The influence of light on copper‐limited growth of an oceanic diatom, Thalassiosira oceanica (Coscinodiscophyceae)

Thalassiosira oceanica ( CCMP 1005) was grown over a range of copper concentrations at saturating and subsaturating irradiance to test the hypothesis that Cu and light were interacting essential resources. Growth was a hyperbolic function of irradiance in Cu‐replete medium (263 fmol Cu′ · L −1 ) with maximum rates achieved at 200 μmol photons · m −2  · s −1 . Lowering the Cu concentration at this irradiance to 30.8 fmol Cu′ · L −1 decreased cellular Cu quota by 7‐fold and reduced growth rate by 50%. Copper‐deficient cells had significantly slower ( P  < 0.0001) rates of maximum, relative photosynthetic electron transport ( rETR max ) than Cu‐sufficient cells, consistent with the role of Cu in photosynthesis in this diatom. In low‐Cu medium (30.8 fmol Cu′ · L −1 ), growth rate was best described as a positive, linear function of irradiance and reached the maximum value measured in Cu‐replete cells when irradiance increased to 400 μmol photons · m −2  · s −1 . Thus, at high light, low‐Cu concentration was no longer limiting to growth: Cu concentration and light interacted strongly to affect growth rate of T. oceanica ( P  < 0.0001). Relative ETR max and Cu quota of cells grown at low Cu also increased at 400 μmol photons · m −2  · s −1 to levels measured in Cu‐replete cells. Steady‐state uptake rates of Cu‐deficient and sufficient cells were light‐dependent, suggesting that faster growth of T. oceanica under high light and low Cu was a result of light‐stimulated Cu uptake.