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Lanzilli M, Esercizio N, Vastano M, Xu Z, Nuzzo G, Gallo C, Manzo E, Fontana A, d’Ippolito G. Effect of Cultivation Parameters on Fermentation and Hydrogen Production in the Phylum Thermotogae. Int J Mol Sci 2020; 22:ijms22010341. [PMID: 33396970 PMCID: PMC7795431 DOI: 10.3390/ijms22010341] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2020] [Revised: 12/21/2020] [Accepted: 12/23/2020] [Indexed: 01/19/2023] Open
Abstract
The phylum Thermotogae is composed of a single class (Thermotogae), 4 orders (Thermotogales, Kosmotogales, Petrotogales, Mesoaciditogales), 5 families (Thermatogaceae, Fervidobacteriaceae, Kosmotogaceae, Petrotogaceae, Mesoaciditogaceae), and 13 genera. They have been isolated from extremely hot environments whose characteristics are reflected in the metabolic and phenotypic properties of the Thermotogae species. The metabolic versatility of Thermotogae members leads to a pool of high value-added products with application potentials in many industry fields. The low risk of contamination associated with their extreme culture conditions has made most species of the phylum attractive candidates in biotechnological processes. Almost all members of the phylum, especially those in the order Thermotogales, can produce bio-hydrogen from a variety of simple and complex sugars with yields close to the theoretical Thauer limit of 4 mol H2/mol consumed glucose. Acetate, lactate, and L-alanine are the major organic end products. Thermotagae fermentation processes are influenced by various factors, such as hydrogen partial pressure, agitation, gas sparging, culture/headspace ratio, inoculum, pH, temperature, nitrogen sources, sulfur sources, inorganic compounds, metal ions, etc. Optimization of these parameters will help to fully unleash the biotechnological potentials of Thermotogae and promote their applications in industry. This article gives an overview of how these operational parameters could impact Thermotogae fermentation in terms of sugar consumption, hydrogen yields, and organic acids production.
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Affiliation(s)
- Mariamichela Lanzilli
- Istituto di Chimica Biomolecolare (ICB), CNR, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy; (M.L.); (N.E.); (M.V.); (G.N.); (C.G.); (E.M.); (A.F.)
| | - Nunzia Esercizio
- Istituto di Chimica Biomolecolare (ICB), CNR, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy; (M.L.); (N.E.); (M.V.); (G.N.); (C.G.); (E.M.); (A.F.)
| | - Marco Vastano
- Istituto di Chimica Biomolecolare (ICB), CNR, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy; (M.L.); (N.E.); (M.V.); (G.N.); (C.G.); (E.M.); (A.F.)
| | - Zhaohui Xu
- Department of Biological Sciences, Bowling Green State University, Bowling Green, OH 43403, USA;
| | - Genoveffa Nuzzo
- Istituto di Chimica Biomolecolare (ICB), CNR, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy; (M.L.); (N.E.); (M.V.); (G.N.); (C.G.); (E.M.); (A.F.)
| | - Carmela Gallo
- Istituto di Chimica Biomolecolare (ICB), CNR, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy; (M.L.); (N.E.); (M.V.); (G.N.); (C.G.); (E.M.); (A.F.)
| | - Emiliano Manzo
- Istituto di Chimica Biomolecolare (ICB), CNR, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy; (M.L.); (N.E.); (M.V.); (G.N.); (C.G.); (E.M.); (A.F.)
| | - Angelo Fontana
- Istituto di Chimica Biomolecolare (ICB), CNR, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy; (M.L.); (N.E.); (M.V.); (G.N.); (C.G.); (E.M.); (A.F.)
| | - Giuliana d’Ippolito
- Istituto di Chimica Biomolecolare (ICB), CNR, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy; (M.L.); (N.E.); (M.V.); (G.N.); (C.G.); (E.M.); (A.F.)
