Premise: Symbioses between lichen-forming fungi and brown algae (phaeophytes) are extremely rare. We investigated the interactions between the marine fungus Collemopsidium pelvetiae and its two photosynthetic partners, the brown alga Pelvetia canaliculata and a cyanobacterial symbiont to address questions on symbiosis biology, lichenization, and the diversity of fungal–photosynthetic associations in marine environments. Methods: We combined light microscopy, fluorescence microscope in structural illumination microscopy mode (SIM), and transmission electron microscopy (TEM) to characterize thallus architecture, symbiont interfaces, and ultrastructural interactions. Amplicon-based 16S rRNA sequencing profiled cyanobacteria and heterotrophic bacteria from two regions (northern Spain and Brittany, France). Results: Collemopsidium pelvetiae had distinct interactions with its two photosynthetic partners and with heterotrophic bacteria. Interactions with Pelvetia canaliculata were restricted to the region beneath the perithecia, where hyphae penetrated the outer amorphous cell-wall layer, without any evident host defense response. In contrast, the interaction with cyanobacteria involved the formation of intracellular haustoria, which ultimately lead to the death of the cyanobionts, suggesting a controlled parasitic relationship or a transitional stage toward lichenization. Unexpectedly, C. pelvetiae also produced haustorium-like projections into heterotrophic bacterial cells, a structure not previously reported in lichen symbioses. Microbiome analysis identified Pleurocapsa as the most likely cyanobiont genus associated with C. pelvetiae and revealed consistent heterotrophic bacterial communities, suggesting a species-specific assemblage. Conclusions: This marine tripartite symbiosis involves distinct interaction modes, challenging strict lichen definitions. We propose that lichen symbioses form a multidimensional continuum of strategies rather than a single mutualistic model.