Iron bioavailability to phytoplankton: an empirical approach

Area of Science:

• Marine Biology
• Biogeochemistry
• Physiological Ecology

Background:

• Iron is a critical micronutrient for phytoplankton, often limiting primary productivity in aquatic ecosystems.
• Understanding iron bioavailability is essential for modeling phytoplankton growth and ocean biogeochemical cycles.

Purpose of the Study:

• To quantify iron (Fe) bioavailability to various phytoplankton species using different iron substrates.
• To establish a framework for comparing Fe uptake efficiencies across phytoplankton groups.

Main Methods:

• Measuring iron uptake rate constants for diverse phytoplankton species under iron-limited conditions.
• Utilizing model iron substrates: unchelated iron (Fe') and a strong iron-binding molecule (FeDFB).
• Correlating iron uptake rates with phytoplankton surface area (S.A.).

Main Results:

• Eukaryotic phytoplankton showed uptake rate constants proportional to S.A. for both Fe' and FeDFB.
• FeDFB uptake constants were approximately 1000 times lower than Fe' uptake constants.
• An empirical "bioavailability envelope" was defined, with Fe' and FeDFB as upper and lower limits, respectively.
• Cyanobacteria exhibited similar Fe' uptake but lower FeDFB uptake compared to eukaryotes.

Conclusions:

• The "bioavailability envelope" provides a robust tool for assessing iron bioavailability and phytoplankton iron uptake capabilities.
• Phytoplankton iron uptake appears to have reached a universal upper limit, suggesting constraints in the uptake mechanism.