Effect of humic acids on electricity generation integrated with xylose degradation in microbial fuel cells

Power generation from wastewater using microbial fuel cells: A review

As the world grapples with an imminent energy crisis brought on by the depletion of nonrenewable resources, such as petroleum, the necessity for alternative and eco-friendly power sources becomes increasingly apparent. In this regard harnessing knowledge gained from natural microorganisms to produce electricity using economical substrates is a promising solution through microbial fuel cells (MFCs). Microbial fuel cells leverage microbes' catabolic abilities to break down organic matter and release electrons that are subsequently transported across an external circuit for electricity generation. This article delves into the fundamental components involved in MFC construction and explores crucial factors that impact their performance including substrate oxidation, electron transfer, and internal resistance. Additionally, it offers a comprehensive analysis of existing microbial fuel cell designs while highlighting their respective strengths and weaknesses. Finally, the article showcases cost-effective MFC models based on thorough studies conducted worldwide while illuminating potential practical applications of this renewable energy technology.

Effects of substrate type on variation of sludge organic compounds, bioelectric production and microbial community structure in bioelectrochemically-assisted sludge treatment wetland

A bioelectrochemical assisted sludge treatment wetland (BE-STW) is a promising technology used in the elimination of organic compounds and recovery of bio-energy. In this study, four BE-STW systems were constructed to investigate the effects of some substrates (i.e. graphite particles, zeolite, ceramsite, and gravel) on organic compounds biodegradation and transformation, electricity production, and anodic bacterial community. The maximum output voltages were 0.939, 0.870, 0.741 and 0.835 V, and the maximum power densities were 0.467, 0.143, 0.110, and 0.131 W/m³ for the graphite particles (BS-GP), zeolite (BS-Z), ceramsite (BS-C), and gravel (BS-G) systems, respectively. Also, the dissolved organic carbon (DOC) removal rates were 61.84%, 28.54%, 25.56%, and 18.34% in BS-GP, BS-G, BS-Z, and BS-C, respectively. The degradation of aromatic compounds in sludge extracellular biological organic matter (EBOM) was mainly due to the decrease of hydrophilic fraction (HPI) and transphilic acid fraction (TPI-A) contents. Moreover, aromatic proteins were preferentially removed in BS-Z. For BS-C, the tyrosine-like proteins and humic acid-like substances in TPI-A were totally removed. An excitation-emission matrix (EEM) analysis showed that the fluorescent intensity of the humic acid-like substances was the lowest in BS-GP, and no fluorescence peaks of fulvic acid-like substances were observed. Finally, at the genus level, Longilinea, Terrimonas, Ottowia, and Saccharibacteria_genera_incertae_sedis were the dominant bacteria in BE-STW, and Methylophilus was also only detected in BS-GP. These results confirmed that substrate materials have a significant impact on the preferentially degraded organic matter in BE-STWs, which can provide a theoretical basis for the practical application of STW in the future.

Insight into the organic matter degradation enhancement in the bioelectrochemically-assisted sludge treatment wetland: Transformation of the organic matter and microbial community evolution

Sludge treatment wetland (STW) has been widely used to dewater and mineralize the various sludge, but the low degradation ability of organic matter can limit its application. Bioelectrochemistry has been proven to accelerate the degradation of organic compounds and recover bioenergy from the sludge. In this study, a bioelectrochemical-assisted sludge treatment wetland (BE-STW) system was constructed to determine the most common types of degraded organic matter and the functional bacterial community. It was found that the bioelectrochemistry process contributed to a further removal of the total chemical oxygen demand (TCOD) by 19% (±0.6) and the additional soluble chemical oxygen demand (SCOD) value was 64.10% (±0.63), with a voltage output of 0.961 V and a power density of 0.351 W/m³. The hydrophilic and hydrophobic acid fractions of the sludge were preferentially removed in BE-STW. The tryptophan-like protein and fulvic acid-like substances were totally removed, whereas, the hydrolysis of aromatic organic compounds in the neutral and hydrophobic acid fractions was enhanced. Also, the enrichment of Longilinea and Methylophilus improved the hydrolysis of organic matter. Moreover, the high relative abundance of Thauera, Dechloromonas, and Syntrophorhabdus could accelerate the degradation of aromatic compounds in the BE-STW system. The bacteria from the genus Geobacter was predominantly detected (2.48%) in the anodic biofilm on BE-STW. The results showed that bioelectrochemistry could improve the sludge stabilization degree in STW, accelerate the organic matter degradation and hydrolysis efficiency, and harvest bioelectricity, simultaneously. This technology can provide a new pathway to increase the efficiency of the traditional STW systems.

