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Copper-containing plastocyanin used for electron transport by an oceanic diatom

Abstract

The supply of some essential metals to pelagic ecosystems is less than the demand, so many phytoplankton have slow rates of photosynthetic production and restricted growth1. The types and amounts of metals required by phytoplankton depends on their evolutionary history2 and on their adaptations to metal availability3,4, which varies widely among ocean habitats. Diatoms, for example, need considerably less iron (Fe) to grow than chlorophyll-b-containing taxa2, and the oceanic species demand roughly one-tenth the amount of coastal strains5,6,7. Like Fe, copper (Cu) is scarce in the open sea, but notably higher concentrations of it are required for the growth of oceanic than of coastal isolates8. Here we report that the greater Cu requirement in an oceanic diatom, Thalassiosira oceanica, is entirely due to a single Cu-containing protein, plastocyanin, which—until now—was only known to exist in organisms with chlorophyll b and cyanobacteria. Algae containing chlorophyll c, including the closely related coastal species T. weissflogii, are thought to lack plastocyanin and contain a functionally equivalent Fe-containing homologue, cytochrome c6 (ref. 9). Copper deficiency in T. oceanica inhibits electron transport regardless of Fe status, implying a constitutive role for plastocyanin in the light reactions of photosynthesis in this species. The results suggest that selection pressure imposed by Fe limitation has resulted in the use of a Cu protein for photosynthesis in an oceanic diatom. This biochemical switch reduces the need for Fe and increases the requirement for Cu, which is relatively more abundant in the open sea.

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Figure 1: Growth rate of Thalassiosira as a function of intracellular Cu concentration.
Figure 2: Isolation of plastocyanin from the oceanic diatom Thalassiosira oceanica.
Figure 3: PSII fluorescence transients of Thalassiosira.

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Acknowledgements

We thank R. Popovic for use of the PEA and M. R. Di Falco for purifying the trypsin-digested peptides. This work was supported by grants from the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Centre for Environmental Bio-Inorganic Chemistry (CEBIC, Princeton University).

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Correspondence to Graham Peers.

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The sequences reported here have been deposited in the UniProt knowledgebase under accession number P84800. Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

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Peers, G., Price, N. Copper-containing plastocyanin used for electron transport by an oceanic diatom. Nature 441, 341–344 (2006). https://doi.org/10.1038/nature04630

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