1. Department of Biology, McGill University, 1205 Avenue Dr Penfield, Montreal, Quebec H3A 1B1, Canada

Correspondence to: Graham Peers¹ Correspondence and requests for materials should be addressed to G.P. (Email: graham.peers@mail.mcgill.ca). The sequences reported here have been deposited in the UniProt knowledgebase under accession number P84800.

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 availability^{3, 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 strains^{5, 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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