- Correspondence: ; Tel.: +39-081-8675096
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Uncoupling Fermentative Synthesis of Molecular Hydrogen from Biomass Formation in Thermotoga maritima. Appl Environ Microbiol 2018; 84:AEM.00998-18. [PMID: 29959252 DOI: 10.1128/aem.00998-18] [Citation(s) in RCA: 30] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2018] [Accepted: 06/24/2018] [Indexed: 01/08/2023] Open
Abstract
When carbohydrates are fermented by the hyperthermophilic anaerobe Thermotoga maritima, molecular hydrogen (H2) is formed in strict proportion to substrate availability. Excretion of the organic acids acetate and lactate provide an additional sink for removal of excess reductant. However, mechanisms controlling energy management of these metabolic pathways are largely unexplored. To investigate this topic, transient gene inactivation was used to block lactate production as a strategy to produce spontaneous mutant cell lines that overproduced H2 through mutation of unpredicted genetic targets. Single-crossover homologous chromosomal recombination was used to disrupt lactate dehydrogenase (encoded by ldh) with a truncated ldh fused to a kanamycin resistance cassette expressed from a native P groESL promoter. Passage of the unstable recombinant resulted in loss of the genetic marker and recovery of evolved cell lines, including strain Tma200. Relative to the wild type, and considering the mass balance of fermentation substrate and products, Tma200 grew more slowly, produced H2 at levels above the physiologic limit, and simultaneously consumed less maltose while oxidizing it more efficiently. Whole-genome resequencing indicated that the ABC maltose transporter subunit, encoded by malK3, had undergone repeated mutation, and high-temperature anaerobic [14C]maltose transport assays demonstrated that the rate of maltose transport was reduced. Transfer of the malK3 mutation into a clean genetic background also conferred increased H2 production, confirming that the mutant allele was sufficient for increased H2 synthesis. These data indicate that a reduced rate of maltose uptake was accompanied by an increase in H2 production, changing fermentation efficiency and shifting energy management.IMPORTANCE Biorenewable energy sources are of growing interest to mitigate climate change, but like other commodities with nominal value, require innovation to maximize yields. Energetic considerations constrain production of many biofuels, such as molecular hydrogen (H2) because of the competing needs for cell mass synthesis and metabolite formation. Here we describe cell lines of the extremophile Thermotoga maritima that exceed the physiologic limits for H2 formation arising from genetic changes in fermentative metabolism. These cell lines were produced using a novel method called transient gene inactivation combined with adaptive laboratory evolution. Genome resequencing revealed unexpected changes in a maltose transport protein. Reduced rates of sugar uptake were accompanied by lower rates of growth and enhanced productivity of H2.
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Hirano SI, Matsumoto N. Electrochemically applied potentials induce growth and metabolic shift changes in the hyperthermophilic bacterium Thermotoga maritima MSB8. Biosci Biotechnol Biochem 2017; 81:1619-1626. [DOI: 10.1080/09168451.2017.1329618] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
Abstract
Bioelectrochemical systems are an attractive technology for regulating microbial activity. The effect of an applied potential on hydrolysis of starch in Thermotoga maritima as a model bacterium was investigated in this study. A cathodic potential (−0.6 and −0.8 V) induced 5-h earlier growth initiation of T. maritima with starch as the polymeric substrate than that without electrochemical regulation. Moreover, metabolic patterns of starch consumption were altered by the cathodic potential. While acetate, H2, and CO2 were the major products of starch consumption in the control experiment without electrolysis, lactate accumulation was detected rather than decreased acetate and H2 levels in the bioelectrochemical system experiments with the cathodic potential. These results indicate that the applied potential could control microbial activities related to the hydrolysis of polymeric organic substances and shift carbon and electron flux to a lactate-producing reaction in T. maritima.
