The First Generation Magnox Storage Pond (FGMSP) at the Sellafield Nuclear Facility in the UK is in active use on site, and will shortly be undergoing decommissioning. High levels of radioactivity and alkaline pH at 11.4 provide extreme conditions in the pond. Despite such an extreme environment, seasonal blooms are frequently reported. The blooms restrict the visibility of pond water, which significantly hinders the maintenance and decommissioning operations. Previous investigations on the bloom identified that the dominant bloom organism was a cyanobacterium affiliated with a Pseudanabaena species. A mixed laboratory culture of Pseudanabaena catenata with a similar microbial community to the pond sample, was used to investigate the adaptive responses to the abundant radionuclide Cs. The investigation found that P. catenata can uptake Cs using the K transport system. A high K+ affinity Kdp-ATPase system was switched off by 1 mM of Cs. A moderate K+ affinity Trk system was highly stimulated by Cs which may increase the uptake of Cs+/K+ and stimulated a series of metabolic pathways, such as photosynthesis. Uptake of Cs induced common response proteins and proteins relate to nutrient limitation. In addition, Cs can be accumulated by P. catenata and concentrated in discrete zones. High concentration of K+ significantly reduced the growth of P. catnata. A chemostat experiment indicated that KOH performed better than NaOH in controlling the growth of the P. catanata dominated culture and so can be suggested as an alternative to the use of NaOH in the pond system to control the bloom and simultaneously maintain the alkaline conditions of pond water.