Enhancement of the electron transfer and ion transport phenomena in microbial fuel cells containing humic acid-modified bioanodes

Although microbial fuel cells (MFCs) are an attractive alternative to environmental remediation and power generation, there are still significant limitations related to power density and coulombic efficiency. Previous works have shown that the addition of humic acids (HA, a type of organic matter contained in soils and composting-by-products), improves the fuel to electricity conversion at the porous bioanodes (ba)|anolyte junction. In this work, MFCs having HA-modified bioanodes (MFC/baHA) were prepared and electrochemically analyzed utilizing discharge curves (cell potential vs current density plots) and electrochemical impedance spectroscopy (EIS). This investigation was motivated by looking for a deeper understanding of the functional effects of HA molecules on the operation of baHA-containing Microbial Fuel Cells (MFC/baHA). Our results revealed that the modification of bioanodes with HA molecules decreases the activation energy of the acetate ion oxidation, increasing by a factor of three the consumption rate of this fuel at the baHA|anolyte interface, and enhancing the diffusive transport of these ions through the pores of the baHA permeated by the anolyte.

Microbial Fuel Cell: Recent Developments in Organic Substrate Use and Bacterial Electrode Interaction

A new bioelectrochemical approach based on metabolic activities inoculated bacteria, and the microbial fuel cell (MFC) acts as biocatalysts for the natural conversion to energy of organic substrates. Among several factors, the organic substrate is the most critical challenge in MFC, which requires long-term stability. The utilization of unstable organic substrate directly affects the MFC performance, such as low energy generation. Similarly, the interaction and effect of the electrode with organic substrate are well discussed. The electrode-bacterial interaction is also another aspect after organic substrate in order to ensure the MFC performance. The conclusion is based on this literature view; the electrode content is also a significant challenge for MFCs with organic substrates in realistic applications. The current review discusses several commercial aspects of MFCs and their potential prospects. A durable organic substrate with an efficient electron transfer medium (anode electrode) is the modern necessity for this approach.

Interactions between Humic Substances and Microorganisms and Their Implications for Nature-like Bioremediation Technologies

The state of the art of the reported data on interactions between microorganisms and HSs is presented herein. The properties of HSs are discussed in terms of microbial utilization, degradation, and transformation. The data on biologically active individual compounds found in HSs are summarized. Bacteria of the phylum Proteobacteria and fungi of the phyla Basidiomycota and Ascomycota were found to be the main HS degraders, while Proteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes were found to be the predominant phyla in humic-reducing microorganisms (HRMs). Some promising aspects of interactions between microorganisms and HSs are discussed as a feasible basis for nature-like biotechnologies, including the production of enzymes capable of catalyzing the oxidative binding of organic pollutants to HSs, while electron shuttling through the utilization of HSs by HRMs as electron shuttles may be used for the enhancement of organic pollutant biodegradation or lowering bioavailability of some metals. Utilization of HSs by HRMs as terminal electron acceptors may suppress electron transfer to CO2, reducing the formation of CH4 in temporarily anoxic systems. The data reported so far are mostly related to the use of HSs as redox compounds. HSs are capable of altering the composition of the microbial community, and there are environmental conditions that determine the efficiency of HSs. To facilitate the development of HS-based technologies, complex studies addressing these factors are in demand.

Recovery of chromium, copper and vanadium combined with electricity generation in two-chambered microbial fuel cells

Microbial fuel cells (MFCs) offer a promising solution towards recovery and treatment of heavy metal pollutants. In this study, two-chambered MFCs were employed for recovery of chromium, copper and vanadium (Cr (VI), Cu (II) and V (V)). One g/L concentrations of K2Cr2O7, CuCl2 and NaVO3 served as catholytes, while a mixed culture was used as anolyte. Cr (VI), Cu (II) and V (V) were reduced biologically into less toxic forms of Cr (III), Cu and V (IV) respectively. Power density and cathodic efficiency were calculated for each of the catholytes. Cr (VI) gave the maximum power density and cathodic efficiency due to its high redox potential. Current produced depended on the concentration of the catholyte. Over a period of time, biological reduction of catholytes lead to decrease in the metal concentrations, which demonstrated the application of MFC technology towards heavy metal treatment and recovery in a reasonably cost-effective manner.

