Cyanobacteria are photosynthetic microorganisms that play a central role in global carbon and nitrogen cycling, particularly in aquatic ecosystems. A subset of these organisms, heterocystous cyanobacteria, form multicellular filaments in which specialized heterocysts fix atmospheric nitrogen and exchange metabolites with neighboring vegetative cells, releasing a portion of fixed nitrogen to the surrounding environment. Multicellular growth and differentiation in cyanobacteria depend on the controlled exchange of metabolites between cells and, in symbiotic systems, between organisms. Although septal junctions are known to mediate intercellular molecular exchange and communication in heterocystous cyanobacteria, how exchange is regulated across cell types, filament architectures and symbiotic lifestyles remains unclear – particularly in marine representatives that are difficult to culture. This thesis investigates how physical connectivity and metabolic dependency together shape functional integration in heterocystous cyanobacteria.

In Chapter I, we quantified intercellular molecular exchange in live filaments using fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP), enabling complementary assessment at single-cell and filament levels. Comparing the freshwater model Anabaena sp. PCC 7120, a communication-impaired Anabaena mutant and the marine facultative symbiont Richelia rhizosoleniae SC01 revealed local diffusion rates are conserved despite differences in filament architecture and heterocyst positioning. Nonetheless, exchange was cell type-dependent, with faster transfer between vegetative cells than at junctions involving heterocysts. FLIP further revealed distance-dependent and asymmetric fluorescence decay patterns, indicating that intercellular exchange is dynamically regulated rather than determined solely by filament architecture. Ultrastructural analyses of septal peptidoglycan disks in Richelia SC01 revealed nanopore abundance and diameter comparable to well-studied freshwater heterocystous cyanobacteria, consistent with the observed exchange rates.

To place intercellular exchange in a symbiotic context, the metabolic basis of host-symbiont relation was examined in the endosymbiont of a Hemiaulus diatom, Richelia euintracellularis HH01, in Chapter II. Functional characterization of membrane transporters and a heterocyst-specific invertase identified uptake pathways of host-derived sugars, amino acids, and polyamines, with environmental transcript data confirming potential activity of these pathways in nature.

Together, this work corroborates that intercellular exchange in heterocystous cyanobacteria rests on conserved nanopore architecture but is dynamically regulated by cell type rather than filament architecture alone. For the Hemiaulus-Richelia endosymbiosis, we provide a mechanistic framework explaining how the endosymbiont’s metabolic requirements for effective nitrogen fixation are met, through characterization of symbiont transport systems that reveal a broad metabolic dependency on the host spanning carbon, nitrogen, and polyamine acquisition.