C-source priority and availability limits bidirectional electron transfer in freshwater mixed culture EAB biofilms

Question

1. What is the significance of bidirectional EET in mixed EAB cultures and how was it investigated?

2. What is the duration of the half-batch polarization reversal (HBPR) and periodic polarization reversal (PPR) in the Materials & Methods section?

3. How were COD measurements conducted in the BES study?

4. What factors influenced the maximum current densities in BES during bio lm cultivation?

Accepted Answer

Bidirectional EET in mixed EAB cultures is significant as it allows for both electrogenic and electroautotrophic pathways, enabling combined bioelectrosynthesis and power generation in a single device. This research paved the way for better cultivation and control of EAB for bioelectrosynthesis applications. To investigate bidirectional EET in mixed EAB cultures, the cultures were grown heterotrophically under an applied potential favoring electrogenic activity, followed by switching to a more negative potential in the presence of organic, inorganic carbon, or a mixture thereof. This was done to see if fast switching is possible while maintaining bio lm stability. Additionally, electrochemical and HPLC measurements were conducted to evaluate the adaptation and differentiation of EAB within the experiments. The composition of the microbial community in the BES was also determined to round out the findings.

Accepted Answer

The duration of the half-batch polarization reversal (HBPR) and periodic polarization reversal (PPR) in the Materials & Methods section is as follows: For HBPR, the nominal cathodic polarization was applied for 81 hours, followed by CV with a potential range from -0.5 V to +0.2 V ending at +0.2 V with a scan rate of 1 mV s-1 for 4 cycles. For PPR, the rst CA was followed by a switch in polarization to the other applied potential, i.e., -0.5 to 0.2 or 0.2 to -0.5V. The BES were operated without CV operations in between the CA and periods of anodic and cathodic polarization, with each period being 27.8 hours long. This was repeated for 8 batches, with the concentration of carbon sources decreased by a half for the sixth batch and onwards for the PPR experiments.

Accepted Answer

COD measurements were conducted using cuvette tests (LCK 514 and LCK 314) in a spectral photometer (DR3900) to measure the COD of samples before and after each cycle. The cuvettes and photometer were provided by Hach Lange, Germany. The samples were taken from fresh media, at the end of each batch from each BES, and after half of each HBPR batch. This process allowed for the analysis of changes in media composition in relation to the applied electrochemical conditions.

Accepted Answer

Several factors influenced the maximum current densities in BES during bio lm cultivation. Firstly, the initial average maximum current densities were relatively low, ranging up to 0.25 mA cm-2. However, significant differences were observed between replicates in terms of maximum current densities and turnover duration. The best performance was observed in the last two cycles, with three out of nine BES exceeding a maximum current density of 1.2 mA cm-2. Acetate consumption and diminishing current density uctuations were also observed during the cultivation process. Stationary conditions were reached after 6 batches (42 days), with reproducible anodic current generation and full acetate consumption. The maximum average current density of 0.70 mA cm-2 was achieved after 45 hours in the last two cycles. Additionally, COD removal at the end of the cultivation stage exceeded 80% in the majority of BES, with two BES at about 10% lower. To ensure equal initial conditions for HBPR and PPR stages, the BES were grouped into three sets based on performance. These sets were then used for bidirectional EET experiments and fed with different carbon sources.

Accepted Answer

The carbon source provided to the BES significantly influences their behavior. Reductive current was produced only by the BES fed with carbonate (S1), while no reductive current was observed in the BES fed with acetate and mixed carbon media (S2 & S3). Similarly, no oxidative current was observed in the S1-BES at 0.2 V. These observations were consistent for all the batches in Half-batch polarization reversal (HBPR). The limited amount of alternative terminal electron acceptors and the high ratio of reduction to oxidation rate may have led to an accumulation of reduced molecules, resulting in a local concentration of negative charge. This thermodynamically favored electron ux from the biofilm to the working electrode at low potentials. The S2 and S3-BES showed increased capacity to store charge and immediate reductive or oxidative discharge at low potentials. The equilibrium potentials of S2 and S3-BES were close to -0.5 V, resulting in a sharp increase in current at very low potentials. The S1-BES showed increased reduction currents after being polarized at 0.2 V, approximately 0.1 V more than their equilibrium potential. The formal potentials of the EET components were determined based on the analysis of voltammograms and their respective first derivatives. Most voltammograms had two extrema in the first derivatives or peak systems in the case of CVs from non-turnover states.

