To investigate the effects of potassium on photosynthetic activity in the green alga Ulva pertusa, we enzymatically isolated protoplasts from thallus samples. Photochemical quenching showed that protoplasts were capable of electron transport by photosystem II (PS II) during illumination. This quenching was dependent on external pH, with a reduced electron transport rate at pH >6.8 and less ability to use ${HCO_3}^-$ under alkaline conditions. In the presence of external $Na^+$, $K^+$ enhanced PS II quantum yield, indicating a functional role for $K^+$ during photosynthesis. That yield was enhanced in a [$K^+$]-dependent manner, with maximum activity at 100 mM. However, potassium alone did not maintain photochemical activity, and its addition supported photosynthetic $O_2$ evolution only in the presence of $Na^+$. A deficiency of $K^+$ led to the production of superoxide anions. Because of that generation, activities of superoxide dismutase and ascorbate peroxidase, two key enzymes involved in scavenging reactive oxygen species in the water-water cycle, also increased during such stress. These results strongly suggest that a series of ROS-scavenging systems are initiated in Ulva chloroplasts in response to $K^+$ deficiency and that enzyme activities might protect algal cell photosynthesis.