Cyanobacteria are the most ancient Gram-negative oxygen-producing photosynthetic organisms [[1], [2], [3]] and are assumed to be as the progenitor of plant chloroplast [4,5]. Cyanobacteria can utilize solar energy with high efficiency (3–9%) [6], which powers the photoautotrophic mechanism that can fix huge amounts of nitrogen (N2, NH4, NO2, NO3, or urea) and inorganic carbon (CO2, NaHCO3, and Na2CO3), [[7], [8], [9]] into huge amount of biomass [10] which sustains a major part of the food chain. They have been recognized as potential microcell factories that could produce green, value-added products for healthcare and industrial uses [7].

Cyanobacteria are invariably subject to a variety of challenges, including drought, herbicides, heavy metals, ultraviolet (UV) radiation (UVR), fluctuations in light intensity and quality, availability of inorganic nutrients, high and low temperatures, salt, and pH [[11], [12], [13], [14]]. These are photoautotrophs, therefore, inevitably exposed to UVR that affects growth, cell differentiation, pigments, and N2 metabolism. It also affects the electron transport chain of photosynthetic machinery by changing the redox state, resulting in the generation of reactive oxygen species (ROS) [11,15,16]. UVR is directly absorbed by DNA, which severely damages the DNA molecules, resulting in mutagenesis, carcinogenesis, and even apoptosis, or cell death in the organisms [13,[17], [18], [19], [20]]. UV radiation induces lesions in the double-stranded DNA, and the two major lesions in the DNA are cyclobutane pyrimidine dimer (CPD) and (6–4) pyrimidine primidone photoproduct [17,[21], [22], [23]]. In response to this stress, cyanobacteria have evolved different strategies [18]. They exhibit a variety of metabolisms and morphologies [24,25], and many species can differentiate into a special kind of cells called akinetes that can survive under extreme environments [26,27]. Additionally, cyanobacteria produce a broad range of bioactive metabolites which have potential health benefits [7]. They can be easily genetically modified to produce the metabolites of interest.

UVR mainly damages the cells and affects the DNA, proteins, lipids, and plasma membrane. It affects the photosynthetic performance of cyanobacteria, algae, and plants, reducing their fitness and productivity. UVR also have a mutagenic effect on organisms that results in cell death. Therefore, in this study, we have focused on the hypothesis that for how many hours the cyanobacterium Synechocystis sp. PCC 6803 can tolerate the PAB exposure under laboratory conditions. We also wanted to see if it recovers under normal PAR conditions. We have investigated the post-irradiated (PIR) recovery of the cyanobacterium under PAR for 7 days and quantified the growth in terms of Chl a. This will help in employing the cyanobacterium to produce metabolites of interest in response to changing environmental conditions.