Guberman-Pfeffer (2024) affirms that the Geobacter filaments are cytochromes and aims the conductivity-mismatch critique mainly at the measurements, while leaving sample identity open as one branch of a disjunctive conclusion

electromicrobiologygeobactermicrobial-nanowirescytochromeredox-conductionscientific-controversy

Guberman-Pfeffer's paper answers its own title question — "to be or not to be a cytochrome" — in the affirmative: the Geobacter filaments are cytochromes. Directly: "The foregoing evidence suggests answering in the affirmative: A handful of cytochrome filaments resolved by CryoEM can carry the entire metabolic flux of electrons from a Geobacter cell at rates that are both consistent with kinetic analyses on other multi-heme proteins and do not pose the rate-limiting step to cellular respiration. Cytochrome filaments are proposed to be physiologically relevant." Because of this, the 10²–10⁵-fold discrepancy the paper reports (claim-2024-paper-disputes-geobacter-cytochrome-nanowire-conductivity) is presented largely as a problem with how conductivity was measured, rather than as a reason to doubt that the cryo-EM-resolved filaments are cytochrome polymers (claim-geobacter-conductive-filaments-are-omcs-cytochrome-polymers).

A proposed artifact source is the conducting-probe AFM (CP-AFM) contact geometry: "In addition to the abiological electron transport conditions used experimentally, the reported conductivities may be artificially large because the 40–60 nm-wide tip used in CP-AFM can contact ~10 filament subunits" — so the measured current may sum contributions across many parallel heme stacks rather than reflect single-filament redox hopping. The paper also characterizes the measurement conditions themselves as physiologically irrelevant: electrode-adsorbed, air-dried, and mechanically compressed filaments "exposed to electric fields unscreened by mobile ions," under which "heme-Fe redox activity... and concentration gradients... are not relevant."

The measurement critique is not, however, the paper's only exit. Its stated conclusion is disjunctive: "that earlier claims of inappropriate experimental design are true..., biologically irrelevant phenomena have been measured, and/or the conformation or composition of the characterized proteins is something other than the known cytochrome filaments." The third branch is a sample-identity doubt, not a technique doubt — the abstract likewise blames "experimental artifacts and sample impurities" — and the paper is explicit about which side that helps: having set aside whether e-pili exist as "not the issues at hand," it adds that "By failing to find some structural and electrical characterizations consistent with cytochromes, however, lends circumstantial support to the e-pilus hypothesis."

This sharpens the shape of the Geobacter nanowire dispute without removing the paper from it. The critic grants the structural identity that the older pili model denied and the cryo-EM work established (claim-malvankar-lab-overturned-pili-model-without-lovley) — cytochrome filaments exist and suffice for physiological flux — and his primary target is the electrical characterization technique rather than the material. But he leaves open, and by his own account lends circumstantial support to, the possibility that what the conductivity experiments measured was not those filaments at all. It is a later, differently aimed line of attack than the pili-versus-cytochrome fight, not a withdrawal from it.