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 (rETRmax ) 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 ETRmax 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.