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Affiliation(s)
- Shin-ichi Hirano
- Biotechnology Sector, Environmental Science Research Laboratory, Central Research Institute of Electric Power Industry, Abiko-shi, Japan
| | - Norio Matsumoto
- Biotechnology Sector, Environmental Science Research Laboratory, Central Research Institute of Electric Power Industry, Abiko-shi, Japan
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Boileau C, Auria R, Davidson S, Casalot L, Christen P, Liebgott PP, Combet-Blanc Y. Hydrogen production by the hyperthermophilic bacterium Thermotoga maritima part I: effects of sulfured nutriments, with thiosulfate as model, on hydrogen production and growth. BIOTECHNOLOGY FOR BIOFUELS 2016; 9:269. [PMID: 28018486 PMCID: PMC5168592 DOI: 10.1186/s13068-016-0678-8] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/08/2016] [Accepted: 12/01/2016] [Indexed: 06/06/2023]
Abstract
BACKGROUND Thermotoga maritima and T. neapolitana are hyperthermophile bacteria chosen by many research teams to produce bio-hydrogen because of their potential to ferment a wide variety of sugars with the highest theoretical H2/glucose yields. However, to develop economically sustainable bio-processes, the culture medium formulation remained to be optimized. The main aim of this study was to quantify accurately and specifically the effect of thiosulfate, used as sulfured nutriment model, on T. maritima growth, yields and productivities of hydrogen. The results were obtained from batch cultures, performed into a bioreactor, carefully controlled, and specifically designed to prevent the back-inhibition by hydrogen. RESULTS Among sulfured nutriments tested, thiosulfate, cysteine, and sulfide were found to be the most efficient to stimulate T. maritima growth and hydrogen production. In particular, under our experimental conditions (glucose 60 mmol L-1 and yeast extract 1 g L-1), the cellular growth was limited by thiosulfate concentrations lower than 0.06 mmol L-1. Under these conditions, the cellular yield on thiosulfate (Y X/Thio) could be determined at 3617 mg mmol-1. In addition, it has been shown that the limitations of T. maritima growth by thiosulfate lead to metabolic stress marked by a significant metabolic shift of glucose towards the production of extracellular polysaccharides (EPS). Finally, it has been estimated that the presence of thiosulfate in the T. maritima culture medium significantly increased the cellular and hydrogen productivities by a factor 6 without detectable sulfide production. CONCLUSIONS The stimulant effects of thiosulfate at very low concentrations on T. maritima growth have forced us to reconsider its role in this species and more probably also in all thiosulfato-reducer hyperthermophiles. Henceforth, thiosulfate should be considered in T. maritima as (1) an essential sulfur source for cellular materials when it is present at low concentrations (about 0.3 mmol g-1 of cells), and (2) as both sulfur source and detoxifying agent for H2 when thiosulfate is present at higher concentrations and, when, simultaneously, the pH2 is high. Finally, to improve the hydrogen production in bio-processes using Thermotoga species, it should be recommended to incorporate thiosulfate in the culture medium.
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Affiliation(s)
- Céline Boileau
- Aix Marseille Université, CNRS, Université de Toulon, IRD, MIO UM 110, 13288 Marseille, France
| | - Richard Auria
- Aix Marseille Université, CNRS, Université de Toulon, IRD, MIO UM 110, 13288 Marseille, France
| | - Sylvain Davidson
- Aix Marseille Université, CNRS, Université de Toulon, IRD, MIO UM 110, 13288 Marseille, France
| | - Laurence Casalot
- Aix Marseille Université, CNRS, Université de Toulon, IRD, MIO UM 110, 13288 Marseille, France
| | - Pierre Christen
- Aix Marseille Université, CNRS, Université de Toulon, IRD, MIO UM 110, 13288 Marseille, France
| | - Pierre-Pol Liebgott
- Aix Marseille Université, CNRS, Université de Toulon, IRD, MIO UM 110, 13288 Marseille, France
| | - Yannick Combet-Blanc
