The Effect of Copper\nLimitation on the Bioavailability\nof Cu-Organic Complexes

We examined the bioavailability of synthetic organic\nCu complexes\n(Cu-L) and inorganic Cu species (Cu′) to Thalassiosira oceanica growing under Cu-limiting and Cu-inhibiting conditions. Copper bioavailability\ndepended on the phytoplankton Cu nutritional state and whether Cu′\ndiffusion to the cell surface was sufficient to meet the cellular\ndemands for growth. Under Cu-limiting conditions when [Cu′]\nwas less than the diffusion concentration threshold (DCT: 10<sup>–14.13</sup> M), growth rate was a hyperbolic function of [Cu-L]. Increasing\n[Cu′] above the DCT caused growth rate to increase proportionally,\nbut growth rate also increased with increasing [Cu-L] so that both\nCu′ and Cu-L were bioavailable. Short-term photosynthesis assays\nconducted under Cu-limiting conditions showed a similar response to\nCu speciation. In contrast, the growth rate of Cu-inhibited cells\nat high [Cu′] varied inversely with [Cu<sup>2+</sup>] and was\nindependent of [Cu-L], as previously reported. The change in Cu-L\nbioavailability correlated with expression levels of genes encoding\nthe reduction-dependent, high-affinity Cu uptake pathway, which was\nregulated by [Cu′]. Our analysis shows that at [Cu′]\ntypical of the open sea, T. oceanica-like phytoplankton are diffusion limited and must rely on Cu-organic\ncomplexes to fulfill their Cu requirements for growth.