# Non‐Respiratory Extracellular Electron Transfer Competes with Nitrogenase for Electrons in Rhodopseudomonas Palustris

## TL;DR

This study shows how electron transfer in a photosynthetic bacterium competes with nitrogen fixation, affecting both energy production and nitrogen availability.

## Contribution

The study reveals that outward EET suppresses nitrogen fixation by competing for electrons in photosynthetic bacteria.

## Key findings

- Outward EET maintains redox balance but suppresses biological nitrogen fixation.
- Lumichrome and cytochromes are key in indirect and direct electron transfer pathways.
- The pio operon is involved in outward extracellular electron transfer.

## Abstract

Biological nitrogen fixation (BNF) is a pivotal process that reduces nitrogen to ammonium within the nitrogen cycle. Extracellular electron transfer (EET) between diazotrophs and the extracellular environment influences the occurrence and efficiency of BNF. Although extracellular electron acceptors can function as a component of the electron transport chain, providing energy for chemotrophic nitrogen fixation via extracellular respiration, the function and mechanism of outward EET in photosynthetic diazotrophs remain unclear. Here, using Rhodopseudomonas palustris TIE‐1, a photosynthetic bioelectrochemical nitrogen fixation system is established for simultaneous nitrogen fixation and current generation, to dissect the complex interaction between these two processes. Outward EET functions are found to maintain redox balance, rather than serving as an extracellular respiration pathway. It significantly suppresses BNF by competing with nitrogenase for electrons. Lumichrome serves as the primary electron shuttle for indirect electron transfer, while cytochromes play an important role in direct electron transfer. Notably, the pio operon participates in outward EET. This study reveals the interaction mechanism between photosynthetic BNF and outward EET, providing new insight into the regulatory mechanisms of nitrogen fixation in anoxygenic phototrophs across diverse environmental conditions.

This study develops a photosynthetic bioelectrochemical nitrogen fixation system for simultaneous nitrogen fixation and current generation using Rhodopseudomonas palustris. Notably, extracellular electron transfer (EET) serves as a redox balancing strategy, suppressing nitrogen fixation by competing with nitrogenase for electrons. Lumichrome and cytochromes facilitate indirect and direct EET pathways, respectively.