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Extracellular electron transfer (EET) by electroactive bacteria in anoxic soils and sediments is an intensively researched subject, but EET's function in planktonic ecology has been less considered. Following the discovery of an unexpectedly high prevalence of EET genes in a bog lake's bacterioplankton, we hypothesized that the redox capacities of dissolved organic matter (DOM) enrich for electroactive bacteria by mediating redox chemistry. We developed the bioinformatics pipeline FEET (Find EET) to identify and summarize predicted EET protein‐encoding genes from metagenomics data. We then applied FEET to 36 bog and thermokarst lakes and correlated gene occurrence with environmental data to test our predictions. Our results provide indirect evidence that DOM may participate in bacterioplankton EET. We found a similarly high prevalence of genes encoding putative EET proteins in most of these lakes, where oxidative EET strongly correlated with DOM. Numerous novel clusters of multiheme cytochromes that may enable EET were identified. Taxa previously not considered EET‐capable were found to carry EET genes. We propose that EET and DOM interactions are of ecologically important to bacterioplankton in small boreal lakes, and that EET, particularly by methylotrophs and anoxygenic phototrophs, should be further studied and incorporated into methane emission models of melting permafrost. [ABSTRACT FROM AUTHOR]

The Mtr respiratory pathway of Shewanella oneidensis strain MR-1 is required to effectively respire both soluble and insoluble forms of oxidized iron. Flavins (riboflavin and flavin mononucleotide) recently have been shown to be excreted by MR-1 and facilitate the reduction of insoluble substrates. Other Shewanella species tested accumulated flavins in supernatants to an extent similar to that of MR-1, suggesting that flavin secretion is a general trait of the species. External flavins have been proposed to act as both a soluble electron shuttle and a metal chelator;, however, at biologically relevant concentrations, our results suggest that external flavins primarily act as electron shuttles for MR-1. Using deletion mutants lacking various Mtr-associated proteins, we demonstrate that the Mtr extracelinlar respiratory pathway is essential for the reduction of flavins and that decaheme cytochromes found on the outer surface of the cell (MtrC and OmcA) are required for the majority of this activity. Given the involvement of external flavins in the reduction of electrodes, we monitored current production by Mtr respiratory pathway mutants in three-electrode bioreactors under controlled flavin concentrations. While mutants lacking MtrC were able to reduce flavins at 50% of the rate of the wild type in cell suspension assays, these strains were unable to grow into productive electrode-reducing biofilms. The analysis of mutants lacking OmcA suggests a role for this protein in both electron transfer to electrodes and attachment to surfaces. The parallel phenotypes of Mtr mutants in flavin and electrode reduction blur the distinction between direct contact and the redox shuttling strategies of insoluble substrate reduction by MR-1. doi: 10.1128/JB.00925-09

Electron transfer from cells to metals and electrodes by the Fe(III)-reducing anaerobe Geobacter sulfurreducens requires proper expression of redox proteins and attachment mechanisms to interface bacteria with surfaces and neighboring cells. We hypothesized that transposon mutagenesis would complement targeted knockout studies in Geobacter spp. and identify novel genes involved in this process. Escherichia coli mating strains and plasmids were used to develop a conjugation protocol and deliver mini-Himar transposons, creating a library of over 8,000 mutants that was anaerobically arrayed and screened for a range of phenotypes, including auxotrophy for amino acids, inability to reduce Fe(III) citrate, and attachment to surfaces. Following protocol validation, mutants with strong phenotypes were further characterized in a three-electrode system to simultaneously quantify attachment, biofilm development, and respiratory parameters, revealing mutants defective in Fe(III) reduction but unaffected in electron transfer to electrodes (such as an insertion in GSU1330, a putative metal export protein) or defective in electrode reduction but demonstrating wild-type biofilm formation (due to an insertion upstream of the NHL domain protein GSU2505). An insertion in a putative ATP-dependent transporter (GSU1501) eliminated electrode colonization but not Fe(III) citrate reduction. A more complex phenotype was demonstrated by a mutant containing an insertion in a transglutaminase domain protein (GSU3361), which suddenly ceased to respire when biofilms reached approximately 50% of the wild-type levels. As most insertions were not in cytochromes but rather in transporters, two-component signaling proteins, and proteins of unknown function, this collection illustrates how biofilm formation and electron transfer are separate but complementary phenotypes, controlled by multiple loci not commonly studied in Geobacter spp.

