludges were distributed in various prokaryotic taxa such

as the phyla Proteobacteria (mainly in the class Deltapro-teobacteria), Chloroflexi, Firmicutes, Spirochaetes, and Bacter-oidetes in the domain Bacteria. Similarly, clones in

the classes Methanomicrobia, Methanobacteria, and Thermo-plasmata in the domain Archaea are those of typical phy-

lotypes found in such sludges [6]. In addition to these

relatively known taxa, phylotypes belonging to a variety of

uncultured candidate phyla (or classes) (known as ‘clone

cluster’) were often detected in these sludges [6,7,8��,9,10�,11�,12]. Within the domain Bacteria, diverse uncul-

tivated taxonomic groups, such as OP10, BA024, OP8,

TM6, EM3, OP3, and OS-K, were detected in these

sludges [6] (note that most of the candidate taxa in this

review are named according to the article by Hugenholtz

[13]). Finding key (or dominant) populations that belong to

such uncultured lineages at various taxonomic levels (from

species to phylum levels) is one of the major advances in

the microbiology of anaerobic digestion processes in the

past few years.

One recent finding in the predominant populations of the

clone clusters is that of the candidate bacterial phylum

WWE1. Chouari et al. [10�] described that 81% of all the

bacterial 16S rRNA gene clones retrieved from sludge

from an anaerobic mesophilic digester were assigned with

the group WWE1. Fluorescence in situ hybridization

(FISH) analysis revealed that WWE1-type cells had

rod and filamentous morphotypes, and the rRNA from

WWE1 cells accounted for 12% of the total bacterial

rRNA [10�]. Although the ecophysiological function of

WWE1-type bacteria remains unknown, their high abun-

dance in the sludge suggests they might play a certain role

in the digestion process.

Another example of the predominant populations that

belong to clone clusters is that of the bacterial phylum

Deferribacteres. The phylum Deferribacteres (formerly

recognized as the phylum ‘Synergistes’) contained pheno-

typically diverse genera such as Deferribacter, Denitrovor-ans, Geovibrio, Flexistipes and ‘Synergistes’ [14]. Within the

phylum, several 16S rRNA gene clones that are distantly

related to known cultivated species, forming distinct

clone clusters at the subphylum (class) levels, have also

been retrieved from anaerobic (methanogenic) sludges.

The distribution of members of this phylum was

thoroughly explored in a total of 93 anaerobic ecosystems,

such as anaerobic digesters, soil and gut samples, indi-

cating that Deferribacteres-type clones were widely dis-

tributed in such anaerobic ecosystems [11�]. The clones

retrieved from methanogenic digester sludges were

assigned to four different subphyla of Deferribacteres, none


of which contains cultured representatives. More recently,

Diaz et al. [15] showed that 34% of all the bacterial clones

detected from a full-scale methanogenic UASB process

treating brewery wastewater were affiliated with uncul-

tured clades of the phylum Deferribacteres. Given their high

occurrence in such methanogenic ecosystems, they might

play a role in part of the food web for the methanogenic

degradation of organic compounds; however, their ecophy-

siology remains unknown.

With respect to the uncultured archaeal lineages, archaeal

16S rRNA gene clones affiliated with the candidate taxon

WSA2 were retrieved in abundance from a mesophilic

methanogenic digester decomposing sewage sludge [9]

(this candidate taxon is also named according to the

review by Hugenholtz [13], although Chouari et al. [9]

refer to the taxon as ‘ArcI’ group in their report). This

group is a clone cluster at the subphylum (or class) level

within the archaeal phylum Euryarchaeota, and phylo-

types of WSA2 were sometimes detected in other

anaerobic treatment processes [6]. Chouari et al. [9] also

obtained highly enriched WSA2 cells using formate- or

hydrogen-containing media, implying that WSA2-related

microorganisms are methanogens.