Storing of exoelectrogenic anolyte for efficient microbial fuel cell recovery

Starting up a microbial fuel cell (MFC) requires often a long-term culture enrichment period, which is a challenge after process upsets. The purpose of this study was to develop low-cost storage for MFC enrichment culture to enable prompt process recovery after upsets. Anolyte of an operating xylose-fed MFC was stored at different temperatures and for different time periods. Storing the anolyte for 1 week or 1 month at +4°C did not significantly affect power production, but the lag time for power production was increased from 2 days to 3 or 5 days, respectively. One month storing at -20°C increased the lag time to 7 days. The average power density in these MFCs varied between 1.2 and 1.7 W/m³. The share of dead cells (measured by live/dead staining) increased with storing time. After 6-month storage, the power production was insignificant. However, xylose removal remained similar in all cultures (99-100%) while volatile fatty acids production varied. The results indicate that fermentative organisms tolerated the long storage better than the exoelectrogens. As storing at +4°C is less energy intensive compared to freezing, anolyte storage at +4°C for a maximum of 1 month is recommended as start-up seed for MFC after process failure to enable efficient process recovery.

Effect of hydraulic retention time on electricity generation using a solid plain-graphite plate microbial fuel cell anoxic/oxic process for treating pharmaceutical sewage

Treatment efficiency and electricity generation were evaluated using a solid plain-graphite plate microbial fuel cell (MFC) anoxic/oxic (A/O) process that treated pharmaceutical sewage using different hydraulic retention times (HRT). Short HRTs increased the volumetric organic loading rate, thereby reducing the MFC performance due to rapid depletion of the substrate (carbon/nitrogen source). The COD removal efficiency decreased from 96.28% at a HRT of 8 h to 90.67% at a HRT of 5 h. The removal efficiency of total nitrogen was reduced from 74.16% at a HRT of 8 h to 53.42% at a HRT of 5 h. A shorter HRT decreased the efficiency in treatment of the pharmaceutical products (PPs), which included acetaminophen, ibuprofen and sulfamethoxazole in an aerobic reactor because these antibiotic compounds inhibited the microbial activity of the aerobic activated sludge in the MFC A/O system. The average power density and coulombic efficiency values were 162.74 mW m^-2 and 7.09% at a HRT of 8 h and 29.12 mW m^-2 and 2.23% at a HRT of 5 h, respectively. The dominant bacterial species including Hydrogenophaga spp., Rubrivivax spp. and Leptothrix spp., which seem to be involved in PP biodegradation; these were identified in the MFC A/O system under all HRT conditions for the first time using next generation sequencing. Bacterial nanowires were involved in accelerating the transfer of electrons and served as mediators in the SPGRP biofilm. In conclusion, a SPGRP MFC A/O system at a HRT of 8 h gave better removal of COD, T-N and PPs, as well as generated more electricity.

Raw hematite based Fe(III) bio-reduction process for humified landfill leachate treatment

Microorganisms from paddy soils and raw hematite are used for enhancing natural Fe(III) bio-reduction, in order to remove macromolecular organic pollutants from humified landfill leachate. Based on batch experiments, 60% of refractory organics can be adsorbed by hematite in 12 days. In the presence of Fe(III)-reducing bacteria, 489.60 ± 0.14 mg L^-1 of dissolved organic matters can be degraded to 51.90 ± 3.96 mg L^-1 within 50 days; twelve types of semi volatile organic compounds can be degraded; hereby, the reaction follows a first-order kinetics. Crystalline Fe(III) is transformed into the amorphous form and reduced to Fe(II), hydroquinone functional groups in the humic acid (HA) are transformed to quinone ones, and the formation of HA-hematite ligands is promoted. Comparing with most of the studies about electron shuttling of HA, the transformation of quinone in the HA to hydroquinone could not be observed in the present bio-system. Based on column evaluations, more than 93% of chemical oxygen demand (influent concentration of 658 ± 19 mg L^-1) could be removed microbially under flow conditions, when the hydraulic retention time was 45 h. Raw hematite-based Fe(III) bio-reduction has a promising potential for the removal of humic and benzene series in humified landfill leachate.