- Aix Marseille Université, CNRS, Université de Toulon, IRD, MIO UM 110, 13288 Marseille, France
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Auria R, Boileau C, Davidson S, Casalot L, Christen P, Liebgott PP, Combet-Blanc Y. Hydrogen production by the hyperthermophilic bacterium Thermotoga maritima Part II: modeling and experimental approaches for hydrogen production. BIOTECHNOLOGY FOR BIOFUELS 2016; 9:268. [PMID: 28018485 PMCID: PMC5168804 DOI: 10.1186/s13068-016-0681-0] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/08/2016] [Accepted: 12/02/2016] [Indexed: 06/06/2023]
Abstract
BACKGROUND Thermotoga maritima is a hyperthermophilic bacterium known to produce hydrogen from a large variety of substrates. The aim of the present study is to propose a mathematical model incorporating kinetics of growth, consumption of substrates, product formations, and inhibition by hydrogen in order to predict hydrogen production depending on defined culture conditions. RESULTS Our mathematical model, incorporating data concerning growth, substrates, and products, was developed to predict hydrogen production from batch fermentations of the hyperthermophilic bacterium, T. maritima. It includes the inhibition by hydrogen and the liquid-to-gas mass transfer of H2, CO2, and H2S. Most kinetic parameters of the model were obtained from batch experiments without any fitting. The mathematical model is adequate for glucose, yeast extract, and thiosulfate concentrations ranging from 2.5 to 20 mmol/L, 0.2-0.5 g/L, or 0.01-0.06 mmol/L, respectively, corresponding to one of these compounds being the growth-limiting factor of T. maritima. When glucose, yeast extract, and thiosulfate concentrations are all higher than these ranges, the model overestimates all the variables. In the window of the model validity, predictions of the model show that the combination of both variables (increase in limiting factor concentration and in inlet gas stream) leads up to a twofold increase of the maximum H2-specific productivity with the lowest inhibition. CONCLUSIONS A mathematical model predicting H2 production in T. maritima was successfully designed and confirmed in this study. However, it shows the limit of validity of such mathematical models. Their limit of applicability must take into account the range of validity in which the parameters were established.
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Affiliation(s)
- Richard Auria
- Aix Marseille Université, Université de Toulon, CNRS, IRD, MIO UM 110, Mediterranean Institute of Oceanography, 13288 Marseille, France
| | - Céline Boileau
- Aix Marseille Université, Université de Toulon, CNRS, IRD, MIO UM 110, Mediterranean Institute of Oceanography, 13288 Marseille, France
| | - Sylvain Davidson
- Aix Marseille Université, Université de Toulon, CNRS, IRD, MIO UM 110, Mediterranean Institute of Oceanography, 13288 Marseille, France
| | - Laurence Casalot
- Aix Marseille Université, Université de Toulon, CNRS, IRD, MIO UM 110, Mediterranean Institute of Oceanography, 13288 Marseille, France
| | - Pierre Christen
- Aix Marseille Université, Université de Toulon, CNRS, IRD, MIO UM 110, Mediterranean Institute of Oceanography, 13288 Marseille, France
| | - Pierre Pol Liebgott
- Aix Marseille Université, Université de Toulon, CNRS, IRD, MIO UM 110, Mediterranean Institute of Oceanography, 13288 Marseille, France
| | - Yannick Combet-Blanc
- Aix Marseille Université, Université de Toulon, CNRS, IRD, MIO UM 110, Mediterranean Institute of Oceanography, 13288 Marseille, France
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Orgill JJ, Chen C, Schirmer CR, Anderson JL, Lewis RS. Prediction of methyl viologen redox states for biological applications. Biochem Eng J 2015. [DOI: 10.1016/j.bej.2014.11.005] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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Xue J, Isern NG, Ewing RJ, Liyu AV, Sears JA, Knapp H, Iversen J, Sisk DR, Ahring BK, Majors PD. New generation NMR bioreactor coupled with high-resolution NMR spectroscopy leads to novel discoveries in Moorella thermoacetica metabolic profiles. Appl Microbiol Biotechnol 2014; 98:8367-75. [PMID: 24946863 DOI: 10.1007/s00253-014-5847-8] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2014] [Revised: 05/13/2014] [Accepted: 05/22/2014] [Indexed: 11/26/2022]