Another unique, uncultured archaeal taxon that is also

often found in methanogenic sludges is subphylum C2 of

the archaeal phylum Crenarchaeota. For example, 16% of

the archaeal rRNA gene clones analyzed from a meso-

philic methanogenic digester were found to belong to

members of Crenarchaeota, particularly the subphylum C2

[9]. C2-type rRNA gene clones were also detected from a

full-scale, methanogenic (partially sulfidogenic) UASB

process treating paper mill wastewater [16] and a

laboratory-scale UASB reactor treating methanethiol

[17]. Collins et al. [8��] recently surveyed C2-type phylo-

types in anaerobic sludges from a variety of methanogenic

wastewater treatment systems, detecting these phylotypes

in abundance (14–78% of the total archaeal clones

analyzed). From FISH observations in thin-sections of

methanogenic sludge granules, cells of the C2-type phy-

lotypes were found to be rods (1.5 mm in length and 0.7 mm

in width), forming dense microcolonies within the sections.

Interestingly, they were juxtaposed to Methanosaeta cells

[8��]. These findings may imply that they are active

members of the sludge ecosystems, interacting with acet-

iclastic methanogens in situ. Further studies are needed to

clearly describe the ecophysiological functions of these

unique Archaea.

Community changes within anaerobicdigestion processesRecent microbiological studies focus not only on the

description of a microbial community at a particular

operational time, but also on the community shift (popu-

lation dynamics) of anaerobic sludge along with different

operational periods (or conditions).


Microbial anaerobic digestion processes Narihiro and Sekiguchi 275

Such population dynamics of resident microbes in

anaerobic treatment processes have also been analyzed

by using rRNA and rRNA-gene based methods. For

example, Diaz et al. [15] and Zheng et al. [18] studied

the community changes along with the maturation of

sludge granules in UASB reactors using rRNA gene-based

denaturing gradient gel electrophoresis (DGGE) analysis

and FISH. Similarly, Hori et al. [19] analyzed changes

in the microbial community succession in a thermophilic

methanogenic reactor under deteriorative and stable

conditions, which were induced by acidification and

neutralization, using rRNA gene-based single-strand con-

formation polymorphism (SSCP), quantitative PCR,

and FISH. The results indicated that the methanogenic

community in the process was significantly affected by

volatile fatty acid concentrations.

Newly isolated microorganisms fromanaerobic treatment processesThe domestication (cultivation) of uncultured organisms

is of great interest in this field to gain insights into the

function of these organisms. In the past few years, newly

discovered microorganisms have been successfully iso-

lated from anaerobic sludges, and the information regard-

ing their physiology in conjugation with phylogeny is

updated regularly: examples include carbohydrate degra-

ders [20,21], a protein degrader [22], fatty acid oxidizers

[23,24�,25–32], terephthalate oxidizers [33], and metha-

nogens [34–36].

One of the most significant advances in this field is the

isolation of organisms from subphylum I of the bacterial

phylum Chloroflexi. The subphylum I was recognized as a

clone cluster, members of which were frequently detected

in anaerobic environments in abundance [6,15,16,37,38].

Recently, four filamentous strains that belong to the Chlor-oflexi subphylum I were successfully isolated and cultured

from anaerobic wastewater treatment sludges, and these

strains were characterized in detail to give their taxonomic

placements [20,21]. At present, the subphylum contains

two thermophilic species of the genus Anaerolinea, one

mesophilic species of the genus Levilinea, and one meso-

philic species of the genus Leptolinea, and the subphylum

was named as the new class Anaerolineae. These organisms

are known to be one of the major populations in mesophilic

and thermophilic sludge granules of UASB reactors [37,38].

One species (Anaerolinea thermophila) is associated with

filamentous bulking of methanogenic granular sludge [37].

All strains possess filamentous morphotypes, growing fer-

mentatively with a range of carbohydrates and yeast extract

as substrates. These findings suggest that members of the

class might play a key role in the primary degradation of

carbohydrates and cellular materials (such as amino acids)

in methanogenic digestion processes.

