Single medium microbial fuel cell: Stainless steel and graphite electrode materials select bacterial communities resulting in opposite electrocatalytic activities

A graphite electrode and a stainless steel electrode immersed in exactly the same medium and polarised at the same potential were colonised by different microbial biofilms. This difference in electroactive microbial population leads stainless steel and graphite to become a microbial cathode and a microbial anode respectively. The results demonstrated that the electrode material can drive the electrocatalytic property of the biofilm opening perspectives for designing single medium MFC. This new discovery led to of the first demonstration of a “single medium MFC.” Such a single medium MFC designed with a graphite anode connected to a stainless steel cathode, both buried in marine sediments, produced 280 mA m−2 at a voltage of 0.3 V for more than 2 weeks

Lowering the applied potential during successive scratching/re-inoculation improves the performance of microbial anodes for microbial fuel cells

Microbial anodes were formed under polarisation at -0.2 V/SCE on smooth graphite plate electrodes with paper mill effluents. Primary, secondary and tertiary biofilms were formed by a successive scratching and re-inoculation procedure. The secondary and tertiary biofilms formed while decreasing the polarisation potential allowed the anodes to provide current density of 6 A/m² at -0.4 V/SCE. In contrast, applying -0.4 V/SCE initially to form the primary biofilms did not lead to the production of current. Consequently, the scratching/re-inoculation procedure combined with progressive lowering of the applied potential revealed an efficient new procedure that gave efficient microbial anodes able to work at low potential. The observed progressive pH drift to alkaline values above 9 explained the open circuit potentials as low as -0.6 V/SCE. The remarkable performance of the electrode at alkaline pH was attributed to the presence of Desulfuromonas acetexigens as the single dominant species in the tertiary microbial anodes

P2CS:Updates of the prokaryotic two-component systems database

International audienceThe P2CS database (http://www.p2cs.org/) is a comprehensive resource for the analysis of Prokaryotic Two-Component Systems (TCSs). TCSs are comprised of a receptor histidine kinase (HK) and a partner response regulator (RR) and control important prokaryotic behaviors. The latest incarnation of P2CS includes 164 651 TCS proteins, from 2758 sequenced prokaryotic genomes. Several important new features have been added to P2CS since it was last described. Users can search P2CS via BLAST, adding hits to their cart, and ho-mologous proteins can be aligned using MUSCLE and viewed using Jalview within P2CS. P2CS also provides phylogenetic trees based on the conserved signaling domains of the RRs and HKs from entire genomes. HK and RR trees are annotated with gene organization and domain architecture, providing insights into the evolutionary origin of the contemporary gene set. The majority of TCSs are encoded by adjacent HK and RR genes, however, 'orphan' unpaired TCS genes are also abundant and identifying their partner proteins is challenging. P2CS now provides paired HK and RR trees with proteins from the same genetic locus indicated. This allows the appraisal of evolutionary relationships across entire TCSs and in some cases the identification of candidate partners for orphan TCS proteins

Lowering the applied potential during successive scratching/re-inoculation improves the performance of microbial anodes for microbial fuel cells

Microbial anodes were formed under polarisation at -0.2 V/SCE on smooth graphite plate electrodes with paper mill effluents. Primary, secondary and tertiary biofilms were formed by a successive scratching and re-inoculation procedure. The secondary and tertiary biofilms formed while decreasing the polarisation potential allowed the anodes to provide current density of 6 A/m² at -0.4 V/SCE. In contrast, applying -0.4 V/SCE initially to form the primary biofilms did not lead to the production of current. Consequently, the scratching/re-inoculation procedure combined with progressive lowering of the applied potential revealed an efficient new procedure that gave efficient microbial anodes able to work at low potential. The observed progressive pH drift to alkaline values above 9 explained the open circuit potentials as low as -0.6 V/SCE. The remarkable performance of the electrode at alkaline pH was attributed to the presence of Desulfuromonas acetexigens as the single dominant species in the tertiary microbial anodes

Garden compost inoculum leads to microbial bioanodes with potential-independent characteristics

International audienceGarden compost leachate was used to form microbial bioanodes under polarization at 0.4, 0.2 and +0.1 V/SCE. Current densities were 6.3 and 8.9 A m2 on average at 0.4 and +0.1 V/SCE respectively, with acetate 10 mM. The catalytic cyclic voltammetry (CV) showed similar electrochemical characteristics for all bioanodes and indicated that the lower currents recorded at 0.4 V/SCE were due to the slower interfacial electron transfer rate at this potential, consistently with conventional electrochemical kinetics.RNA- and DNA-based DGGE evidenced that the three dominant bacterial groups Geobacter, Anaerophaga and Pelobacter were identical for all bioanodes and did not depend on the polarization potential. Only non-turnover CVs showed differences in the redox equipment of the biofilms, the highest potential promoting multiple electron transfer pathways. This first description of a potential-independent electroactive microbial community opens up promising prospects for the design of stable bioanodes for microbial fuel cells

Suppression of crown and root rot of wheat by the rhizobacterium Paenibacillus polymyxa

A seedling bioassay was developed for screening a wheat root-associated rhizobacterial strain of Paenibacillus polymyxa for ability to suppress crown and root rot pathogens of wheat. The primary aim was to evaluate the ability of P. polymyxa to suppress Fusarium graminearum, F. culmorum, F. verticillioides and Microdochium nivale, the fungal pathogens responsible for Fusarium crown and root rot and head blight of wheat in Algeria. Bioassays conducted under controlled conditions indicated that seed treatments with P. polymyxa strain SGK2 significantly reduced disease symptoms caused by all four fungal pathogens. Plant growth promotion (increased shoot and root dry weights), however, depended on the pathogen tested. Our results indicate that seed treatments with a biocontrol agent could be an additional strategy for management of wheat crown and root rot pathogens

