The study investigates the complementary chromatic adaptation (CCA) in *Fremyella diplasiphon*, a filamentous blue-green alga, under different light conditions. The research focuses on the changes in pigmentation and morphology, particularly the presence and synthesis of C-phycocerythrin (PE), a key pigment in this alga. Key findings include: 1. **Pigmentation and Morphology**: - Red-illuminated cultures lack measurable PE and have significantly shorter filaments compared to fluorescent-illuminated cultures. - Fluorescent light promotes PE synthesis, while red light inhibits it. - The mean filament length in red-illuminated cultures is about 50 μm, compared to 460 μm in fluorescent-illuminated cultures. 2. **Metabolic Experiments**: - **Continuous Fluorescent Illumination**: PE is stable in fluorescent-illuminated cultures, with no significant degradation observed. - **Red to Fluorescent Light Transfer**: Transfer from red to fluorescent light initiates PE synthesis, but only a small fraction of the new PE is derived from pre-existing proteins. - **Fluorescent to Red Light Transfer**: Transfer from fluorescent to red light stops PE synthesis and causes a decrease in PE content and total radioactivity, likely due to transcellular filament breakage. 3. **Conclusion**: - Light influences intracellular PE levels by regulating its synthesis. The decrease in PE content in red-illuminated cultures is not due to degradation but to filament fragmentation, which results in cell lysis. This study provides insights into how light conditions affect the metabolic and morphological changes in *Fremyella diplasiphon*, highlighting the role of PE in complementary chromatic adaptation.The study investigates the complementary chromatic adaptation (CCA) in *Fremyella diplasiphon*, a filamentous blue-green alga, under different light conditions. The research focuses on the changes in pigmentation and morphology, particularly the presence and synthesis of C-phycocerythrin (PE), a key pigment in this alga. Key findings include: 1. **Pigmentation and Morphology**: - Red-illuminated cultures lack measurable PE and have significantly shorter filaments compared to fluorescent-illuminated cultures. - Fluorescent light promotes PE synthesis, while red light inhibits it. - The mean filament length in red-illuminated cultures is about 50 μm, compared to 460 μm in fluorescent-illuminated cultures. 2. **Metabolic Experiments**: - **Continuous Fluorescent Illumination**: PE is stable in fluorescent-illuminated cultures, with no significant degradation observed. - **Red to Fluorescent Light Transfer**: Transfer from red to fluorescent light initiates PE synthesis, but only a small fraction of the new PE is derived from pre-existing proteins. - **Fluorescent to Red Light Transfer**: Transfer from fluorescent to red light stops PE synthesis and causes a decrease in PE content and total radioactivity, likely due to transcellular filament breakage. 3. **Conclusion**: - Light influences intracellular PE levels by regulating its synthesis. The decrease in PE content in red-illuminated cultures is not due to degradation but to filament fragmentation, which results in cell lysis. This study provides insights into how light conditions affect the metabolic and morphological changes in *Fremyella diplasiphon*, highlighting the role of PE in complementary chromatic adaptation.