Mechanistic stratification in electroactive biofilms of Geobacter sulfurreducens mediated by pilus nanowires

Abstract

Electricity generation by Geobacter sulfurreducens biofilms grown on electrodes involves matrix-associated electron carriers, such as c-type cytochromes. Yet, the contribution of the biofilm's conductive pili remains uncertain, largely because pili-defective mutants also have cytochrome defects. Here we report that a pili-deficient mutant carrying an inactivating mutation in the pilus assembly motor PilB has no measurable defects in cytochrome expression, yet forms anode biofilms with reduced electroactivity and is unable to grow beyond a threshold distance (∼10 μm) from the underlying electrode. The defects are similar to those of a Tyr3 mutant, which produces poorly conductive pili. The results support a model in which the conductive pili permeate the biofilms to wire the cells to the conductive biofilm matrix and the underlying electrode, operating coordinately with cytochromes until the biofilm reaches a threshold thickness that limits the efficiency of the cytochrome pathway but not the functioning of the conductive pili network.

Evolutionary dynamics of microbial communities in bioelectrochemical systems

“…NuoL, for which metagenomic rank changed most at the M4 anode compared to M1A, is responsible for the reverse electron transfer and H + /estoichiometry (Steimle et al, 2011), which may be important for balancing electron flow between NAD+ and ferredoxin pools. PilB is an ATPase required for polymerization of conductive e-pili (McCallum et al, 2017), and pilB mutants are reported to generate lower current and form thinner biofilms (Steidl et al, 2016). Additionally, it was observed by Ishii et al (2018) that pilA expression increased at lower surface potential.…”

Potential regulates metabolism and extracellular respiration of electroactive Geobacter biofilm

“…Transmembrane electron flow across the envelope relies on cyt clusters such as OmaB-OmcB, OmaC-OmcC, extA-extC/D, and extG-extF, each of which is integrated with a porin for protein localization (Otero, Chan, & Bond, 2018;Shi et al, 2016). For long range electron transfer in the exterior space of cells, redox proteins including OmcE, OmcS, and OmcZ and conductive pili are involved as electron relay and/or terminal reductases (Steidl, Lampa-Pastirk, & Reguera, 2016;Vargas et al, 2013). Besides, other redox proteins may be used when these proteins are absent in mutants (Aklujkar et al, 2013;Mehta, Childers, Glaven, Lovley, & Mester, 2006;Otero et al, 2018).…”

High Biofilm Conductivity Maintained Despite Anode Potential Changes in a Geobacter‐Enriched Biofilm

“…The Nernst-Monod equationa ssumes Ohm's law for EET. [8,65] In contrast, recent publications reported that the conductivity of Geobacter sulfurreducens biofilms increased with increasing temperature and water content, [60,61] supporting redox conduction among extracellularc ofactors (e.g.,c ytochromes) for EET. EET conduction mechanismsa re still on debate between metallic-like and redox conduction for the Geobacter sulfurreducens biofilm anode.…”

“…This metal-like conduction attributed from p-p stacking of aromatic amino acids in Geobacter's pili or nanowire. [8,65] In contrast, recent publications reported that the conductivity of Geobacter sulfurreducens biofilms increased with increasing temperature and water content, [60,61] supporting redox conduction among extracellularc ofactors (e.g.,c ytochromes) for EET. It seems that Geobacter sulfurreducens biofilms would have multiple conduction mechanisms for EET to electrodes, which implies that EET conduction mechanismsw ould be more complicated in Geobacter-enriched mixed cultureb iofilms.…”