The photosynthetic machinery of Spirulina maxima was studied when subjected to induced reactive oxygen species (ROS) to examine the organism's responses to stress. Significant decreases in both photosynthetic efficiency and growth rate were observed. Exposure to 0.01 mmol H2O2/(g cell), which induced the lowest specific intracellular ROS level (siROS) led to a 15% decrease in specific growth rate: an increase in siROS by 70-fold led to a 25% decrease in specific growth rate. Similarly, siROS induced by 0.01 mmol H2O2/(g cell) led to 15% inhibition in phytosynthetic efficiency, while an increase in siROS by 40- or 70-fold led to about 60% inhibition in photosynthetic efficiency. To further understand the effects of induced ROS on photosynthetic machinery, we performed a detailed pigmentation analysis as well as analysed Phycobilisomes (PBS), Photosystem II (PSII), and Photosystem I (PSI), the three important components of cyanobacterial photosynthetic apparatus. We found carotenoids (beta-carotene and lutein) to be most sensitive to siROS. Also, specific levels of phycocyanin and allophycocyanin, which are important PBS pigments, decreased significantly in response to H2O2. Further, electron transport assays revealed that ROS cause damage primarily to PSII, whereas they do not significantly affect PSI in comparison; siROS induced by 0.01 mmol H2O2/(g cell) led to a 15% inhibition of PSII, and increase in siROS by 9-, 40-, and 70-fold led to 22%, 36%, and 46% inhibition, respectively.