Several important proton-reducing syntrophic bacteria

affiliated with the group ‘Desulfotomaculum cluster I’ have


been isolated and cultured from anaerobic wastewater

processes. Imachi et al. [23,39] first isolated Pelotomaculumthermopropionicum from a laboratory-scale UASB reactor

operated under thermophilic conditions. This bacterium

can degrade propionate in co-culture with hydrogeno-

trophic methanogens [23]. Later, the obligately syntrophic,

propionate-oxidizing bacterium Pelotomaculum schinkii[24�], a mesophilic, syntrophic propionate-oxidizing

strain MGP [40��], two mesophilic, syntrophic phthalate-

degrading species Pelotomaculum terephthalicum and Peloto-maculum isophthalicum [33,41], and a mesophilic, syntrophic

benzoate-oxidizing species Sporotomaculum syntrophicum[25] were also isolated from anaerobic wastewater treat-

ment processes. Interestingly, all these syntrophs lack the

ability to dissimilatory reduce sulfate, although other mem-

bers of the ‘Desulfotomaculum cluster I’ are known to

be sulfate reducers. From a set of various molecular

and cultivation-based analyses for non-sulfate-reducing

Desulfotomaculum-type organisms, it was hypothesized that

these microorganisms have recently adopted a syntrophic

lifestyle to thrive in low-sulfate, methanogenic environ-

ments and thus have lost their ancestral ability for dissim-

ilatory sulfate/sulfite reduction [40��]. Importantly, by

using DNA-based stable isotope probing (SIP) analyses

members of the genus Pelotomaculum were found to be a

key contributor to the syntrophic propionate degradation

in natural anaerobic ecosystems (e.g. flooded soil and

freshwater marshes) [42,43�].

Among the characterized species of these syntrophic organ-

isms of the ‘Desulfotomaculum cluster I’, P. thermopropioni-cum has been intensively studied in recent years. Kosaka

et al. [44�] analyzed the genome of P. thermopropionicum and

proposed a novel propionate-oxidizing pathway (methyl-

malomyl coenzyme A pathway), which differs from that of

the previously characterized pathway in other bacteria.

Together with proteomic data, they assumed that fumarase

plays a central role in the regulation of syntrophic propio-

nate degradation. Additionally, it was reported that the

cells of P. thermopropionicum coaggregated when they were

co-cultivated with Methanothermobacter thermautotrophicuscells in the presence of propionate [45�,46�]. Interestingly,

the flagellum-like filaments of P. thermopropionicumparticipate in coaggregation of each cell [45�], suggesting

that these filaments might have a role in keeping the

two organisms closely juxtaposed with each other for

efficient interspecies hydrogen transfer. More recently,

the flagellum-like filaments of P. thermopropionicumwere found to be electrically conductive, suggesting the

possibility of direct exocellular electron transfer through

the flagellum-like filaments (this topic is further discussed

below) [47��,48��]

Although the number of descriptions of these new anae-

robes is increasing, there is still much work to be done to

domesticate the yet-to-be cultured microbes that are

recalcitrant to artificial cultivation. To avoid difficulties



in cultivation, several techniques such as FISH combined

with microautoradiography (MAR–FISH), DNA-based

stable isotope probing (SIP) and RNA-SIP have been

developed and applied to anaerobic sludges [49]. Visua-

lizing substrate uptake pattern within the methanogenic

granule sections with new techniques, such as radioactive

tracer technique plus b imaging [8��], SIP-Raman micro-

scopy [50] and SIP-secondary ion mass spectrometry

(SIMS) [51], might also help to point the way to the

cultivation of uncultured organisms. Collins et al. [8��]attempted to apply the radioactive tracer technique and b

imaging to sludge granules to speculate the function of

uncultured Crenarchaeota cells (mentioned above).

Furthermore, metagenomic approaches for microbial con-

sortia containing yet-to-be cultured microbial cells have

recently been applied to a wide range of environments to

gain new insight into the potential metabolic activities of

predominant microbes based on gene information. For

example, the report by Wexler et al. [52] is, to our knowl-

edge, the first attempt at employing a metagenomic

approach for anaerobic digester sludges.

