# The interplay between the marine diazotroph Vibrio diazotrophicus and its prophage shapes both biofilm structure and nitrogen release

## TL;DR

A marine bacterium that fixes nitrogen interacts with its virus to form biofilms and release nutrients, influencing ocean ecosystems.

## Contribution

Discovery of a prophage in Vibrio diazotrophicus that influences biofilm structure and nitrogen release in marine environments.

## Key findings

- The prophage in Vibrio diazotrophicus can spontaneously excise and form infective particles.
- Phage-mediated lysis increases biofilm production and releases dissolved organic carbon and ammonium.
- This interaction creates microhabitats suitable for diazotrophy and contributes to nutrient cycling.

## Abstract

Marine environments are frequently oligotrophic, characterized by low amount of bioassimilable nitrogen sources. At the global scale, the microbial fixation of N₂, or diazotrophy, represents the primary source of fixed nitrogen in pelagic marine ecosystems, playing a key role in supporting primary production and driving the export of organic matter to the deep ocean. However, given the high energetic cost of N₂ fixation, the active release of fixed nitrogen by diazotrophs appears counterintuitive, suggesting the existence of alternative passive release pathways that remain understudied to date. Here, we show that the marine non-cyanobacterial diazotroph Vibrio diazotrophicus is endowed with a prophage belonging to the Myoviridae family, whose expression is induced under anoxic and biofilm-forming conditions. We demonstrate that this prophage can spontaneously excise from the genome of its host and that it forms intact and infective phage particles. Moreover, phage-mediated host cell lysis leads to increased biofilm production compared with a prophage-free derivative mutant and to increased release of dissolved organic carbon and ammonium. Altogether, the results suggest that viruses may play a previously unrecognized role in oceanic ecosystem dynamics by structuring microhabitats suitable for diazotrophy and by contributing to the recycling of (in)organic matter.

Diazotrophs are key players in ocean functioning by providing fixed nitrogen to ecosystems and fueling primary production. However, from a physiological point of view, the active release of nitrogenous compounds by diazotrophs is paradoxical, since they would invest in an energy-intensive process and supply nutrient to non-sibling cells, with the risk of being outcompeted. Therefore, alternative ways leading to the release of fixed nitrogen must exist. Here, we show that the marine non-cyanobacterial diazotroph Vibrio diazotrophicus possesses one prophage, whose activation leads to cell death, increased biofilm production, and the release of dissolved organic compounds and ammonium. Taken together, our results provide evidence that marine phage–diazotroph interplay leads to the creation of microhabitats suitable for diazotrophy, such as biofilm, and to nutrient cycling, and contributes to better understanding of the role of viruses in marine ecosystems.