Effects of terminal electron acceptors on the biodegradation of waste motor oil using Chlorella vulgaris-Rhodococcus erythropolis consortia: Kinetic and thermodynamic windows of opportunity analysis

The Chlorella vulgaris-Rhodococcus erythropolis consortia was constructed for the biodegradation of waste motor oil (WMO), combined with thermodynamic calculations and stoichiometric analyses. The microalgae-bacteria consortium was constructed as C. vulgaris: R. erythropolis = 1:1 (biomass, cell/mL), pH = 7, 3 g/L WMO. Under the same condition, the terminal electron acceptors (TEAs) play a crucial role in the WMO biodegradation, which follows Fe3+ >SO42- > none. The biodegradation of WMO fitted well with the first-order kinetic model under experimental temperatures with different TEAs (R2 >0.98). The WMO biodegradation efficiency reached 99.2 % and 97.1 % with Fe3+ and SO42-as TEAs at 37 °C, respectively. Thermodynamic methanogenesis opportunity windows with Fe3+ as TEA are 2.72 times fold as large as those with SO42-. Microorganism metabolism equations demonstrated the viability of anabolism and catabolism on WMO. This work lays the groundwork for the implementation of WMO wastewater bioremediation and supports research into the biochemical process of WMO biotransformation.

Anodic electro-fermentation: Empowering anaerobic production processes via anodic respiration

In nature as well as in industrial microbiology, all microorganisms need to achieve redox balance. Their redox state and energy conservation highly depend on the availability of a terminal electron acceptor, for example oxygen in aerobic production processes. Under anaerobic conditions in the absence of an electron acceptor, redox balance is achieved via the production of reduced carbon-compounds (fermentation). An alternative strategy to artificially stabilize microbial redox and energy state is the use of anodic electro-fermentation (AEF). This emerging biotechnology empowers respiration under anaerobic conditions using the anode of a bioelectrochemical system as an undepletable terminal electron acceptor. Electrochemical control of redox metabolism and energy conservation via AEF can steer the carbon metabolism towards a product of interest and avoid the need for continuous and cost-inefficient supply of oxygen as well as the production of mixed reduced by-products, as is the case in aerobic production and fermentation processes, respectively. The great challenge for AEF is to establish efficient extracellular electron transfer (EET) from the microbe to the anode and link it to central carbon metabolism to enhance the synthesis of a target product. This article reviews the advantages and challenges of AEF, EET mechanisms, microbial energy gain, and discusses the rational choice of substrate-product couple as well as the choice of microbial catalyst. Besides, it discusses the potential of the industrial model-organism Bacillus subtilis as a promising candidate for AEF, which has not been yet considered for such an application. This prospective review contributes to a better understanding of how industrial microbiology can benefit from AEF and analyses key-factors required to successfully implement AEF processes. Overall, this work aims to advance the young research field especially by critically revisiting the fundamental aspects of AEF.

How does electron transfer occur in microbial fuel cells?

Microbial fuel cells (MFCs) have emerged as a promising technology for sustainable wastewater treatment coupled with electricity generation. A MFC is a device that uses microbes as catalysts to convert chemical energy present in biomass into electrical energy. Among the various mechanisms that drive the operation of a MFC, extracellular electron transfer (EET) to the anode is one of the most important. Exoelectrogenic bacteria can natively transfer electrons to a conducting surface like the anode. The mechanisms employed for electron transfer can either be direct transfer via conductive pili or nanowires, or mediated transfer that involves either naturally secreted redox mediators like flavins and pyocyanins or artificially added mediators like methylene blue and neutral red. EET is a mechanism wherein microorganisms extract energy for growth and maintenance from their surroundings and transfer the resulting electrons to the anode to generate current. The efficiency of these electron transfer mechanisms is dependent not only on the redox potentials of the species involved, but also on microbial oxidative metabolism that liberates electrons. Attempts at understanding the electron transfer mechanisms will boost efforts in giving rise to practical applications. This article covers the various electron transfer mechanisms involved between microbes and electrodes in microbial fuel cells and their applications.

Biodegradation of petroleum hydrocarbons and changes in microbial community structure in sediment under nitrate-, ferric-, sulfate-reducing and methanogenic conditions

In the present study, the biodegradation behaviors of petroleum hydrocarbons under various reducing conditions were investigated. n-Alkanes and polycyclic aromatic hydrocarbons (PAHs) were degraded with NO₃^-, Fe³⁺, SO₄^2-, or HCO₃^- as terminal electron acceptors (TEAs), which link to four typical reducing conditions (i.e., nitrate-reducing, ferric-reducing, sulfate-reducing and methanogenic conditions, respectively) in sediment. The fastest degradation rates were achieved under sulfate-reducing conditions with half-lives of 49.51 days for n-alkanes and 58.74 days for PAHs. For short-chain n-alkanes and low-molecular weight (LMW) PAHs, relatively higher removal efficiencies were achieved under nitrate- and ferric-reducing conditions. The degradation of long-chain n-alkanes and high-molecular weight (HMW) PAHs coupled to methanogenesis was the most favored as compared with other reducing conditions. Carboxylation was found to be the principle mechanism for regulating n-alkane degradation coupled to denitrification, while the activation of n-alkanes by the addition of fumarate was the principle mechanism for the n-alkane degradation under sulfate-reducing conditions. The anaerobic metabolism of n-alkanes may not proceed via fumarate addition or carboxylation under ferric-reducing and methanogenic conditions. Illumina HiSeq sequencing revealed dissimilar structures of the microbial communities under various reducing conditions. It is hypothesized that the utilization of different TEAs for n-alkane and PAH degradation resulted in distinct microbial community structures, which were highly correlated with the varied degradation behaviors of petroleum hydrocarbons in sediment. The current results may provide reference value on better understanding the biodegradation behaviors of n-alkanes and PAHs in association with the induced microbial communities in sedimentary environments under the four typical reducing conditions.