The effects of copper on growth, photosynthesis and carbonic anhydrase activity in four marine diatoms were investigated under controlled conditions. Two species (Amphora acutiuscula and Nitzschia palea) were collected from the southeast Vietnamese coast, and the other two (Amphora coffeaeformis and Entomoneis paludosa) were native from the French Atlantic coast. Adding 1.5 and 3.0 µM of Cu to artificial seawater did not affect the growth rate, but did reduce the maximum cell density in all four diatom species. Photosynthetic parameters were determined from gross photosynthesis versus irradiance (P vs. E), and relative electron transport rate versus irradiance (rETR vs. E) curves. In A. acutiuscula, a similar pattern was observed for the P vs. E and rETR vs. E curves, indicating that 3.0 µM Cu significantly affected all photosynthetic parameters. In the other three species, the P vs. E and rETR vs. E curves did not show this pattern. The effect of Cu on photosynthesis depended on species, either increasing or reducing the electron transport rate in the thylakoid and oxygen production. External carbonic anhydrase activity followed a similar pattern to gross photosynthesis at growth irradiance, indicating it plays a major role in the supply of inorganic carbon to carboxylase(s).