Enriching distinctive microbial communities from marine sediments via an electrochemical-sulfide-oxidizing process on carbon electrodes

Energy Harvesting Untethered Soft Electronic Devices

Advances in wearable and stretchable electronic technologies have yielded a wide range of electronic devices that can be conformably worn by, or implanted in humans to measure physiological signals. Moreover, various cutting-edge technologies for battery-free electronic devices have led to advances in healthcare devices that can continuously measure long-term biosignals for advanced human-machine interface and clinical diagnostics. This report presents the recent progress in battery-less, wearable devices using a wide range of energy harvesting sources, such as electromagnetic energy, mechanical energy, and biofuels. Additionally, this report also discusses the principles and working mechanisms of near/far-field communications, triboelectric, thermoelectric, and biofuel technologies.

Wiring Up Along Electrodes for Biofilm Formation

Millimeter-length cables of bacteria were discovered growing along a graphite-rod electrode serving as an anode of a microbial electrolysis cell (MEC). The MEC had been inoculated with a culture of Fe-reducing microorganisms enriched from a polluted river sediment (Reconquista river, Argentina) and was operated at laboratory controlled conditions for 18 days at an anode poised potential of 240 mV (vs. Ag/AgCl), followed by 23 days at 480 mV (vs. Ag/AgCl). Anode samples were collected for scanning electron microscopy, phylogenetic and electrochemical analyses. The cables were composed of a succession of bacteria covered by a membranous sheath and were distinct from the known "cable-bacteria" (family Desulfobulbaceae). Apparently, the formation of the cables began with the interaction of the cells via nanotubes mostly located at the cell poles. The cables seemed to be further widened by the fusion between them. 16S rRNA gene sequence analysis confirmed the presence of a microbial community composed of six genera, including Shewanella, a well-characterized electrogenic bacteria. The formation of the cables might be a way of colonizing a polarized surface, as determined by the observation of electrodes extracted at different times of MEC operation. Since the cables of bacteria were distinct from any previously described, the results suggest that bacteria capable of forming cables are more diverse in nature than already thought. This diversity might render different electrical properties that could be exploited for various applications.

Responses of electroactive biofilms to chronic chlorine exposure: Insights from the composition and spatial structure of extracellular polymeric substances

Extensive amounts of chlorine disinfectants have been applied to wastewater system since the outbreak of coronavirus disease 2019 (COVID-19), which inevitably affects the pollutant degradation via interfering with electron transfer mediated by electroactive bacteria. Herein, the response of electroactive biofilm (EAB) to chronic chlorine exposure was investigated. Results showed the EAB formed without exposure (EAB-0) exhibited a 53% and 123% higher current output than that formed with 0.1 mg L⁻¹ (EAB-0.1) and 0.5 mg L⁻¹ (EAB-0.5) chlorine, respectively. The chronic chlorine exposure of EAB boosted the contents of extracellular polymeric substances (EPS) in EAB-0.1 and EAB-0.5 by over secretion of extracellular polysaccharides. The EAB-0.1 and EAB-0.5 also presented lower electron exchange capacities (EECs) of EPS, coincided with reduced relative abundance of Geobacter from 61% in EAB-0 to 52% in EAB-0.5. This study provided new insights into the application of engineered EAB for wastewater treatment in a disinfection environment.

Microorganisms in sediment microbial fuel cells: Ecological niche, microbial response, and environmental function

A sediment microbial fuel cell (SMFC) is a device that harvests electrical energy from sediments rich in organic matter. SMFCs have been attracting increasing amounts of interest in environmental remediation, since they are capable of providing a clean and inexhaustible source of electron donors or acceptors and can be easily controlled by adjusting the electrochemical parameters. The microorganisms inhabiting sediments and the overlying water play a pivotal role in SMFCs. Since the SMFC is applied in an open environment rather than in an enclosed chamber, the effects of the environment on the microbes should be intense and the microbial community succession should be extremely complex. Thus, this review aims to provide an overview of the microorganisms in SMFCs, which few previous review papers have reported. In this study, the anodic and cathodic niches for the microorganisms in SMFCs are summarized, how the microbial population and community interact with the SMFC environment is discussed, a new microbial succession strategy called the electrode stimulation succession is proposed, and recent developments in the environmental functions of SMFCs are discussed from the perspective of microorganisms. Future studies are needed to investigate the electrode stimulation succession, the environmental function and the electron transfer mechanism in order to boost the application of SMFCs for power generation and environmental remediation.

