Growth progression driven by c-di-GMP in photogranules under hydrostatic conditions: polysaccharides biosynthesis and microbial interactions

Hydrostatically formed photogranules (HSPs) are spontaneously formed algal-bacterial aggregates, wherein sticky polysaccharides (PS) intertwine with the microbial moiety to form cohesive structures. However, the mechanism of algal-bacterial self-immobilization for spherical granules remains unclear. Herein, we elucidated cyanobacterial-bacterial interactions by applying multi-omics analysis to reveal the self-aggregation mechanism of HSPs. Under optimal conditions, mature HSPs were formed within 30 days with the success rate of cultivation exceeding 90 %. During the phototrophic bloom and granulation phases, PS secretion was significantly correlated with the levels of c-di-GMP (p < 0.01). Notably, alginate (24.43 % ± 4.23 %) was the most abundant metabolite among the PS compositions, and the cross-feeding between filamentous cyanobacteria (FC) and symbiotic bacteria for alginate constituted the core of intercellular relationships in HSPs. FC secreted c-di-GMP via a Wsp chemosensory-like system and promoted the alginate production. Acidobacteriota-affiliated bacteria exhibited expression of gene algl (4.47 %) to utilize alginate, contributing to PS polymerization and a tight assembly of FC and bacteria. In return, bacteria provided growth factors and facilitated public-goods (vitamins and cofactors) exchanges with FC. Overall, the metabolic characterizations advanced the photogranulation mechanism from the aspect of cross-feeding mediated by c-di-GMP, highlighting the significant role of PS in the growth progressions of HSPs.