Recent issue: microbial fuel cells revisitedMicrobial fuel cells (MFC) have recently received much

attention for their potential to directly recover electricity

as an energy resource from waste and wastewater [48��,53,54]. Interest in the MFC process originally began in

the 1960s, when it was discovered that resident microor-

ganisms in the process oxidize the organic substances in

waste and wastewater as electron donors and transfer

electrons to the anode electrode via soluble electron

mediators such as ferricyanide. However, the addition

of such mediators was considered infeasible for actual

electron recovery because of their toxicity and instability

in a prolonged operation of the MFC process [53,55,56].

Recently, Bond and Lovley [55] and Chaudhuri and

Lovley [56] showed that Geobacter sulfurreducens and Rho-doferax ferrireducens, respectively, directly transfer elec-

trons to the anode surface via components associated with

their cell wall; these findings again shed light on the MFC

process research. The electron transfer efficiency from

acetate to electricity was found to be 96.8% with G.sulfurreducens cells, whereas the electron transfer effi-

ciency from glucose was reported to be 81% with R.ferrireducens cells. Several attempts to produce electricity

with a complex microbial community have also been

reported [54,57,58]. For example, Rabaey et al. [57]

reported that the MFC process enriched with an

anaerobic granular sludge achieved 81% efficiency for

electron transfer from glucose. Most recently, it is

reported that electrically conductive pilus-like filaments

(they called ‘bacterial nanowires’) of the metal-reducing

bacterium Shewanella oneidensis mediated exocellular

electron transfer to metal surface [47��]. As mentioned

above, a similar finding was made when the syntrophic

propionate-oxidizer P. thermopropionicum was cultivated


in pure culture with fumarate or in co-culture with

M. thermautotrophicus with propionate [47��]. This suggests

the possibility of direct exocellular electron transfer

from the syntrophic microbe through such electrically

conductive, flagellum-like filaments [47��,48��].

ConclusionsAs described in this short update, significant advances

have been made in elucidating the diversity of yet-to-be

cultured organisms in anaerobic (methanogenic) diges-

tion processes. Although some important anaerobes have

been cultivated and characterized, there is still work to

be done for a vast number of remaining anaerobes that

have not yet been cultured. For the remainder, further

characterization of their ecophysiological traits with cul-

tivation or with much more elegant methods (such as SIP)

is needed. The accumulation of such information will

offer substantial information for more sophisticated man-

agement of the digestion technology.

Recent attention to the potential of the MFC process to

directly recover electricity as an energy resource from

waste and wastewater could suggest a future direction of

the technological development of the anaerobic digestion

process. Further investigations of the MFC process will

be necessary to improve the process, as well as to fully

understand the role of the microbial community contri-

buting the generation of electricity.

AcknowledgementsThis study was supported by the Project ‘Development of Technologiesfor Analyzing and Controlling the Mechanism of Biodegrading andProcessing’, which was ensured to the New Energy and IndustrialTechnology Development Organization (NEDO).

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� of special interest�� of outstanding interest

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32. Kendall MM, Liu Y, Boone DR: Butyrate- and propionate-degrading syntrophs from permanently cold marinesediments in Skan Bay, Alaska, and description ofAlgorimarina butyrica gen. nov., sp. nov.. FEMS Microbiol Lett2006, 262:107-114.

33. Qiu YL, Sekiguchi Y, Hanada S, Imachi H, Tseng IC, Cheng SS,Ohashi A, Harada H, Kamagata Y: Pelotomaculumterephthalicum sp. nov. and Pelotomaculum isophthalicumsp. nov.: two anaerobic bacteria that degrade phthalateisomers in syntrophic association with hydrogenotrophicmethanogens. Arch Microbiol 2006, 185:172-182.

34. Ma K, Liu XL, Dong XZ: Methanobacterium beijingense sp. nov.,a. novel methanogen isolated from anaerobic digesters..Int J Syst Evol Microbiol 2005, 55:325-329.