Enhanced Hydrocarbons Biodegradation at Deep-Sea Hydrostatic Pressure with Microbial Electrochemical Snorkels

In anaerobic sediments, microbial degradation of petroleum hydrocarbons is limited by the rapid depletion of electron acceptors (e.g., ferric oxide, sulfate) and accumulation of toxic metabolites (e.g., sulfide, following sulfate reduction). Deep-sea sediments are increasingly impacted by oil contamination, and the elevated hydrostatic pressure (HP) they are subjected to represents an additional limitation for microbial metabolism. While the use of electrodes to support electrobioremediation in oil-contaminated sediments has been described, there is no evidence on their applicability for deep-sea sediments. Here, we tested a passive bioelectrochemical system named ”oil-spill snorkel” with two crude oils carrying different alkane contents (4 vs. 15%), at increased or ambient HP (10 vs. 0.1 MPa). Snorkels enhanced alkanes biodegradation at both 10 and 0.1 MPa within only seven weeks, as compared to nonconductive glass controls. Microprofiles in anaerobic, contaminated sediments indicated that snorkels kept sulfide concentration to low titers. Bulk-sediment analysis confirmed that sulfide oxidation by snorkels largely regenerated sulfate. Hence, the sole application of snorkels could eliminate a toxicity factor and replenish a spent electron acceptor at increased HP. Both aspects are crucial for petroleum decontamination of the deep sea, a remote environment featured by low metabolic activity.

Ferric iron stimulation in marine SMFCs: Impact on the microbial structure evolution in contaminated sediments with low and high molecular weight PAHs

The impact of ferric iron stimulation on the evolution of microbial structure in marine sediment microbial fuel cells (SMFCs), operated for the bioremediation of a complex mixture of low and high molecular weight PAHs (naphthalene, fluorene, pyrene and benzo(a)pyrene), was assessed. Microbial evolution profiles showed high relative abundances of exoelectrogenic iron-reducing bacteria throughout the biodegradation, namely Geoalkalibacter, under ferric iron stimulation and anode reducing conditions, irrespective of sulfate reducing bacteria (SRB) inhibition. Highest PAHs removal was measured in the absence of anode reduction, under Fe stimulation and SRB inhibition, reaching 40.85% for benzo(a)pyrene, the most persistent PAH used in this study. Results suggest that amendment of contaminated sediment with ferric iron could constitute a better bioremediation strategy than using SMFCs. This becomes significant when considering the well-established and dominant indigenous SRB population in marine sediments that usually limits the performance of the anode as a terminal electron acceptor in marine SMFCs.

Studies on expression levels of pil Q and fli P genes during bio-electrogenic process in Kluyvera georgiana MCC 3673

The bacterium Kluyvera georgiana MCC 3673 transfers electrons directly to the electrode for bio-electricity generation in microbial fuel cell (MFC). This could be due to the formation of biofilm on the surface of electrode or with through the extracellular appendages, or both. The role of extracellular appendages pili and flagella in exo-electron transfer mechanism was investigated. The expression level of the genes fli P and pil Q for pili and flagella, respectively, in K. georgiana MCC 3673 was compared in MFC and in shake flask. The transcript analysis was done by qRT-PCR at different times and conditions. The expression level of pil Q transcript in K. georgiana MCC 3673 showed over twofold higher expression during bio-electrogenic process, compared to the one inoculated in shake flask. Similarly, fli P had also showed similar kind of expression in MFC compared to that in shake flask. Higher level of pil Q and fli P transcripts were observed throughout bio-electrogenic process. The level of pil Q was found to be nearly fourfold higher in biofilm-forming cells forming compared to the cells in suspension form. The obtained results suggest that flagella have a role in movement of bacterium towards electrode for donating the electron in absence of oxygen, and pili aiding in adhering on the surface of electrode and forming biofilm. The cumulative effect of fli P and pil Q resulted in exo-electron transfer to the electrode and bio-electricity generation process. The open circuit potential (OCV) of + 0.7 V was produced with the maximum power density of 393 ± 14 mW/m² in MFC.

