Bioavailable iron titrations reveal oceanic Synechococcus ecotypes optimized for different iron availabilities.

Area of Science:

• Marine microbial ecology
• Ocean biogeochemistry
• Phytoplankton physiology

Background:

• Iron (Fe) is a critical trace metal controlling phytoplankton activity and diversity in surface oceans.
• High-nutrient, low-chlorophyll (HNLC) waters often experience Fe-limitation, yet cyanobacteria like Synechococcus thrive.
• Evidence suggests the existence of Fe-adapted Synechococcus ecotypes, but experimental proof is limited.

Purpose of the Study:

• To investigate the transcriptional responses of microbial communities to varying iron availabilities in the Southern Ocean.
• To identify and characterize potential Fe-adapted Synechococcus ecotypes.
• To understand the short-term adaptive mechanisms of phytoplankton to Fe supply.

Main Methods:

• Analysis of metatranscriptomes from on-deck incubation experiments with a gradient of Fe availabilities.
• Comparison of transcriptional signatures under low-Fe and high-Fe conditions.
• Integration of transcriptomic data with in situ surface conditions.

Main Results:

• Transcriptomic signatures revealed co-occurring Synechococcus ecotypes adapted to different Fe regimes.
• Functional analyses identified Fe-acquisition mechanisms enabling Synechococcus persistence under Fe-limitation.
• The study confirmed the ecological relevance of these mechanisms and the presence of both ecotypes in situ.

Conclusions:

• Phytoplankton communities in the HNLC Southern Ocean exhibit short-term transcriptional responses to Fe availability.
• Distinct Fe-acquisition strategies allow for the persistence of different Synechococcus ecotypes under varying Fe conditions.
• This research enhances understanding of phytoplankton adaptation to changing Fe supply in marine environments.