Publikationsdatum:
2020-10-12
Beschreibung:
recent study using Fe-limited phytoplankton strains, showed that iron (Fe) uptake rates normalized by cellular
surface area were best related to dissolved iron (dFe) concentrations as the inorganic Fe (Fe’) supply rates were
not sufficient to satisfy the Fe biological demand. Short-term (24 h) shipboard incubations with the in-situ
phytoplankton community were used to measure Fe uptake rates that were normalized per biomass (as particulate
organic carbon, POC). Fe uptake rates measured following 55FeCl3 additions (0.05 to 0.9 nM) were fitted
to different Fe pools (dFe, Felabile, and Fe’) using the Michaelis-Menten equation. Data showed a similar high
conditional stability constant for biological transporters across all sites and phytoplankton size classes, with only
a 2-fold variation in the concentrations of cellular transporters. These observations are in line with previous
reports that eukaryotic phytoplankton takes up Fe close to the limit imposed by transporters cellular density and
uses similar high-affinity Fe uptake systems. To further explore the link between Fe uptake rates and Fe
chemistry, we also studied the effect of Fe additions preequilibrated with different Fe-binding ligands (L) including:
the siderophore desferrioxamine B, two carbohydrates (glucuronic acid and carrageenan) and two
different bacterial exopolycarbohydrates (L6 and L22, referred as EPS). For all stations, phytoplankton were able
to acquire Fe associated to DFB as previously reported, however, different Fe:L ratios prevent quantitative
comparison with other studies. Iron bound to carbohydrates, glucuronic acid, carrageenan and EPS could enhance
or decrease Fe uptake rates in comparison to equimolar FeCl3 addition. These results illustrate that the
effect of such L on Fe uptake rates will depend on the in-situ plankton community and their chemical structure.
The variation of the Fe’ concentrations was able to explain up to 69% of the Fe uptake rates observed for the
Antarctic communities. This relationship with Fe’ was related to the fact that the Fe’ maximal supply, due to the
dissociation of FeL, was enough to satisfy the measured Fe uptakes rates. Calculations using previous reports in
contrasted regions of the Southern Ocean, showed that Fe’ maximal supply was greater than Fe uptake rates
measured in 80% of the cases. Moreover, considering photo- and redox-chemistry as well as kinetical situations
prevailing in the field, Fe’ should not be overlooked as a pool able to satisfy most of the Fe biological demand.
Finally, this study points towards the potential that the GEOTRACES Fe chemical speciation data represent to
explore Fe uptake rates at a larger scale in this vast Fe-limited oceanic region.
Repository-Name:
EPIC Alfred Wegener Institut
Materialart:
Article
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