Iron, Copper and Oceanic Phytoplankton – ICOP

Laboratory cultures pf phytoplankton will be grown at different degrees of Fe-Cu (co-)limitation. The parameters analyzed during these experiments will be used in two models (biotic and abiotic), as well as newly acquired data from the international GEOTRACES program. The coupling of these two models will be included in the global oceanic biogeochemical model PISCES.

The culture and modelling experiments conducted during ICOP will allow to consider phytoplankton dynamics and impact on the major biogeochemical cycles from the cell scale to the global scale. Such questions are central to a better understanding of the climate system, and are recognized of interest by international research programs like GEOTRACES, SOLAS, IMBER.

To come.

To come.

Oceanic phytoplankton plays a key role in the atmospheric concentrations of climate-active gases like carbon dioxide (CO2) and dimethylsulfide (DMS). Diatoms are a major oceanic primary producer and export large quantities of organic carbon and biogenic silica (BSi) to the deep ocean. According to their possible role in DMS production, they may also have to be considered in sulfur budgets.
Diatom growth is however limited by iron (Fe), an essential micronutrient, in more than 40% of the ocean. Iron may also play a role in the silicification of diatoms, and subsequently on biogenic silica degradation and C export. However, we still do not understand how Fe affects the silicification process, and in particular Si-C interactions at the cell level, rendering it difficult to predict how biogenic silica degrades and to quantify accurately C and Si export.
The role of Fe in the elemental composition of other groups of phytoplankton than diatoms has also been largely overlooked. Picophytoplankton are also Fe-limited, and may play an important role in C export.
The biological demand for iron may also be linked to copper (Cu). Phytoplankton may indeed use Cu-containing proteins instead of Fe, and need Cu for Fe acquisition. Species of the diatom Pseudonitzschia produce domoic acid as an organic ligand for Fe and Cu to assimilate these two micronutrients. Cu ligands are also produced by Synechococcus and are not available to eukaryotes: this may be a strategy to compete for Fe.
It seems then more and more likely that iron and copper interact to affect oceanic phytoplankton production, and competition between diatoms and picophytoplankton. There are strong indications that Cu might also not only partly controls the intensity of the Fe impact, but may even control primary production by itself. However to date no study investigated the combined role of Fe and Cu on the major biogeochemical cycles.

Within ICOP, we plan:
- to study the impact of Fe and Cu on (i) the elemental ratios and metal quota of Pseudonitzchia and Synechococcus, and the DMS cycling in cultures of Pseudonitzschia, (ii) the silicification processes in Pseudonitzschia and their impact on BSi degradation,
- to study how Pseudonitzschia and Synechococcus interact when Fe, Cu and Fe-Cu limit their growth, notably via the quantification of Fe and Cu acquisition strategies.
- to evaluate the biogeochemical significance of Fe-Cu interactions at the global scale.

These research topics lean on an internationally acknowledged expertise of LEMAR on oceanic Fe and Si biogeochemical cycles. Within ICOP, young researchers will innovatively extend the research on these cycles by (i) including copper, and its potential impact on Fe and DMS biogeochemistry, (ii) better understanding Si-C interactions in diatoms and their impact on degradation and C export. These axes will also enable to develop a joint research on Fe, Cu, Si and phytoplankton interactions, and to collaborate with other young researchers in France. Also, the education of students at the Master and PhD level will help to build the new generation of young researchers.
This project is strongly interdisciplinary as it will consider phytoplankton dynamics and impact on the major biogeochemical cycles from the cell level to the global, using complementary experimental and modeling approaches. Such questions are central to a better understanding of the climate system, and are recognized of interest by international research programs like GEOTRACES, SOLAS, IMBER. Results from ICOP will be presented during international conferences and published in international peer-reviewed journals. They will also be presented during conferences and on a web site for public outreach.