# Vitamin B12-associated interactions between Mesorhizobium sp. TaiHu and Synechococcus sp. PCC 7002 revealed by multi-omics analysis

## TL;DR

This study shows how a bacterium called Mesorhizobium sp. TaiHu helps a cyanobacterium called Synechococcus sp. PCC 7002 by providing vitamin B12, using multi-omics methods.

## Contribution

The study reveals new insights into vitamin B12-mediated microbial interactions and identifies Mesorhizobium sp. TaiHu as a key partner in a synthetic microbial community.

## Key findings

- Mesorhizobium sp. TaiHu upregulates vitamin B12 biosynthesis and transport genes when interacting with Synechococcus sp. PCC 7002.
- Vitamin B12 complementarity between Mesorhizobium sp. TaiHu and Synechococcus sp. PCC 7002 supports the growth of the cyanobacterium.
- Nitrogen metabolism-related genes are upregulated in the microbial community during vitamin B12 regulation.

## Abstract

The marine cyanobacterium Synechococcus sp. PCC 7002 (Syn7002) is a model organism that lacks the gene cluster required for vitamin B12 biosynthesis, necessitating cooperative interactions with other microbes. In this study, we established a synthetic microbial consortium by co-culturing Syn7002 with a bloom-forming Microcystis community, followed by purification, and subsequently investigated the interactions between Syn7002 and the associated microbial community. Electron microscopy revealed numerous rod-shaped bacteria clustered around Syn7002 cells, indicating close spatial associations between species. Metagenomic analysis showed that the early-stage community consisted mainly of Syn7002, Mesorhizobium sp. TaiHu (MesTH) and Pseudomonas sp. TaiHu (PseTH), although the abundance of PseTH declined after community stabilization. Investigation of vitamin B12 regulation between MesTH and Syn7002 through metatranscriptomic analysis revealed upregulation of nitrogen metabolism-related genes in the microbial community. Transcriptomic data further indicated that vitamin B12 biosynthesis and transport genes were significantly upregulated in MesTH. Combined with vitamin B12-positive control experiments, these results confirm potential vitamin B12 complementarity between the two strains. The results further suggest that MesTH promotes the growth of Syn7002 in the community by providing the small amount of vitamin B12 needed for its growth. These findings provide new insights into vitamin-mediated microbial interactions and reveal additional transcriptional features of the synthetic community.