35. Ma K, Liu XL, Dong XZ: Methanosaeta harundinacea sp. nov., anovel acetate-scavenging methanogen isolated from a UASBreactor.. Int J Syst Evol Microbiol 2006, 56:127-131.



36. Jiang B, Parshina SN, van Doesburg W, Lomans BP, Stams AJM:Methanomethylovorans thermophila sp. nov., a thermophilic,methylotrophic methanogen from an anaerobic reactor fedwith methanol.. Int J Syst Evol Microbiol 2005, 55:2465-2470.

37. Sekiguchi Y, Takahashi H, Kamagata Y, Ohashi A, Harada H: Insitu detection, isolation, and physiological properties of a thinfilamentous microorganism abundant in methanogenicgranular sludges: a novel isolate affiliated with a clonecluster, the green non-sulfur bacteria, subdivision I.Appl Environ Microbiol 2001, 67:5740-5749.

38. Yamada T, Sekiguchi Y, Imachi H, Kamagata Y, Ohashi A,Harada H: Diversity, localization, and physiological propertiesof filamentous microbes belonging to Chloroflexi subphylum Iin mesophilic and thermophilic methanogenic sludgegranules. Appl Environ Microbiol 2005, 71:7493-7503.

39. Imachi H, Sekiguchi Y, Kamagata Y, Ohashi A, Harada H:Cultivation and in situ detection of a thermophilic bacteriumcapable of oxidizing propionate in syntrophic association withhydrogenotrophic methanogens in a thermophilicmethanogenic granular sludge. Appl Environ Microbiol 2000,66:3608-3615.

40.��

Imachi H, Sekiguchi Y, Kamagata Y, Loy A, Qiu YL, Hugenholtz P,Kimura N, Wagner M, Ohashi A: Harada H: Non-sulfate-reducing, syntrophic bacteria affiliated withDesulfotomaculum cluster I are widely distributed inmethanogenic environments. Appl Environ Microbiol 2006,72:2080-2091.

In this paper, the occurrence and abundance of ‘Desulfotomaculumcluster I’ in low-sulfate, methanogenic environments were analyzed using16S rRNA-based molecular approaches, and the new strain MGP wassuccessfully isolated in co-culture with a hydrogenotrophic methanogen.Interestingly, strain MGP could not dissimilatory reduce sulfur com-pounds, but the strain contained (and expressed) dsrAB, key genes inthe sulfate respiration.

41. Qiu YL, Sekiguchi Y, Imachi H, Kamagata Y, Tseng IC, Cheng SS,Ohashi A, Harada H: Identification and isolation of anaerobic,syntrophic phthalate isomer-degrading microbes frommethanogenic sludges treating wastewater fromterephthalate manufacturing. Appl Environ Microbiol 2004,70:1617-1626.

42. Lueders T, Pommerenke B, Friedrich MW: Stable-isotopeprobing of microorganisms thriving at thermodynamiclimits: Syntrophic propionate oxidation in flooded soil.Appl Environ Microbiol 2004, 70:5778-5786.

43.�

Chauhan A, Ogram A: Fatty acid-oxidizing consortiaalong a nutrient gradient in the Florida Everglades.Appl Environ Microbiol 2006, 72:2400-2406.

DNA-based stable isotope proving was employed to investigate the fateof carbon from propionate and butyrate in freshwater marshes.

44.�

Kosaka T, Uchiyama T, Ishii S, Enoki M, Imachi H, Kamagata Y,Ohashi A, Harada H, Ikenaga H, Watanabe K: Reconstructionand regulation of the central catabolic pathway in thethermophilic propionate-oxidizing syntroph Pelotomaculumthermopropionicum. J Bacteriol 2006, 188:202-210.

This paper discusses the whole picture of the possible central catabolicpathway of Pelotomaculum thermopropionicum based on draft genomesequencing.

