Microbial competition for iron determines its availability to the ferrous wheel.

Area of Science:

• Marine biogeochemistry
• Oceanic primary productivity
• Microbial ecology

Background:

• Iron is a critical nutrient regulating primary productivity in marine ecosystems.
• Iron biogeochemistry research traditionally focuses on sourcing new iron, overlooking recycling processes.
• Microbial recycling, termed the "ferrous wheel," is vital for sustaining ocean productivity.

Purpose of the Study:

• To investigate the role of microbes in iron recycling within the Southern Ocean.
• To differentiate iron uptake between heterotrophic bacteria and picophytoplankton.
• To assess the influence of light and depth on microbial iron uptake.

Main Methods:

• Studying iron uptake in microbes smaller than 2 µm in subantarctic waters.
• Quantifying iron uptake rates for heterotrophic bacteria and picophytoplankton separately.
• Examining seasonal and depth-related variations in iron uptake influenced by light.

Main Results:

• Picoeukaryote phytoplankton exhibited 10 times higher iron uptake rates than bacteria, per unit biomass.
• Light exposure stimulated phytoplankton iron uptake 8- to 16-fold and bacterial uptake 4- to 8-fold.
• Iron uptake varied seasonally and with depth, particularly under light influence.

Conclusions:

• Picoeukaryotic phytoplankton play a more significant role in the "ferrous wheel" than previously recognized.
• Light availability strongly influences the rate of iron recycling in the ocean.
• Understanding these microbial dynamics is crucial for predicting ocean productivity and iron cycling.