Factors Affecting the Effectiveness of Bioelectrochemical System Applications: Data Synthesis and Meta-Analysis

Microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) are promising bioelectrochemical systems (BESs) for simultaneous wastewater treatment and energy/resource recovery. Unlike conventional fuel cells that are based on stable chemical reactions, these BESs are sensitive to environmental and operating conditions, such as temperature, pH, external resistance, etc. Substrate type, electrode material, and reactor configuration are also important factors affecting power generation in MFCs and hydrogen production in MECs. In order to discuss the influence of these above factors on the performance of MFCs and MECs, this study analyzes published data via data synthesis and meta-analysis. The results revealed that domestic wastewater would be more suitable for treatment using MFCs or MECs, due to their lower toxicity for anode biofilms compared to swine wastewater and landfill leachate. The optimal temperature was 25–35 °C, optimal pH was 6–7, and optimal external resistance was 100–1000 Ω. Although systems using carbon cloth as the electrodes demonstrated better performance (due to carbon cloth’s large surface area for microbial growth), the high prices of this material and other existing carbonaceous materials make it inappropriate for practical applications. To scale up and commercialize MFCs and MECs in the future, enhanced system performance and stability are needed, and could be possibly achieved with improved system designs.

Microbial fuel cells as an alternative energy source: current status

Microbial fuel cell (MFC) technology is an emerging area for alternative renewable energy generation and it offers additional opportunities for environmental bioremediation. Recent scientific studies have focused on MFC reactor design as well as reactor operations to increase energy output. The advancement in alternative MFC models and their performance in recent years reflect the interests of scientific community to exploit this technology for wider practical applications and environmental benefit. This is reflected in the diversity of the substrates available for use in MFCs at an economically viable level. This review provides an overview of the commonly used MFC designs and materials along with the basic operating parameters that have been developed in recent years. Still, many limitations and challenges exist for MFC development that needs to be further addressed to make them economically feasible for general use. These include continued improvements in fuel cell design and efficiency as well scale-up with economically practical applications tailored to local needs.

Influence of the fuel and dosage on the performance of double-compartment microbial fuel cells

This manuscript focuses on the evaluation of the use of different types and dosages of fuels in the performance of double-compartment microbial fuel cell equipped with carbon felt electrodes and cationic membrane. Five types of fuels (ethanol, glycerol, acetate, propionate and fructose) have been tested for the same organic load (5,000 mg L(-1) measured as COD) and for one of them (acetate), the range of dosages between 500 and 20,000 mg L(-1) of COD was also studied. Results demonstrate that production of electricity depends strongly on the fuel used. Carboxylic acids are much more efficient than alcohols or fructose for the same organic load and within the range 500-5,000 mg L(-1) of acetate the production of electricity increases linearly with the amount of acetate fed but over these concentrations a change in the population composition may explain a worse performance.

A high performance xylose microbial fuel cell enabled by Ochrobactrum sp. 575 cells

A new bacterium, Ochrobactrum sp. 575, is applied as a high performance MFC while resolving xylose, and the formation of fumaric acid is observed during the discharging process.

Humus-reducing microorganisms and their valuable contribution in environmental processes

Humus constitutes a very abundant class of organic compounds that are chemically heterogeneous and widely distributed in terrestrial and aquatic environments. Evidence accumulated during the last decades indicating that humic substances play relevant roles on the transport, fate, and redox conversion of organic and inorganic compounds both in chemically and microbially driven reactions. The present review underlines the contribution of humus-reducing microorganisms in relevant environmental processes such as biodegradation of recalcitrant pollutants and mitigation of greenhouse gases emission in anoxic ecosystems, redox conversion of industrial contaminants in anaerobic wastewater treatment systems, and on the microbial production of nanocatalysts and alternative energy sources.

