Direct Heme Uptake by Phytoplankton-Associated Roseobacter Bacteria

Area of Science:

• Marine microbiology and biogeochemistry
• Iron acquisition mechanisms in bacteria
• Phytoplankton-bacterial interactions

Background:

• Iron is a vital micronutrient limiting marine microbial growth.
• Organic iron acquisition pathways, like heme transport, are crucial for marine bacteria.
• Understanding iron cycling in ocean ecosystems is key to productivity.

Purpose of the Study:

• Investigate the molecular basis of heme transport in phytoplankton-associated Roseobacter.
• Explore the role of bacterial heme uptake in the marine environment.
• Determine if host cues influence heme transport regulation.

Main Methods:

• Genomic analysis of 153 Roseobacter genomes for heme transport systems.
• Coculture transcriptome analysis of Sulfitobacter sp. strain SA11.
• Generation and competition experiments with a Ruegeria sp. strain TM1040 hmuR mutant.

Main Results:

• Nearly half of Roseobacter genomes possess complete heme transport systems.
• Heme transport gene expression is downregulated during mutualistic growth with diatoms, suggesting host cue influence.
• The hmuR gene is essential for heme and hemoprotein uptake; wild-type bacteria outcompete mutants for lysed diatom iron.

Conclusions:

• Heme transport is an effective strategy for phytoplankton-associated bacteria to access host iron post-lysis.
• Marine metagenomes show limited heme transport systems, suggesting specialized populations.
• Direct heme transport facilitates rapid iron recycling from decaying phytoplankton into the microbial loop.