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Implications of the mesopelagic Remineralization for the OceaN Iron Cycle

Final Report Summary - IRON-IC (Implications of the mesopelagic Remineralization for the OceaN Iron Cycle)

The IRON-IC project seeks to better understand the functioning of the ocean’s iron (Fe) cycle through investigations of the bacterial remineralization of particulate iron (PFe) within the mesopelagic zone. The bioavailability of Fe has been shown to limit primary production in up to 50% of the ocean’s waters. As a result of its function in modulating Earth’s climate, the role of Fe supply on the oceanic carbon cycle has received widespread attention over the last two decades. However, much of the research to date has focused on new sources of Fe, and little attention has been directed toward controls on the supply of recycled Fe. Within the mesopelagic zone (100-1000 m), heterotrophic bacterial activity results in a dramatic decrease in the organic carbon exported from surface waters. While understanding the processes that occur in this layer is critically important for predicting future changes in the ocean’s carbon cycle, bacterial remineralization and the fate of PFe in the mesopelagic are, paradoxically, poorly understood.
Considering the objectives of the IRON-IC project (i.e. quantify the release of dissolved iron (DFe) during the bacterial remineralization), an instrument able to measure simultaneously the bacterial remineralization rates and DFe release in the mesopelagic was needed. The RESPIRE (Respiration of Sinking Particles In the subsuRface ocEan) trap allows in situ measurements of the bacterial remineralization of the sinking particles in the mesopelagic zone by measuring the depletion of oxygen due to the bacterial degradation of particles. A trace metal (TM) clean version of the RESPIRE trap has been developed by the fellow in order to measure in situ and simultaneously the bacterial remineralization rates and DFe release within the mesopelagic zone.

The in situ measurements of the PFe remineralization rate in the mesopelagic, which represent a major experimental challenge, has been performed for the first time on the R/V Investigator in March-April 2016 during the Eddies cruise (Southern Ocean). The RESPIRE traps have been deployed within the upper mesopelagic (150-200 m depth) of two cyclonic and anticyclonic eddies. Several additional parameters have been measured in order to characterize the downward particulate flux (particulate organic carbon, particulate trace elements, thorium isotopes), the pool of dissolved elements (dissolved trace metal and iron-binding ligands concentrations), and the heterotrophic community activity (bacterial activity and abundance, excess non-lithogenic particulate barium). In addition, a second set of deployments has been performed successfully at the SOTS (Southern Ocean Time-Series) site in March 2017. These two sets of deployments performed in the Southern Ocean allow a better understanding of iron and carbon cycling in an ocean region that is central to the climate.

Within the framework of the Peacetime project http://peacetime-project.org/ the TM RESPIRE traps have been deployed in the Mediterranean Sea in May 2017. The goal of this project is to study the impact of Saharan dust deposition on the cycle of chemical (trace) elements, on the biogeochemical functioning of the pelagic ecosystem and finally on carbon export to the deep ocean. During this cruise, the TM RESPIRE traps were deployed before, during, and after Saharan dust deposition events. The comparison of the iron remineralization patterns within these two contrasting regions (i.e. the Southern Ocean and the Mediterranean Sea) characterized by contrasting Fe supply mechanisms (recycled vs. new iron) enables us to determine whether the fate of PFe at depth is primarily driven by particle properties (biogenic vs. lithogenic) that are imprinted within the surface ocean or in mesopelagic particle transformations.

Since the speciation (the physico-chemical state of an element) of DFe released through bacterial remineralization has major consequences on its bioavailability and residence time in the dissolved phase, the concurrent release during bacterial particle remineralization of iron-binding ligands, their size class (soluble or colloidal), and the bioavailability of the released DFe to the surface community has been investigated using field work and in vitro experiments. Two different in vitro experiments have been performed at sea. In April 2015, suspended (1-51 µm) and sinking (>51 µm) particles were collected in the upper mesopelagic (43°25’S, 148°25’E). After the resuspension of these particles in filtered seawater (<0.2 µm), the bacterial degradation of the particles and the subsequent release of trace elements and iron-binding ligands within the soluble (<0.02 µm) and colloidal (0.02-0.2 µm) phases were followed during two weeks at in situ temperature in the dark.
In March-April 2016, a complementary experiment has been performed in order to determine the bioavailability of the DFe released during mesopelagic remineralization. After a one-week incubation period of particles (along with the particle-attached bacteria community) collected in the upper mesopelagic, the DFe released during remineralization was mixed with the surface community. The response of the different communities (heterotrophic bacteria, nano and micro-phytoplankton) was followed for a week. Results from the two in vitro experiments demonstrated that (1) small suspended particles (1-51 µm) constitute the main source of DFe in the mesopelagic, while sinking particles (>51 µm) represent a scavenging substrate for DFe, and (2) DFe released during mesopelagic remineralization constitute a significant source of bioavailable DFe for phytoplankton and heterotrophic bacteria communities in the surface ocean (after either upwelling or the deepening of the surface mixed layer).

Finally, the effect of iron limitation on phytoplankton stickiness, cell aggregation and in fine the export of these aggregates toward the deep ocean has been investigated during a laboratory experiment. Two diatom cultures (Chaetoceros neglectus and Pseudo-Nitzschia heimii) where iron is the limiting nutrient have been set up. These iron-limited (and iron-replete) cultures have been used to assess the impact of iron limitation on the cell stickiness, the release of exopolysaccharides substances (EPS) and transparent exopolymeric particles (TEP), and on the aggregation process. The experimental set-up, using a roller table and a couette chamber, was developed during the first year of the project. The fellow demonstrated that iron limitation triggers the aggregation of Pseudo-Nitzschia heimii by increasing the cell stickiness. This increase in cell stickiness is mainly driven by an increase in the EPS bound to the cell surface. At the opposite, the stickiness of Chaetoceros neglectus is not influenced by iron limitation since aggregation of this species is mainly driven by its morphological characteristics (e.g. presence of spines). These results could explain some important findings observed during ocean iron fertilization experiments and help to better understand the differences observed in term of carbon export following the decline of algal blooms.

For the first time, the IRON-IC project provides in situ measurements of the bacterial remineralization rate of PFe within the mesopelagic, investigate the residence time of the recycled iron via the measurement of the concurrent release of iron-binding ligands in the soluble and colloidal phases, and finally assess how environmental parameters, such as atmospheric dust deposition and surface primary production are impacting the iron cycle in the mesopelagic zone. By providing needed parameterization of key processes in global biogeochemical models, the results obtained will be beneficial to the oceanic biogeochemistry community as well as the entire “climate change” research community.