Redox mediator enhanced simultaneous decolorization of azo dye and bioelectricity generation in air-cathode microbial fuel cell

Enhanced simultaneous decolorization of Congo red and bioelectricity generation with anthraquinone-2,6-disulphonic disodium salt (AQDS), riboflavin (RF) and humic acid (HA) as mediators in air-cathode microbial fuel cell (MFC) was demonstrated. Compared with mediator-free MFC, the MFC with added 0.005 mM AQDS, 0.005 mM RF or 1g/L HA showed 36%, 26% and 15% increase in maximum power density along with 394%, 450%, and 258% increases in decolorization rates of Congo red, respectively. Addition of mediators at higher concentration further increased power and Congo red decolorization but the increases were not proportional to the rise in mediator concentration. Based on decreases of anode charge transfer resistance and increases of Congo red decolorization, the mediators kinetically promote the extracellular electron transfer between bacteria, anode and Congo red. Microbial analysis showed that addition of mediators changed the composition of anodic microbial community and stimulated the growth of species belonging to Chlorobi, Endomicrobia and Firmicutes.

Evaluation of electricity production from alkaline pretreated sludge using two-chamber microbial fuel cell

Electricity production from alkaline pretreated sludge was evaluated using a two-chamber microbial fuel cell (MFC). The electricity production was found to be stable over a long period of time (approximately 17 d) with voltage outputs and power densities of 0.47-0.52 V and 46.80-55.88 mW/m(2), respectively. The anode resistance was the main internal resistance (73.2%) of MFC in the stable stage. Most soluble organic matters (proteins and carbohydrates) in the anode chamber were first degraded and converted into volatile fatty acids (0-15 d), which were then degraded and converted into electricity and methane (15-29 d). The insoluble organics were solubilized thereby decreasing the sludge concentration and reducing the sludge mass. Methane was produced in the anode chamber owing to the growth of methanogens, which did not obviously affect the electricity production. The change in humic-like substances displayed a positive correlation with the electricity production of the MFC. Microbial analysis showed that methanogens and electricity-producing bacteria co-existed mostly on the surface as well as inside the anode. Decreasing the anode resistance and increasing the anode utilization could enhance the electricity production.

Electron transfer mechanisms, new applications, and performance of biocathode microbial fuel cells

Broad application of microbial fuel cells (MFCs) requires low cost and high operational sustainability. Microbial-cathode MFCs, or cathodes using only bacterial catalysts (biocathodes), can satisfy these demands and have gained considerable attention in recent years. Achievements with biocathodes over the past 3-4 years have been particularly impressive not only with respect to the biological aspects but also the system-wide considerations related to electrode materials and solution chemistry. The versatility of biocathodes enables us to use not only oxygen but also contaminants as possible electron acceptors, allowing nutrient removal and bioremediation in conjunction with electricity generation. Moreover, biocathodes create opportunities to convert electrical current into microbially generated reduced products. While many new experimental results with biocathodes have been reported, we are still in the infancy of their engineering development. This review highlights the opportunities, limits, and challenges of biocathodes.

Electricity generation and microbial community response to substrate changes in microbial fuel cell

The effect of substrate changes on the performance and microbial community of two-chamber microbial fuel cells (MFCs) was investigated in this study. The MFCs enriched with a single substrate (e.g., acetate, glucose, or butyrate) had different acclimatization capability to substrate changes. The MFC enriched with glucose showed rapid and higher power generation, when glucose was switched with acetate or butyrate. However, the MFC enriched with acetate needed a longer adaptation time for utilizing glucose. Microbial community was also changed when the substrate was changed. Clostridium and Bacilli of phylum Firmicutes were detected in acetate-enriched MFCs after switching to glucose. By contrast, Firmicutes completely disappeared and Geobacter-like species were specifically enriched in glucose-enriched MFCs after feeding acetate to the reactor. This study further suggests that the type of substrate fed to MFC is a very important parameter for reactor performance and microbial community, and significantly affects power generation in MFCs.

A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production

Microbial fuel cells (MFCs) have gained a lot of attention in recent years as a mode of converting organic waste including low-strength wastewaters and lignocellulosic biomass into electricity. Microbial production of electricity may become an important form of bioenergy in future because MFCs offer the possibility of extracting electric current from a wide range of soluble or dissolved complex organic wastes and renewable biomass. A large number of substrates have been explored as feed. The major substrates that have been tried include various kinds of artificial and real wastewaters and lignocellulosic biomass. Though the current and power yields are relatively low at present, it is expected that with improvements in technology and knowledge about these unique systems, the amount of electric current (and electric power) which can be extracted from these systems will increase tremendously providing a sustainable way of directly converting lignocellulosic biomass or wastewaters to useful energy. This article reviews the various substrates that have been explored in MFCs so far, their resulting performance, limitations as well as future potential substrates.

