Bioelectrochemical systems (BESs) are hybrid systems using electroactive bacteria and solid electrodes, which serve as electron donor or acceptor for microorganisms. When forming a biofilm on the electrode, bacteria secrete extracellular polymeric substances (EPSs). However, EPS excretion of electroactive biofilms in BES has been rarely studied so far. Consequently, the aim of this study is to develop a routine including the electrochemical cultivation, biofilm harvesting, fractionation, and biochemical analysis of the EPS secreted by Geobacter sulfurreducens under electroactive conditions. G. sulfurreducens was cultivated in microbial fuel cell mode on graphite-based electrodes polarized to +400 mV versus Ag/AgCl for 8 d. A maximum current density of 172 ± 29 μA cm–2 was reached after 7 d. The EPS secreted from the biofilms were harvested and fractioned into soluble, loosely bound, and tightly bound EPS and biochemically analyzed. Electroactive cultures secreted significantly more EPSs compared to cells grown under standard heterotrophic conditions (fumarate respiration). With 116 pg per cell, the highest amount of EPSs was measured for the soluble EPS fraction of G. sulfurreducens using anodic respiration, followed by the tightly bound (18 pg cell–1) and loosely bound (11 pg cell–1) fractions of the EPS. Proteins were found to dominate all EPS fractions of the biofilms grown under electrochemical conditions. To the best of the authors’ knowledge, these experiments are the first approach toward a complete analysis of the main EPS components of G. sulfurreducens under anode-respiring conditions.
Acidithiobacillus ferrooxidansandAcidithiobacillus ferrivoransare used in bioleaching to recover metals such as copper or gold from low-grade ores.At. ferrooxidansis one of the best studied bioleaching microorganisms.At. ferrivoransis known for its ability to leach ores at low temperature. Both microorganisms are involved in acid mine drainage (AMD) formation. The cells are embedded in extracellular polymeric substances (EPS), which play an important role in the attachment to metal-sulfide-surfaces and in catalysis of their dissolution. EPS amounts and compositions differ, depending on the energy source, as it was shown forAt. ferrooxidansR1. Recently,At. ferrooxidansstrains were reclassified into four different subgroups, which are considered to correspond to four different species. As well, a biofilm study, using three of these closely related iron-oxidizingAcidithiobacillusspecies, demonstrated differences regarding attachment to pyrite among them. Consequently, this study focuses on the investigation ofAt. ferrooxidansandAt. ferrivoransspecies and their EPS composition with regard to varying cultivation conditions. Also different energy sources like, elemental sulfur or pyrite were tested. EPS were extracted by the use of a cation exchange resin (DOWEX). Obtained EPS were analyzed for their total amount of proteins, extracellular DNA (eDNA) and carbohydrates.
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