Single walled carbon nanotubes were carboxylated by microwave assisted acid oxidation (f-SWCNTs) and examined for their ecotoxicity on marine alga chlorophyte Dunaliella tertiolecta. Toxicity was evaluated based on growth, photosynthetic activities, oxidative stress, and intracellular glutathione in the concentration range of 0.1-20 mg/L f-SWCNT. Physical interactions between the f-SWCNT and alga were examined using light microscopy and scanning electron microscope. Increasing the nanotube concentration increased the toxic effects where growth inhibition was as high as 30%, photosynthetic yield decreased by as much as 18%, and intracellular glutathione reduction reached 95%. The results from f-SWCNTs were somewhat different when compared to our previous study using the same algae and functionalized multiwalled carbon nanotubes, where exposure led to longer lag phase and higher growth rate inhibition.