We examined the bioavailability of synthetic organic Cu complexes (Cu-L) and inorganic Cu species (Cu') to Thalassiosira oceanica growing under Cu-limiting and Cu-inhibiting conditions. Bioavailability depended on Cu nutritional state and whether [Cu'] diffusion to the cell surface was sufficient to meet the cellular demands for growth. Under Cu limiting conditions when [Cu'] was less than the diffusion concentration threshold (DCT: 10^-14.13 M), growth rate was a hyperbolic function of [Cu-L].  Increasing [Cu'] above the DCT caused growth rate to increase proportionally, but growth rate also increased with increasing [Cu-L] so that both Cu' and Cu-L were bioavailable.  Short-term photosynthesis assays conducted under Cu-limiting conditions showed a similar response to Cu speciation.  In contrast, growth rate of Cu-inhibited cells at high [Cu'] varied inversely with [Cu²⁺] ([Cu']) and was independent of [Cu-L], as previously reported.  The change in Cu-L bioavailability correlated with expression levels of genes encoding the reduction-dependent, high affinity Cu uptake pathway, which was regulated by Cu' availability.  Our analysis shows that at [Cu'] typical of the open sea, T. oceanica-like phytoplankton are diffusion limited and must rely on Cu-organic complexes to fulfill their Cu requirements for growth.