Cyanobacteria are among the oldest extant oxygenic phototrophs and remain integral to global biogeochemical cycles, particularly carbon and nitrogen fluxes in aquatic ecosystems. A subset of these organisms, the bloom-forming cyanobacteria, have adapted unique photosynthetic strategies that enable ecological dominance in nutrient-rich and light-variable environments. This chapter presents a comprehensive examination of the structural, physiological, and molecular mechanisms that facilitate efficient light capture and energy conversion in these taxa. Key adaptations include the deployment of specialized pigment–protein complexes, such as phycobilisomes, and dynamic regulatory processes like state transitions and chromatic acclimation (e.g., CA4, CA6 systems), which allow modulation of light energy distribution between Photosystem I and Photosystem II. Technological advances particularly in time-resolved fluorescence spectroscopy, fluorescence lifetime imaging microscopy (FLIM), and Förster resonance energy transfer (FRET) have enabled unprecedented insights into real-time energy transfer and protein–protein interactions within cyanobacterial photosystems. Concurrently, the advent of CRISPR-Cas and synthetic biology has opened new frontiers in dissecting and engineering photosynthetic pathways for enhanced CO2 fixation and biofuel synthesis. The ecological relevance of these adaptations is further contextualized through their roles in harmful algal bloom (HAB) dynamics, competition for light and nutrients, and climate resilience. Bloom-forming genera such as Microcystis, Dolichospermum, and Planktothrix are discussed in terms of their photophysiological plasticity and environmental triggers. We also explore emerging applications, including the development of biohybrid systems, biophotovoltaics, and carbon-negative biofactories utilizing engineered cyanobacterial strains. The chapter concludes by identifying critical gaps in our understanding of regulatory networks governing photosynthetic responses under multifactorial stressors and proposes integrative approaches combining omics, remote sensing, and ecological modeling. Collectively, these insights underscore the centrality of cyanobacterial photosynthesis not only in ecosystem functioning but also in biotechnological solutions to energy, climate, and sustainability challenges.