PHYTOIRON

Marine micro-algae account for nearly half of the photosynthetic primary production on Earth. The major limiting nutrient in the ocean is iron, an element that is required in particular abundance by photosynthetic organisms. Iron fertilization of the oceans is known to induce the blooming of some species in vast oceanic areas, resulting in increased carbon fixation from atmospheric carbon dioxide, with unknown effects on the marine ecosystem. Reciprocally, the increase in atmospheric carbon dioxide results in a decrease in ocean seawater pH, which affects iron speciation, with again unknown consequences on the ecology of phytoplankton. Studies of the physiology of marine phytoplankton have made little progress in the last 50 years. In particular, nearly nothing is known about the mechanisms of iron assimilation by phytoplankton. All the iron uptake systems described so far in prokaryotes and eukaryotes have affinity constants in the micromolar range –a factor of 103-106 above seawater concentrations of iron. Therefore, there should be completely new mechanisms of iron uptake that await discovery in phytoplanktonic algae. With this project, we propose to elucidate the strategies/mechanisms of iron assimilation by representative species of key eukaryotic phytoplankton, diatoms, coccolithophorids, and alveolates. We have preliminary evidences indicating that iron can be assimilated through either a reductive or a nonreductive mechanism of uptake in some of these species. We will study how these mechanisms are regulated (according to iron availability, and according to the circadian light/dark cycles), and identify and characterize the molecular components involved in iron uptake and storage by these species. Specifically, we want to examine how iron (or iron starvation) impacts cell physiology, as a function of the mechanisms of uptake and storage at play in the different species, and how one or another mechanism of iron uptake might constitute a selective advantage for some species in defined environmental conditions –in particular in conditions of iron repletion/scarcity. Practically, the present project articulates around four specific tasks: 1) Iron requirements (form and amount of iron) for each of the selected species and general view of the iron uptake systems involved in each species (reductive or nonreductive strategy of uptake with or without the involvement of siderophores); 2) Molecular mechanisms of iron uptake in each species; 3) Intracellular iron storage and cell response to iron deficiency; 4) Adaptation of the species and ecotypes to their environment and inter-specific competition with respect to iron availability and to the relative efficiency of different uptake systems. We plan to validate our observations in situ, through meta-transcriptomic analysis of samples collected during a natural fertilization event (KEOPS 2). Transversally to these different tasks, we will focus on the probable involvement of the circadian clock in regulating iron uptake and metabolism along the day night/cycle, and reciprocally, on the involvement of iron in regulating the metabolism through the circadian clock. The present project constitutes the first attempt to characterize on a large-scale the iron uptake systems and the responses (at the cellular and population level) to iron supply/limitation in eukaryotic phytoplankton.