Cyanobacteria are key primary producers in almost every illuminated environment and often represent a significant proportion of natural microbial communities. Consequently, they serve as sources of organic carbon for many other surrounding microorganisms. While the resulting bacterial interactions are often antagonistic, commensal and mutualistic adaptations are also known. A cyanobacterium of the genus Anabaena has been shown to closely interact with the heterotrophic proteobacterium Hoeflea sp., constituting a conspicuous form of bacterial heterologous multicellularity. Within this multicellular consortium the Hoeflea epibiont adheres in high numbers specifically to the heterocysts of the filamentous cyanobacterial partner. This model system offers unique opportunities for the study of symbiosis as (1) it involves two bacteria with medium-sized genomes, (2) each partner can be grown axenically in the presence of oxygen, (3) combining pure cultures of both restores the consortium phenotype.

We applied genomics, transcriptomics, metabolomics, metallomics, fluorescent and electron microscopic techniques to study the biotic interactions underlying this bacterial heterologous multicellularity. Following each partner in axenic culture, or as consortium, we demonstrated that specific metabolites and metals are sequestered by either species prior to being shared, which in turn enhances their combined growth. Cell signalling pathways were further identified via gene expression data that suggest this consortium to be related (and perhaps a predecessor) to the symbiosis of plants with nodulating rhizobia. Since the Anabaena and Hoeflea can be cultivated independently, and then mixed to form consortia, diversity assays were conducted to check the host and epibiont range by exchanging either partner for alternative species. In this way, we discovered that several members of Nostocales could form consortia, whereas each required the specific species of Hoeflea, providing greater context to the possible symbiosis detected in nature.