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Here, we compared the facultative symbiont\n                      <jats:italic toggle=\"yes\">Richelia rhizosoleniae<\/jats:italic>\n                      SC01 (hereafter\n                      <jats:italic toggle=\"yes\">Richelia<\/jats:italic>\n                      ; terminal heterocysts) with the free-living\n                      <jats:italic toggle=\"yes\">Anabaena<\/jats:italic>\n                      sp. PCC 7120 (hereafter\n                      <jats:italic toggle=\"yes\">Anabaena<\/jats:italic>\n                      sp.; intercalary heterocysts) and a septal junction mutant of\n                      <jats:italic toggle=\"yes\">Anabaena<\/jats:italic>\n                      with impaired intercellular communication (CSVT22, \u2206\n                      <jats:italic toggle=\"yes\">fraC<\/jats:italic>\n                      -\u2206\n                      <jats:italic toggle=\"yes\">fraD<\/jats:italic>\n                      ), using fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP).\n                      <jats:italic toggle=\"yes\">Richelia<\/jats:italic>\n                      SC01 and\n                      <jats:italic toggle=\"yes\">Anabaena<\/jats:italic>\n                      showed similar recovery (\n                      <jats:italic toggle=\"yes\">R<\/jats:italic>\n                      ) rates, indicating comparable average molecular exchange through septal junctions, with no significant correlation between\n                      <jats:italic toggle=\"yes\">R<\/jats:italic>\n                      and filament length. By contrast, CSVT22 showed reduced recovery and accelerated, irregular decay, indicative of both disrupted septal connectivity and altered regulation of septal junctions, resulting in heterogeneous and less efficient intercellular exchange. Differences in fluorescence decay speed across cell types and strains suggest that cell identity and filament organization, including filament taper, shape patterns of intercellular exchange. FLIP analyses showed that\n                      <jats:italic toggle=\"yes\">Anabaena<\/jats:italic>\n                      consistently exhibited greater exchange asymmetry, whereas\n                      <jats:italic toggle=\"yes\">Richelia<\/jats:italic>\n                      SC01 vegetative cells displayed comparatively more symmetric exchange, despite pronounced filament tapering and terminal heterocyst organization. Transmission electron microscopy further showed that septal nanopores in\n                      <jats:italic toggle=\"yes\">Richelia<\/jats:italic>\n                      SC01 are similar in size to those of\n                      <jats:italic toggle=\"yes\">Anabaena<\/jats:italic>\n                      but exhibited marked variation in number per septal disk, suggesting a potential structural basis for variation in intercellular exchange patterns. Collectively, our findings show that combining FRAP and FLIP provides a quantitative framework to dissect the kinetics and spatial organization of intercellular exchange in filamentous cyanobacteria.\n                    <\/jats:p>\n                    <jats:sec>\n                      <jats:title>IMPORTANCE<\/jats:title>\n                      <jats:p>\n                        Filamentous cyanobacteria rely on intercellular exchange through septal junctions to coordinate metabolism and development, yet the principles governing multicellular communication in bacterial filaments remain poorly understood. Here, we show that intercellular exchange emerges from the interplay between septal junction properties, cell identity, and filament architecture, generating distinct communication patterns across cyanobacterial lifestyles. Notably, despite its pronounced filament tapering and terminal heterocyst organization,\n                        <jats:italic toggle=\"yes\">Richelia<\/jats:italic>\n                        SC01 exhibited comparatively symmetric exchange, demonstrating that filament polarity alone does not determine communication patterns. Our findings reveal that cyanobacterial multicellularity arises from the integration of structural organization and cell-specific properties, providing a framework for understanding coordinated physiology in filamentous cyanobacteria, including symbiotic and nitrogen-fixing systems.\n                      <\/jats:p>\n                    <\/jats:sec>\n                  <\/jats:sec>","DOI":"10.1128\/aem.00935-26","type":"journal-article","created":{"date-parts":[[2026,9,10]],"date-time":"2026-09-10T13:01:35Z","timestamp":1789045295000},"update-policy":"https:\/\/doi.org\/10.1128\/asmj-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Cell identity and filament architecture shape intercellular communication in the terminal heterocystous cyanobacterium\n                    <i>Richelia rhizosoleniae<\/i>\n                    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