Bacteria of the phylum Planctomycetota are common colonizers of phototrophs in marine and limnic habitats. They frequently co-occur with a variety of microalgal taxa [1] and have been shown to be abundant in the particle-attached fraction of cyanobacterial blooms [2]. On the surfaces of macroalgae and on the leaves of seagrass they can even dominate the microbial community. To study the interactions of Planctomycetota bacteria with phototrophs, we performed co-cultivation experiments with the marine microalga Emiliania huxleyi, the limnic cyanobacterium Microcystis aeruginosa and the marine macroalga Ulva with a selection of Planctomycetota bacteria. Furthermore, we introduced a second heterotrophic bacterium, Phaeobacter inhibens (Rhodobacterales) to our co-cultivations.

Here, we present new insights in the cross-feeding of nutrients, such as carbon and vitamins, in dual and tripartite algal-bacterial interactions. We further investigated how chemical mediators influence the morphology of bacteria as well as phototrophs. We show that algal exudates trigger the elongation of P. inhibens cells, thereby influencing the buoyancy. We hypothesize that this allows P. inhibens to stay in a nutrient-rich zone and to prevent a fast sinking to deeper ocean layers where nutrients are scarce. Furthermore, we show that the cyanobacterium M. aeruginosa underwent morphological changes upon co-cultivation with a planctomycetal model strain. This suggest a stress response triggered by the bacterial partner, possibly involving chemical mediators produced by Planctomycetota in response to the toxin microcystin.

Our results highlight the complex interactions between Planctomycetota bacteria and phototrophs, including both symbiotic and competitive aspects. The discovery of cross-feeding, chemical mediators and morphological adaptations contributes significantly to the understanding of microbial dynamics in aquatic ecosystems and opens new perspectives for the study of bacterial factors influencing algal growth and ecology.

[1] B.U. Pushpakumara, K. Tandon, A. Willis, H. Verbruggen, Unravelling microalgal-bacterial interactions in aquatic ecosystems through 16S rRNA gene-based co-occurrence networks, Scientific Reports, 13 (2023) 2743.

[2] V. Van Le, M. Kang, S.-R. Ko, C.-Y. Park, J.J. Lee, I.-C. Choi, H.-M. Oh, C.-Y. Ahn, Response of particle-attached and free-living bacterial communities to Microcystis blooms, Applied Microbiology and Biotechnology, 108 (2024) 42.