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Algal-bacterial granular consortia are aggregates of aerobic bacterial granules and the microalgae.Fast formation of algal-bacterial granules was studied systematically in the present work in terms of the granulation process,mechanisms involved and the communities.An aerobic bacterial granular sequencing batch reactor(SBR)was configured under natural light exposure(12/12 h of light/dark regimes)and an extremely low settling time of 2 min for spontaneous granulation.The morphological appearances indicated that there were two major phases for the granulation periods.Step one(day 1-3)witnessed abundant filamentous bacteria bridging with the initialing water-born algae on the surface of the granules,and overproduction of extracellular polymeric substances(especially polysaccharides(PS),tryptophan & protein-like,and humic acid-like substances)was observed for binding function.Step two(day 4-7)experienced the maturation of the spontaneous algal-bacterial consortia,when the algae grew into the inner side of the granules.The yellowish aerobic bacterial granules turned green on day 3,and soon became dark green when the granules matured on day 7.The growth of the water-born algae decreased the settleability,mean sizes of the granules,but stimulated the bioactivity through interactions between algae and bacteria.The overgrowth of filamentous bacteria and algae significantly decreased the biomass retention in terms of both mixed liquor suspended sludge(MLSS)and mixed liquor volatile suspended sludge(MLVSS)during the first stage,while stabilized on stage two.Symbiotic growth of algae occurred during granulation process for the increase of chl concentrations including Chla,Chlb and total Chl.Illumina results revealed that the introduction of algae reduced the richness and diversity of bacterial community.Whereas,no obvious shifts of the dominant groups at phylum and classes levels were found,though their relative abundances changed to different degrees.In contrast,the naturally initialized algae experienced thorough displace from Diatomea to green algae Chlorophyceae.The present work showed the spontaneous formation of algal-bacterial could be rapidly achieved under natural sunlight irradiation by seeding aerobic bacterial granules.