Generation of electricity and analysis of microbial communities in wheat straw biomass-powered microbial fuel cells

Electricity generation from wheat straw hydrolysate and the microbial ecology of electricity-producing microbial communities developed in two-chamber microbial fuel cells (MFCs) were investigated. The power density reached 123 mW/m(2) with an initial hydrolysate concentration of 1,000 mg chemical oxygen demand (COD)/liter, while coulombic efficiencies ranged from 37.1 to 15.5%, corresponding to the initial hydrolysate concentrations of 250 to 2,000 mg COD/liter. The suspended bacteria found were different from the bacteria immobilized in the biofilm, and they played different roles in electricity generation from the hydrolysate. The bacteria in the biofilm were consortia with sequences similar to those of Bacteroidetes (40% of sequences), Alphaproteobacteria (20%), Bacillus (20%), Deltaproteobacteria (10%), and Gammaproteobacteria (10%), while the suspended consortia were predominately Bacillus (22.2%). The results of this study can contribute to improving understanding of and optimizing electricity generation in microbial fuel cells.

Reducing organic loads in wastewater effluents from paper recycling plants using microbial fuel cells

Many industries are charged fees based on the organic loads in effluents. Therefore, it can be advantageous to reduce the wastewater strength prior to discharge. We investigated the use of microbial fuel cells (MFCs) to reduce the chemical oxygen demand (COD) of a paper-plant wastewater while at the same time producing electricity in a continuous flow system. At a hydraulic retention time (HRT) of six hours, COD removal using an unamended wastewater (506 mg/L COD) (organic loading rate, OLR = 2.0 kg COD/(m3 d)) was 26 +/- 2%, with a power density of 5.9 +/- 0.2 W/m3 (210 +/- 7 mW/m2). This amount of power was similar to the maximum power density (5.2 +/- 0.4 W/m3) produced in fed-batch tests using a slightly lower strength wastewater in the same device (405 mg/L COD). Increasing the HRT to 25 h (OLR = 0.5 kg COD/(m3 d)) increased COD removal (41 +/- 2%) but substantially decreased power (2.8 +/- 0.3 W/m3). While wastewater strength affected removal rates, the solution conductivity (0.8 mS/cm) was primarily a factor in low power production. These results demonstrate that MFCs can be used to reduce organic loads in effluents at relatively short HRTs, while at the same time generating power.

The effect of different substrates and humic acid on power generation in microbial fuel cell operation

Electricity production from acetate, glucose and xylose with humic acid as mediator was investigated in two chambers microbial fuel cells (MFCs). Acetate produced the highest voltage (570 mV with 1000 Omega) and maximum power density (P(maxd)=123 mW/m(2)) due to a simpler metabolism than with glucose and xylose. Glucose and xylose resulted in P(maxd) of 28 mW/m(2) and 32 mW/m(2) at lower voltage of 380 mV and 414 mV, respectively. P(maxd) increased by 84% and 30%, for glucose and xylose respectively, when humic acid (2g/l) was present in the medium. No significant effect was found with acetate since the internal resistance possessed a limiting effect. The increase of P(maxd) due to humic acid presence was attributed to its ability to act as mediator. Even though pH decreased to 5 with glucose and xylose, due to production of acetate and propionate, the voltage remained on the same level of 250-350 mV.

Xylose anaerobic conversion by open-mixed cultures

Xylose is, after glucose, the dominant sugar in agricultural wastes. In anaerobic environments, carbohydrates are converted into volatile fatty acids and alcohols. These can be used as building blocks in biotechnological or chemical processes, e.g., to produce bioplastics. In this study, xylose fermentation by mixed microbial cultures was investigated and compared with glucose under the same conditions. The product spectrum obtained with both substrates was comparable. It was observed that, in the case of xylose, a higher fraction of the carbon was converted into catabolic products (butyrate, acetate, and ethanol) and the biomass yield was approximately 20% lower than on glucose, 0.16 versus 0.21 Cmol X/Cmol S. This lower yield is likely related to the need of an extra ATP during xylose uptake. When submitted to a pulse of glucose, the population cultivated on xylose could instantaneously convert the glucose. No substrate preference was observed when glucose and xylose were fed simultaneously to the continuously operated bioreactor.