Temporal-Spatial Changes in Viabilities and Electrochemical Properties of Anode Biofilms

Abstract

Sustained current generation by anodic biofilms is a key element for the longevity and success of bioelectrochemical systems. Over time, however, inactive or dead cells can accumulate within the anode biofilm, which can be particularly detrimental to current generation. Mixed and pure culture (Geobacter anodireducens) biofilms were examined here relative to changes in electrochemical properties over time. An analysis of the three-dimensional metabolic structure of the biofilms over time showed that both types of biofilms developed a live outer-layer that covered a dead inner-core. This two-layer structure appeared to be mostly a result of relatively low anodic current densities compared to other studies. During biofilm development, the live layer reached a constant thickness, whereas dead cells continued to accumulate near the electrode surface. This result indicated that only the live outer-layer of biofilm was responsible for current generation and suggested that the dead inner-layer continued to function as an electrically conductive matrix. Analysis of the electrochemical properties and biofilm thickness revealed that the diffusion resistance measured using electrochemical impedance spectroscopy might not be due to acetate or proton diffusion limitations to the live layer, but rather electron-mediator diffusion.

Current density reversibly alters metabolic spatial structure of exoelectrogenic anode biofilms

“…Anodes were polished 1.5 Â 1 Â 0.3 cm graphite plates (4.5 cm 2 surface area) [16] while the cathodes (1.5 Â 1 cm) were made of stainless steel mesh (Type 304, mesh size 90 Â 90).…”

“…The viability of pure and mixed culture biofilms can vary within the biofilm, producing different structures: a live outer-layer with a dead inner-core; a dead outerlayer with a live inner-core; and a completely viable biofilm [16,23,24]. So far, most electroactive biofilm studies have reported either completely viable biofilms or biofilms containing a dead inner layer [24e31].…”

“…So far, most electroactive biofilm studies have reported either completely viable biofilms or biofilms containing a dead inner layer [24e31]. Current density was identified as a key factor in the development of a dead inner layer, as biofilms grown at a higher current density were completely viable (based on using live/ dead stains), whereas lower current densities resulted in a dead inner layer [16]. One limitation of this deduction was that fluorescent dyes used to assess viability were based on membrane integrity, not actual activity or viability [32].…”

“…The current was varied by using different applied voltages to the circuit, rather than using set potentials or different substrate concentrations, as this approach allowed for simple reactor operation and otherwise relatively similar operating conditions. SYTO9 and propidium iodide (PI) were used to infer viability based on staining color relative to membrane integrity, as done in previous studies [16,26,29,33]. An additional dye, fluorescein diacetate, was used to directly measure cell activity based on the conversion of fluorescein diacetate (FDA), a non-fluorescent esterase, into a fluorescent product (fluorescein) as this only occurs in live cells.…”

Biofilms 2015: Multidisciplinary Approaches Shed Light into Microbial Life on Surfaces

“…Dr. Logan discussed the functional consequence of inactive or dead cells accumulating over time in anode Geobacter anodireducens biofilms. This accumulation results in a two-layer structure with a live outer layer, responsible for current generation, covering an inactive inner core layer that functions as an electrically conductive matrix (37). Howard Stone (Princeton University, Princeton, NJ) showed how particular flow and surface structure influence bacterial biofilm dynamics.…”