45.�

Ishii S, Kosaka T, Hori K, Hotta Y, Watanabe K: Coaggregationfacilitates interspecies hydrogen transfer betweenPelotomaculum thermopropionicum andMethanothermobacter thermautotrophicus.Appl Environ Microbiol 2005, 71:7838-7845.

On the basis of beautiful scanning electron microscopic images,the authors suggested that the thermophilic propionate-oxidizingbacterium Pelotomaculum thermopropionicum and the hydrogenotrophic


methanogen Methanothermobacter thermautotrophicus coaggregated viaflagellum-like filaments when they grow in co-culture with propionate.

46.�

Ishii S, Kosaka T, Hotta Y, Watanabe K: Simulating thecontribution of coaggregation to interspecies hydrogen fluxesin syntrophic methanogenic consortia. Appl Environ Microbiol2006, 72:5093-5096.

This paper describes a simple model for simulating the contribution ofcoaggregation to interspecies hydrogen transfer fluxes between syn-trophic bacteria and methanogens.

47.��

Gorby YA, Yanina S, McLean JS, Rosso KM, Moyles D,Dohnalkova A, Beveridge TJ, Chang IS, Kim BH, Kim KS et al.:Electrically conductive bacterial nanowires produced byShewanella oneidensis strain MR-1 and othermicroorganisms. Proc Natl Acad Sci USA 2006,103:11358-11363.

Using scanning tunneling microscopy, the authors report that threebacterial species, Shewanella oneidensis, Synechocystis sp., and Pelo-tomaculum thermopropionicum, produced electrically conductive pilus-like filaments. Because P. thermopropionicum uses these electricallyconductive filaments to coaggregate with methanogens, it is suggestedthat P. thermopropionicum possibly mediates interspecies electrontransfer with methanogens not only via protons but also via the filaments.

48.��

Logan BE, Regan JM: Electricity-producing bacterialcommunities in microbial fuel cells. Trends Microbiol 2006,14:512-518.

This paper reviews current knowledge of the microbial communitiesfound in microbial fuel cells.

49. Hatamoto M, Imachi H, Ohashi A, Harada H: Identification andcultivation of anaerobic, syntrophic long-chain fatty aciddegrading microbes from mesophilic and thermophilicmethanogenic sludges. Appl Environ Microbiol 2007,73:1332-1340.

50. Huang WE, Griffiths RI, Thompson IP, Bailey MJ, Whiteley AS:Raman microscopic analysis of single microbial cells.Anal Chem 2004, 76:4452-4458.

51. Derito CM, Pumphrey GM, Madsen EL: Use of field-based stableisotope probing to identify adapted populations and trackcarbon flow through a phenol-degrading soil microbialcommunity. Appl Environ Microbiol 2005, 71:7858-7865.

52. Wexler M, Bond PL, Richardson DJ, Johnston AWB: A wide host-range metagenomic library from a waste water treatmentplant yields a novel alcohol/aldehyde dehydrogenase.Environ Microbiol 2005, 7:1917-1926.

53. Logan BE, Hamelers B, Rozendal R, Schrorder U, Keller J,Freguia S, Aelterman P, Verstraete W, Rabaey K: Microbial fuelcells: methodology and technology. Environ Sci Technol 2006,40:5181-5192.

54. Lovley DR: Microbial fuel cells: novel microbial physiologiesand engineering approaches. Curr Opin Biotechnol 2006,17:327-332.

55. Bond DR, Lovley DR: Electricity production by Geobactersulfurreducens attached to electrodes. Appl Environ Microbiol2003, 69:1548-1555.

56. Chaudhuri SK, Lovley DR: Electricity generation by directoxidation of glucose in mediatorless microbial fuel cells.Nat Biotechnol 2003, 21:1229-1232.

57. Rabaey K, Boon N, Siciliano SD, Verhaege M, Verstraete W:Biofuel cells select for microbial consortia that self-mediateelectron transfer. Appl Environ Microbiol 2004, 70:5373-5382.

58. Logan BE: Simultaneous wastewater treatment and biologicalelectricity generation. Water Sci Technol 2005, 52:31-37.


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