Amplified cyanobacterial bloom is derived by polyphosphate accumulation triggered by ultraviolet light

• UV light induces polyP accumulation to result in luxury phosphorus uptake of cyanobacteria. • UV light promotes the phycobilins’ synthesis to produce more ATP for polyP synthesis. • Cyanobacterial vertical migration changes its UV exposure dose to prompt its growth. • Cyanobacterial growth and TP accumulation triggered by UV light expedites its expansion. Cyanobacterial blooms appear more strongly, constantly and globally, yet the positive effect of surface solar ultraviolet radiation (UV) on cyanobacterial bloom in natural freshwater habitats is largely ignored. Here in-situ and laboratory studies were jointly designed to probe the mechanism of cyanobacterial bloom promoted by solar UV light. The results showed that solar UV light is a key trigger factor for the accumulation of total phosphorus, dissolved inorganic phosphorus and polyphosphate (polyP) in blooming cyanobacterial cells. The increase of UV dose induces polyP accumulation to result in the excessive phosphorus uptake of blooming cyanobacteria, which provides sufficient phosphorus for cyanobacterial growth in suitable environment. Solar UV light also can promote the contents of phycocyanin, allophycocyanin, and phycoerythrin, producing sufficient ATP by photosynthesis for polyP synthesis in cyanobacterial cells in lake enviroment. The frequent variations of UV irradiance exposure prompts cyanobacteria to absorb excessive phosphorus from suspended solid or sediment. Cyanobacterial intracellular phosphorus is accumulated for their growth. UV light promotes polyP accumulation in blooming cyanobacterial cells to avoid damage. The adsorption amount of phosphorus increases for exuberant growth and then more surface blooming cyanobacteria are exposed to UV light to absorb ample phosphorus. Thus, the positive feedback occurs in lake water bodies with abundant phosphorus. This amplified cycle of cyanobacterial density and phosphorus due to solar UV light in eutrophic water bodies is analogous to a triode to amplify visible photosynthesis by UV light as a base electric current in the energy flow process in lake environment, therefore, “Cyanobacterial Phosphorus Assimilation Ultraviolet Effect” is used to describe this phenomenon. A new explanation is provided for the continuing proliferating mechanism of cyanobacterial bloom. Besides, a new perspective appears on the outbreak of cyanobacterial blooms in natural eutrophic lake water bodies worldwide.