It Takes Two to Tango: Unraveling Trophic Interactions in the Phycosphere

In nature, microbes frequently self-organize into spatially structured communities, where different types of cells inhabit distinct spatial domains. This spatial arrangement plays a pivotal role in influencing various biological functions, including community growth, stability, metabolite cross-feeding, and diversity. An important example is the phycosphere—the region around phytoplankton (e.g., cyanobacteria, microalgae) in the ocean, in which trophic interactions with surrounding bacteria strongly influence carbon/nutrient cycling and aquatic food webs. Laboratory studies typically focus on these microbial systems in well-mixed cultures, which provide valuable information on cellular processes, but do not capture the spatial arrangement of different cell types often found in nature. Thus, here, we address this gap in knowledge using direct visualization of spatially-structured bacteria-cyanobacteria communities in transparent hydrogel matrices. Our experimental platform enables byproduct exchange, mirroring the interactions and spatial organization found in diverse marine and terrestrial ecosystems. Our experiments reveal the emergence of complex dynamical spatio-temporal interactions between bacteria and cyanobacteria, driven by the exchange of byproducts. These interactions can be either beneficial or antagonistic for the communities. These dynamics strongly depend on environmental conditions, cell motility, and cell density, which we recapitulate with a minimal theoretical model. Our results provide quantitative principles to predict and control the trophic interactions in the phycosphere that play crucial roles in the environment and global ecology.