The mechanism of fumarate reduction in Geobacter sulfurreducens was investigated. The genome contained genes encoding a heterotrimeric fumarate reductase, FrdCAB, with homology to the fumarate reductase of Wolinella succinogenes and the succinate dehydrogenase of Bacillus subtilis. Mutation of the putative catalytic subunit of the enzyme resulted in a strain that lacked fumarate reductase activity and was unable to grow with fumarate as the terminal electron acceptor. The mutant strain also lacked succinate dehydrogenase activity and did not grow with acetate as the electron donor and Fe(III) as the electron acceptor. The mutant strain could grow with acetate as the electron donor and Fe(III) as the electron acceptor if fumarate was provided to alleviate the need for succinate dehydrogenase activity in the tricarboxylic acid cycle. The growth rate of the mutant strain under these conditions was faster and the cell yields were higher than for wild type grown under conditions requiring succinate dehydrogenase activity, suggesting that the succinate dehydrogenase reaction consumes energy. An orthologous frdCAB operon was present in Geobacter metallireducens, which cannot grow with fumarate as the terminal electron acceptor. When a putative dicarboxylic acid transporter from G. sulfurreducens was expressed in G. metallireducens, growth with fumarate as the sole electron acceptor was possible. These results demonstrate that, unlike previously described organisms, G. sulfurreducens and possibly G. metallireducens use the same enzyme for both fumarate reduction and succinate oxidation in vivo.

A report of complete oxidation of organic compounds linked to electrode reduction by an isolate outside of the Proteobacteria is presented. The finding that Geothrix fermentans can oxidize acetate and other substrates with an electrode serving as the role electron acceptor suggests that the enrichment of organisms closely related to G. fermentans on energy-harvesting electrodes in freshwater sediment might be due to these organisms gaining an energetic advantage by growth on or near the electrode.

Electrical energy can be generated from marine sediments when a graphite electrode emplaced in marine sediment is connected to a cathode using an electrical circuit and there is a specific enrichment of delta-proteobacteria on the anode surface. Studies were conducted in order to examine how desulfobulbaceae interacts with anodes in sediments.

Research on Butyrivibrio fibrisolvens show that the organism did not produce significant amounts of conjugated linoleic acid (CLA) until the concentration of linoleic acid was high enough to inhibit biohydrogenation, inhibit growth, and cause cell lysis. This could indicate that CLA production is not a normal end product but an artifact in cells that cannot biohydrogenate.

The rate of H+ influx into Streptococcus bovis is about 20 times higher than has ever been recorded in lactic acid bacteria. This bacterium still ferments sugars in its non-growing state at a rate that exceeds its actual needs.

Proton motive force ((delta)p) and the energy spilling rate are apparently responsive to fluctuations in the intracellular inorganic phosphate (P(sub i)) concentration. This was concluded from the results of experiments designed to determine the effect of glucose availability and amino nitrogen on intracellular fructose-1,6-diphosphate (FDP) and phosphate concentrations, ATP hydrolysis and (delta)p. FDP increased, intracellular P(sub i) decreased and energy spilling rate increased when the rate of glucose addition to S. bovis cell suspensions was increased.

Fructose 1,6-diphosphate (FDP) from cells of Streptococcus bovis activate lactate dehydrogenase that leads to energy spilling. FDP activates membrane-bound ATPase of S. bovis and produces a 5-fold decrease in the K(sub m) for ATP. Energy-spilling occurs only when the production of lactate is high. The utilization of ammonia helps in lactate production and ATP-spilling since amino acid elimination, leaving ammonia as the sole nitrogen source, leads to lactate dehydrogenase production.

Shewanella oneidensis is a dissimilatory metal reducing bacterium and model for extracellular electron transfer (EET), a respiratory mechanism in which electrons are transferred out of the cell. In the last 10 years, migration to insoluble electron acceptors for EET has been shown to be nonrandom and tactic, seemingly in the absence of molecular or energy gradients that typically allow for taxis. As the ability to sense, locate, and respire electrodes has applications in bioelectrochemical technology, a better understanding of taxis in S. oneidensis is needed. While the EET conduits of S. oneidensis have been studied extensively, its taxis pathways and their interplay with EET are not yet understood, making investigation into taxis phenomena nontrivial. Since S. oneidensis is a member of an EET‐encoding clade, the genetic circuitry of taxis to insoluble acceptors may be conserved. We performed a bioinformatic analysis of Shewanella genomes to identify S. oneidensis chemotaxis orthologs conserved in the genus. In addition to the previously reported core chemotaxis gene cluster, we identify several other conserved proteins in the taxis signaling pathway. We present the current evidence for the two proposed models of EET taxis, "electrokinesis" and flavin‐mediated taxis, and highlight key areas in need of further investigation. [ABSTRACT FROM AUTHOR]

The functional expression of proteorhodopsin and light-induced changes in membrane potential in the bacterium Shewanella oneidensis strain MR-1 which showed significant increases in electrical current generation during illumination of electrochemical chambers is described. The ability of expression of proteorhodopsin to preserve the viability of the bacterium under nutrient-limited conditions provide evidence for the hypothesis that fulfillment of basic energy needs of organisms that might explain the widespread distribution of proteorhodopsin in marine environments.

Stirred three-electrode anaerobic bioreactors controlled by potentiostats and nondestructive voltammetry techniques are used for describing the growth and characterization of thin viable biofilms of Geobacter sulfurreducens. The technique has provided a basis to catalog quantifiable, defined electron transfer phenotypes as a function of potential, electrode material, growth phase, and culture conditions and also a framework to compare with other species.