Pathways of copper acquisition in phytoplankton

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Metal uptake by phytoplankton is fundamentally important because it links the reactivity of metals in the environment to their physiological and ecological effects in the organisms. Understanding how metals are taken up enables us to predict which chemical forms of metals are bioavailable and which types of phytoplankton will respond to changes in their concentrations. Diatoms and cyanobacteria are two groups of phytoplankton of great significance to aquatic ecosystems. They grow in the open sea in surface waters containing some of lowest levels of metals on Earth although they are exposed to high concentrations transiently because of inputs from dust and upwelling from deep waters. Copper (Cu) is of interest because it is an essential metal for phytoplankton growth and one of the most toxic. Phytoplankton adjust their uptake systems to buffer changes in metal concentrations in the environment to maintain optimal cellular levels, but the details of the uptake pathways are only now being revealed. Copper is utilized by oceanic diatoms by an uptake pathway that uses a chemically reduced form of Cu, Cu(I). The Cu(I) is produced by cellular reduction, but little is known about how cellular reduction is regulated or whether other phytoplankton acquire Cu in the same way. Other groups, like cyanobacteria, may use different pathways for Cu uptake and hence react to different chemical forms of Cu than diatoms. Cyanobacteria, for example, produce compounds that bind Cu to detoxify it, changing the chemistry of Cu in the water and thereby altering its availability to other species. The aims of this research proposal are: 1) to measure the bioavailability of Cu chemical species as a function of Cu nutritional state of diatoms and cyanobacteria, 2) to assess the prevalence of the Cu(II) reduction uptake pathway among different taxonomic groups of phytoplankton and its activation by light, and 3) to describe the Cu-regulated transcriptome of two cyanobacteria and identify differentially expressed genes including putative Cu transport genes induced by Cu deficiency. The proposed research is part of a continuing effort to elucidate the factors regulating phytoplankton growth in the sea. It will provide a mechanistic understanding of Cu uptake by representatives of two ecologically important groups of phytoplankton and identify the potential ecological and biogeochemical consequences of variations in Cu availability. The work is timely given that ocean acidification is predicted to greatly increase the concentration of some inorganic species of Cu and affect Cu complexation by dissolved organic matter. How phytoplankton will be affected by these changes is uncertain as we have a rudimentary understanding of their mechanisms of Cu uptake and know little about the types of Cu species that are available for cellular transport. The research outlined here will begin to fill these gaps in knowledge.