The Effect of Copper Limitation on the Bioavailability of Cu-Organic Complexes

We examined the bioavailability of synthetic organic Cu complexes (Cu-L) and inorganic Cu species (Cu') to <i>Thalassiosira oceanica</i> growing under Cu-limiting and Cu-inhibiting conditions. Copper bioavailability depended on the phytoplankton 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, the growth rate of Cu-inhibited cells at high [Cu'] varied inversely with [Cu²⁺] 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']. Our analysis shows that at [Cu'] typical of the open sea, <i>T. oceanica</i>-like phytoplankton are diffusion limited and must rely on Cu-organic complexes to fulfill their Cu requirements for growth.