Variation in electrode redox potential selects for different microorganisms under cathodic current flow from electrodes in marine sediments

Extracellular electron transport (EET) is a microbial process that allows microorganisms to transport electrons to and from insoluble substrates outside of the cell. Although progress has been made in understanding how microbes transfer electrons to insoluble substrates, the process of receiving electrons has largely remained unexplored. We investigated redox potentials favourable for donating electrons to dissolved and insoluble components in Catalina Harbor marine sediment by combining electrochemical techniques with geochemistry and molecular methods. Working electrodes buried in sediment microcosms were poised at seven redox potentials between -300 and -750 mV versus Ag/AgCl using a three-electrode system. In electrode biofilms recovered after 2-month incubations, overall community diversity increased with more negative redox potentials. Abundances of known EET-capable groups (e.g., Alteromonadales and Desulfuromonadales) varied with redox potential. Motility and chemotaxis genes were found in greater abundance in electrode communities, suggesting a possible selective advantage of these pathways for colonization and utilization of the electrode. Our enrichments demonstrated the validity of this approach in capturing groups known, as well as novel groups (e.g., Campylobacterales) that perform EET. The diverse nature of the enriched cathode communities suggest that insoluble substrate oxidation may be a critical, although poorly described microbial metabolic process in marine sediment.

Using metabolic charge production in the tricarboxylic acid cycle (QTCA) to evaluate the extracellular-electron-transfer performances of Shewanella spp

Using an electrochemical cell equipped with carbon felt electrodes (poised at +0.63 V vs. SHE), the current production capabilities of two Shewanella strains-NTOU1 and KR-12-were examined under various conditions with lactate as an electron donor. The metabolic charge produced in the tricarboxylic acid cycle (Q_TCA) was calculated by mass-balance. The data showed a linear relation between the electric coulomb production (Q_EL) and Q_TCA with an R² of 0.65. In addition, a large amount of pyruvate accumulation was observed at pH = 6, rendering Q_TCA negative. The results indicate an occurrence of an undesired cataplerotic reaction. It was also found that Q_TCA provides important information showing the oxygen-boosting TCA cycle and anodic-current generation of Shewanella spp. Linear dependence of the change in charge for biomass growth (4.52FΔn_Cell) on Q_TCA was also found as expressed by 4.52FΔn_Cell = 1.0428 Q_TCA + 0.0442, indicating that these two charge quantities are inherently identical under most of the experimental conditions. In the mediator-spiked experiments, the external addition of the mediators (ferricyanide, anthraquinone-2, 6-disulfonate, and riboflavin) beyond certain concentrations inhibited the activity of the TCA cycle, indicating that the oxidative phosphorylation is deactivated by excessive amounts of mediators, yet Shewanella spp. are constrained with regard to carrying out the substrate-level phosphorylation.

Assessing Marine Microbial Induced Corrosion at Santa Catalina Island, California

High iron and eutrophic conditions are reported as environmental factors leading to accelerated low-water corrosion, an enhanced form of near-shore microbial induced corrosion. To explore this hypothesis, we deployed flow-through colonization systems in laboratory-based aquarium tanks under a continuous flow of surface seawater from Santa Catalina Island, CA, USA, for periods of 2 and 6 months. Substrates consisted of mild steel – a major constituent of maritime infrastructure – and the naturally occurring iron sulfide mineral pyrite. Four conditions were tested: free-venting “high-flux” conditions; a “stagnant” condition; an “active” flow-through condition with seawater slowly pumped over the substrates; and an “enrichment” condition where the slow pumping of seawater was supplemented with nutrient rich medium. Electron microscopy analyses of the 2-month high flux incubations document coating of substrates with “twisted stalks,” resembling iron oxyhydroxide bioprecipitates made by marine neutrophilic Fe-oxidizing bacteria (FeOB). Six-month incubations exhibit increased biofilm and substrate corrosion in the active flow and nutrient enriched conditions relative to the stagnant condition. A scarcity of twisted stalks was observed for all 6 month slow-flow conditions compared to the high-flux condition, which may be attributable to oxygen concentrations in the slow-flux conditions being prohibitively low for sustained growth of stalk-producing bacteria. All substrates developed microbial communities reflective of the original seawater input, as based on 16S rRNA gene sequencing. Deltaproteobacteria sequences increased in relative abundance in the active flow and nutrient enrichment conditions, whereas Gammaproteobacteria sequences were relatively more abundant in the stagnant condition. These results indicate that (i) high-flux incubations with higher oxygen availability favor the development of biofilms with twisted stalks resembling those of marine neutrophilic FeOB and (ii) long-term nutrient stimulation results in substrate corrosion and biofilms with different bacterial community composition and structure relative to stagnant and non-nutritionally enhanced incubations. Similar microbial succession scenarios, involving increases in nutritional input leading to the proliferation of anaerobic iron and sulfur-cycling guilds, may occur at the nearby Port of Los Angeles and cause potential damage to maritime port infrastructure.