Accepted Answer

During the HBPR experiment, continuous polarization switching led to changes in the recorded voltammograms. The potential window for microbial electron transport chains expanded towards more negative values. Redox potentials for S1, S2, and S3 were -434, -461, and -487 mV, respectively. New reduction and oxidation peaks appeared, indicating adjustments in electron transfer strategies. Known redox pairs such as NAD+/NADH and FAD+/FADH2 were identified, along with the activation of the Riboflavin/FMN+/FMNH2 system and cytochromes A and C. Some redox pairs were active in all systems, while others were specific to certain systems.

Accepted Answer

Periodic polarization reversal with the concentration of carbon sources as in HBPR did not affect the outcome, i.e. the tested systems demonstrated similar behavior in terms of current generation. Reductive current in S1-BES was still likely linked to mediated EET. During periodic polarization, the evolution of oxidative current during polarization at -0.5V occurred, and similarly to HBPR distinctive of S3-BES. However, this effect was more evident, with oxidative current densities reaching 0.19 mA cm-2. The availability of acetate was preventing the S2 and S3-BES from switching to an electroautotrophic metabolism. Decreasing the carbon source concentrations proved partially successful, with S1-BES showing improved reductive performance and S2 and S3-BES showing reductive current when acetate was depleted. The lack of mediated oxidative electrode respiration in S1 seems to be due to the presence of alternate electron acceptors in the medium. Periodic polarization reversal resulted in oxygen production at the counter electrode, which could serve as an alternative electron acceptor for aerobic microorganisms.

Accepted Answer

The study shows that mixed EAB cultures can transition to at least electroautotrophic activity, but further investigation is needed to determine if they can achieve bidirectional EET. The cultures differentiated over the course of the experiments, and substrate or c-source priority limits their transition. However, the study suggests that it is possible for them to transition to bidirectional EET based metabolism. Future investigations should focus on answering questions related to bidirectional EET, biofilm detachment triggers, and optimizing cultivation procedures for bidirectional EAB.

C-source priority and availability limits bidirectional electron transfer in freshwater mixed culture EAB biofilms

Chat with Paper

AI Agents for this Paper

Most frequently asked questions

1. What is the significance of bidirectional EET in mixed EAB cultures and how was it investigated?

2. What is the duration of the half-batch polarization reversal (HBPR) and periodic polarization reversal (PPR) in the Materials & Methods section?

3. How were COD measurements conducted in the BES study?

4. What factors influenced the maximum current densities in BES during bio lm cultivation?

5. What is the effect of carbon source on BES behavior?

6. What changes in redox potentials occurred during the HBPR experiment?

7. What is the effect of periodic polarization reversal on the behavior of S1-BES, S2, and S3-BES in terms of current generation and reductive/oxidative current?

8. Can mixed EAB cultures transition to bidirectional EET?

References

Microbial electrosynthesis — revisiting the electrical route for microbial production

Electroactive microorganisms in bioelectrochemical systems

Secretion of Flavins by Shewanella Species and Their Role in Extracellular Electron Transfer

Microbial electron transport and energy conservation - the foundation for optimizing bioelectrochemical systems.

Methanogenesis facilitated by electric syntrophy via (semi)conductive iron-oxide minerals

Related Papers (5)

2,6-Di-tert-butyl-p-benzoquinone enhances biofilm redox activity and extracellular electron transfer

Effects of Specific Surface Area of Artificial Carriers on Carbon Metabolism Activity of Biofilm

Metabolic spatial variability in electrode-respiring Geobacter sulfurreducens biofilms

Quantification of electron transfer rates to a solid phase electron acceptor through the stages of biofilm formation from single cells to multicellular communities.

Carbon Transformations and Activity in Biofilms