Phytoplakton metallomics: effect of Ocean acidification on iron sequestration – PHYTOMET

Rising atmospheric CO2 affects the ocean and marine ecosystems through changes in the carbon cycle and through acidification of surface waters. The present project deals with this latter process, which is expected to change metal speciation and availability. It is of particular relevance to anticipate the changes in iron availability to phytoplankton as a result of ocean acidification and to understand how phytoplankton species will adapt more generally with respect to the metabolism of transition metals to this changing environment. Our project is organized around 3 work packages (WPs). WP#1 aims at characterizing the major iron pool associated with model marine micro-algae: the surface iron pool. We have preliminary evidence showing that iron binds massively and specifically to the cell wall of diatoms and coccolithophores. Both silicifying and calcifying phytoplankton drive the ocean's biological carbon pump, so if substantial amounts of iron are associated with their cell walls, the biological carbon pump will also directly affect the biogeochemical cycling of iron. We will use different chemical and physical techniques to study the nature of this surface iron in model species of calcifying and silicifying micro-algae (E. huxleyi, P. tricornutum, T. oceanica). In particular, i) we will generate a comprehensive sorption database to fully quantify the relative importance of the main processes involved in the speciation of iron and competing metal ions during sorption and interaction with living organisms and ii) we will apply a combination of synchrotron-based techniques to identify the cellular distribution of metals and their chemical forms in micro-algae, and more generally to study bio-mineralization processes. In a second stage, we will study the fate of this iron pool in acidifying conditions (higher pCO2/lower pH). With WP #2, we will characterize iron uptake mechanisms in representative marine micro-algae, in connection with surface iron and with the cell metallome. This WP is tightly related to WP1 (binding), since iron binding to the cell wall is most probably part of the strategy used by phytoplankton to take up iron. The use of stable (54Fe, 56Fe, 57Fe, 58Fe) and radioactive (55Fe, 59Fe) isotopes will allow us to follow the fate of the surface iron pool, to investigate the role of proteins induced by iron starvation in iron uptake and sequestration, and to study the role of other transition metals (Zn, Cu) in iron metabolism as a function of the pCO2. We will also pay special attention to siderophore-based iron uptake, since species that are able to use siderophores as iron sources should be less affected than others by changes in iron speciation resulting from ocean acidification, and our recent findings suggest that siderophores could play a larger role in the ocean carbon cycle than currently appreciated. In WP #3 we will investigate the molecular mechanisms involved in the adaptation of marine micro-algae to acidification by using Ostreococcus as a model. We will analyze ecotypes described as more tolerant to high CO2 concentrations for their iron requirements and iron uptake mechanisms. Reciprocally, we will check if the low iron requirement ecotypes on which we are currently working are more or less adapted to more acidic conditions, and we will use O. tauri as a model organism to study the response of cells to increased pCO2, by transcriptomic (RNA-seq) proteomic and metallomic approaches.
This research will be supported by innovative state-of-the art analytical techniques addressing the aspects of iron speciation in different compartments of micro-algae species and a global (metallomics) approach linking the iron concentrations with those of other transition elements. Specialists of analytical chemistry and of biophysics (Lobinski, Isaure), of geochemistry (Benedetti), of molecular biology (Bowler) and of biochemistry (Lesuisse, Sutak) will bring their skills and expertise in this collaborative project