Abstract
An in situ nuclear magnetic resonance (NMR) bioreactor was developed and employed to monitor microbial metabolism under batch growth conditions in real time. We selected Moorella thermoacetica ATCC 49707 as a test case. M. thermoacetica (formerly Clostridium thermoaceticum) is a strictly anaerobic, thermophilic, acetogenic, gram-positive bacterium with potential for industrial production of chemicals. The metabolic profiles of M. thermoacetica were characterized during growth in batch mode on xylose (a component of lignocellulosic biomass) using the new generation NMR bioreactor in combination with high-resolution NMR (HR-NMR) spectroscopy. In situ NMR measurements were performed using water-suppressed H-1 NMR spectroscopy at 500 MHz, and aliquots of the bioreactor contents were taken for 600-MHz HR-NMR spectroscopy at specific intervals to confirm metabolite identifications and expand metabolite coverage. M. thermoacetica demonstrated the metabolic potential to produce formate, ethanol, and methanol from xylose, in addition to its known capability of producing acetic acid. Real-time monitoring of bioreactor conditions showed a temporary pH decrease, with a concomitant increase in formic acid during exponential growth. Fermentation experiments performed outside of the magnet showed that the strong magnetic field employed for NMR detection did not significantly affect cell metabolism. Use of the in situ NMR bioreactor facilitated monitoring of the fermentation process, enabling identification of intermediate and endpoint metabolites and their correlation with pH and biomass produced during culture growth. Real-time monitoring of culture metabolism using the NMR bioreactor in combination with HR-NMR spectroscopy will allow optimization of the metabolism of microorganisms producing valuable bioproducts.
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Affiliation(s)
- Junfeng Xue
- Bioproducts, Sciences and Engineering Laboratory, Washington State University Tri-Cities, 2710 Crimson Way, Richland, WA, 99354-1671, USA
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Members of the Order Thermotogales: From Microbiology to Hydrogen Production. MICROBIAL BIOENERGY: HYDROGEN PRODUCTION 2014. [DOI: 10.1007/978-94-017-8554-9_9] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
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Higgins SR, Lopez RJ, Pagaling E, Yan T, Cooney MJ. Towards a hybrid anaerobic digester-microbial fuel cell integrated energy recovery system: an overview of the development of an electrogenic biofilm. Enzyme Microb Technol 2013; 52:344-51. [PMID: 23608503 DOI: 10.1016/j.enzmictec.2013.02.017] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2012] [Revised: 02/26/2013] [Accepted: 02/26/2013] [Indexed: 10/27/2022]
Abstract
An electrogenic biofilm was developed on a macroporous chitosan-carbon nanotube (CHIT-CNT) electrode under constant poised potential (-0.25V versus Ag/AgCl reference electrode) and flow through conditions utilizing the effluent of an anaerobic digester as both the inoculant and substrate for the electrogenic biofilm. After 125 days of inoculation the bioelectrode demonstrated an open circuit potential of -0.62V and a current density of 9.43μAcm(-3) (at -0.25V). Scanning electron microscopy images indicate thorough surface coverage of the biofilm with a high density of bacterial nanowires physically connecting bacteria to bacteria and bacteria to carbon nanotube (electrode surface) suggesting the nanowires are electrically conductive. DGGE was used to identify the major bacterial and archaeal populations.
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Affiliation(s)
- Scott R Higgins
- 1680 East West Rd., POST 109, Hawaii Natural Energy Institute, University of Hawaii, Honolulu HI 96822, United States.