Forming microbial anodes with acetate addition decreases their capability to treat raw paper mill effluent

Microbial anodes were formed under polarization at −0.3 V/SCE on graphite plates in effluents from a pulp and paper mill. The bioanodes formed with the addition of acetate led to the highest current densities (up to 6 A/m2) but were then unable to oxidize the raw effluent efficiently (0.5 A/m2). In contrast, the bioanodes formed without acetate addition were fully able to oxidize the organic matter contained in the effluent, giving up to 4.5 A/m2 in continuous mode. Bacterial communities showed less bacterial diversity for the acetate-fed bioanodes compared to those formed in raw effluents. Deltaproteobacteria were the most abundant taxonomic group, with a high diversity for bioanodes formed without acetate addition but with almost 100% Desulfuromonas for the acetate-fed bioanodes. The addition of acetate to form the microbial anodes induced microbial selection, which was detrimental to the treatment of the raw effluent

Modulation of Metabolism and Switching to Biofilm Prevail over Exopolysaccharide Production in the Response of Rhizobium alamii to Cadmium

Heavy metals such as cadmium (Cd2+) affect microbial metabolic processes. Consequently, bacteria adapt by adjusting their cellular machinery. We have investigated the dose-dependent growth effects of Cd2+ on Rhizobium alamii, an exopolysaccharide (EPS)-producing bacterium that forms a biofilm on plant roots. Adsorption isotherms show that the EPS of R. alamii binds cadmium in competition with calcium. A metabonomics approach based on ion cyclotron resonance Fourier transform mass spectrometry has showed that cadmium alters mainly the bacterial metabolism in pathways implying sugars, purine, phosphate, calcium signalling and cell respiration. We determined the influence of EPS on the bacterium response to cadmium, using a mutant of R. alamii impaired in EPS production (MSΔGT). Cadmium dose-dependent effects on the bacterial growth were not significantly different between the R. alamii wild type (wt) and MSΔGT strains. Although cadmium did not modify the quantity of EPS isolated from R. alamii, it triggered the formation of biofilm vs planktonic cells, both by R. alamii wt and by MSΔGT. Thus, it appears that cadmium toxicity could be managed by switching to a biofilm way of life, rather than producing EPS. We conclude that modulations of the bacterial metabolism and switching to biofilms prevails in the adaptation of R. alamii to cadmium. These results are original with regard to the conventional role attributed to EPS in a biofilm matrix, and the bacterial response to cadmium

Evolutionary history expands the range of signaling interactions in hybrid multikinase networks

Two-component systems (TCSs) are ubiquitous signaling pathways, typically comprising a sensory histidine kinase (HK) and a response regulator, which communicate via intermolecular kinase-to-receiver domain phosphotransfer. Hybrid HKs constitute non-canonical TCS signaling pathways, with transmitter and receiver domains within a single protein communicating via intramolecular phosphotransfer. Here, we report how evolutionary relationships between hybrid HKs can be used as predictors of potential intermolecular and intramolecular interactions (‘phylogenetic promiscuity’). We used domain-swap genes chimeras to investigate the specificity of phosphotransfer within hybrid HKs of the GacS–GacA multikinase network of Pseudomonas brassicacearum. The receiver domain of GacS was replaced with those from nine donor hybrid HKs. Three chimeras with receivers from other hybrid HKs demonstrated correct functioning through complementation of a gacS mutant, which was dependent on strains having a functional gacA. Formation of functional chimeras was predictable on the basis of evolutionary heritage, and raises the possibility that HKs sharing a common ancestor with GacS might remain components of the contemporary GacS network. The results also demonstrate that understanding the evolutionary heritage of signaling domains in sophisticated networks allows their rational rewiring by simple domain transplantation, with implications for the creation of designer networks and inference of functional interactions

Diversifying Anaerobic Respiration Strategies to Compete in the Rhizosphere

The rhizosphere is the interface between plant roots and soil where intense, varied interactions between plants and microbes influence plants' health and growth through their influence on biochemical cycles, such as the carbon, nitrogen, and iron cycles. The rhizosphere is also a changing environment where oxygen can be rapidly limited and anaerobic zones can be established. Microorganisms successfully colonize the rhizosphere when they possess specific traits referred to as rhizosphere competence. Anaerobic respiration flexibility contributes to the rhizosphere competence of microbes. Indeed, a wide range of compounds that are available in the rhizosphere can serve as alternative terminal electron acceptors during anaerobic respiration such as nitrates, iron, carbon compounds, sulfur, metalloids, and radionuclides. In the presence of multiple terminal electron acceptors in a complex environment such as the rhizosphere and in the absence of O2, microorganisms will first use the most energetic option to sustain growth. Anaerobic respiration has been deeply studied, and the genes involved in anaerobic respiration have been identified. However, aqueous environment and paddy soils are the most studied environments for anaerobic respiration, even if we provide evidence in this review that anaerobic respiration also occurs in the plant rhizosphere. Indeed, we provide evidence by performing a BLAST analysis on metatranscriptomic data that genes involved in iron, sulfur, arsenate and selenate anaerobic respiration are expressed in the rhizosphere, underscoring that the rhizosphere environment is suitable for the establishment of anaerobic respiration. We thus focus this review on current research concerning the different types of anaerobic respiration that occur in the rhizosphere. We also discuss the flexibility of anaerobic respiration as a fundamental trait for the microbial colonization of roots, environmental and ecological adaptation, persistence and bioremediation in the rhizosphere. Anaerobic respiration appears to be a key process for the functioning of an ecosystem and interactions between plants and microbes