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Carere CR, Rydzak T, Verbeke TJ, Cicek N, Levin DB, Sparling R. Linking genome content to biofuel production yields: a meta-analysis of major catabolic pathways among select H2 and ethanol-producing bacteria. BMC Microbiol 2012; 12:295. [PMID: 23249097 PMCID: PMC3561251 DOI: 10.1186/1471-2180-12-295] [Citation(s) in RCA: 55] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2012] [Accepted: 12/12/2012] [Indexed: 12/16/2022] Open
Abstract
Background Fermentative bacteria offer the potential to convert lignocellulosic waste-streams into biofuels such as hydrogen (H2) and ethanol. Current fermentative H2 and ethanol yields, however, are below theoretical maxima, vary greatly among organisms, and depend on the extent of metabolic pathways utilized. For fermentative H2 and/or ethanol production to become practical, biofuel yields must be increased. We performed a comparative meta-analysis of (i) reported end-product yields, and (ii) genes encoding pyruvate metabolism and end-product synthesis pathways to identify suitable biomarkers for screening a microorganism’s potential of H2 and/or ethanol production, and to identify targets for metabolic engineering to improve biofuel yields. Our interest in H2 and/or ethanol optimization restricted our meta-analysis to organisms with sequenced genomes and limited branched end-product pathways. These included members of the Firmicutes, Euryarchaeota, and Thermotogae. Results Bioinformatic analysis revealed that the absence of genes encoding acetaldehyde dehydrogenase and bifunctional acetaldehyde/alcohol dehydrogenase (AdhE) in Caldicellulosiruptor, Thermococcus, Pyrococcus, and Thermotoga species coincide with high H2 yields and low ethanol production. Organisms containing genes (or activities) for both ethanol and H2 synthesis pathways (i.e. Caldanaerobacter subterraneus subsp. tengcongensis, Ethanoligenens harbinense, and Clostridium species) had relatively uniform mixed product patterns. The absence of hydrogenases in Geobacillus and Bacillus species did not confer high ethanol production, but rather high lactate production. Only Thermoanaerobacter pseudethanolicus produced relatively high ethanol and low H2 yields. This may be attributed to the presence of genes encoding proteins that promote NADH production. Lactate dehydrogenase and pyruvate:formate lyase are not conducive for ethanol and/or H2 production. While the type(s) of encoded hydrogenases appear to have little impact on H2 production in organisms that do not encode ethanol producing pathways, they do influence reduced end-product yields in those that do. Conclusions Here we show that composition of genes encoding pathways involved in pyruvate catabolism and end-product synthesis pathways can be used to approximate potential end-product distribution patterns. We have identified a number of genetic biomarkers for streamlining ethanol and H2 producing capabilities. By linking genome content, reaction thermodynamics, and end-product yields, we offer potential targets for optimization of either ethanol or H2 yields through metabolic engineering.
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Affiliation(s)
- Carlo R Carere
- Department of Biosystems Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6
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Novel [NiFe]- and [FeFe]-hydrogenase gene transcripts indicative of active facultative aerobes and obligate anaerobes in earthworm gut contents. Appl Environ Microbiol 2011; 77:5842-50. [PMID: 21784904 DOI: 10.1128/aem.05432-11] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The concomitant occurrence of molecular hydrogen (H(2)) and organic acids along the alimentary canal of the earthworm is indicative of ongoing fermentation during gut passage. Fermentative H(2) production is catalyzed by [FeFe]-hydrogenases and group 4 [NiFe]-hydrogenases in obligate anaerobes (e.g., Clostridiales) and facultative aerobes (e.g., Enterobacteriaceae), respectively, functional groups that might respond differently to contrasting redox conditions. Thus, the objectives of this study were to assess the redox potentials of the alimentary canal of Lumbricus terrestris and analyze the hydrogenase transcript diversities of H(2) producers in glucose-supplemented gut content microcosms. Although redox potentials in the core of the alimentary canal were variable on an individual worm basis, average redox potentials were similar. The lowest redox potentials occurred in the foregut and midgut regions, averaging 40 and 110 mV, respectively. Correlation plots between hydrogenase amino acid sequences and 16S rRNA gene sequences indicated that closely related hydrogenases belonged to closely related taxa, whereas distantly related hydrogenases did not necessarily belong to distantly related taxa. Of 178 [FeFe]-hydrogenase gene transcripts, 177 clustered in 12 Clostridiales-affiliated operational taxonomic units, the majority of which were indicative of heretofore unknown hydrogenases. Of 86 group 4 [NiFe]-hydrogenase gene transcripts, 79% and 21% were affiliated with organisms in the Enterobacteriaceae and Aeromonadaceae, respectively. The collective results (i) suggest that fermenters must cope with variable and moderately oxidative redox conditions along the alimentary canal, (ii) demonstrate that heretofore undetected hydrogenases are present in the earthworm gut, and (iii) corroborate previous findings implicating Clostridiaceae and Enterobacteriaceae as active fermentative taxa in earthworm gut content.
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