Dominant oceanic bacteria secure phosphate using a large extracellular buffer.

Area of Science:

• Marine microbiology
• Biogeochemistry
• Oceanography

Background:

• SAR11 and Prochlorococcus are abundant marine bacteria crucial for nutrient cycling.
• These microbes inhabit phosphate-limited environments like the North Atlantic subtropical gyre.
• Their phosphate uptake mechanisms in varying oceanic productivity zones remain incompletely understood.

Purpose of the Study:

• To investigate how SAR11 and Prochlorococcus bacteria manage phosphate acquisition in different oceanic conditions.
• To determine the role of extracellular phosphate buffering in bacterial growth and survival.
• To elucidate the relationship between phosphate availability, cellular demand, and uptake kinetics.

Main Methods:

• Utilized flow sorting to isolate and analyze phosphate-pulsed and chased cells.
• Quantified labile extracellular phosphate buffers in Prochlorococcus and SAR11.
• Employed mathematical modeling to interpret phosphate uptake dynamics and buffer saturation.

Main Results:

• Demonstrated that Prochlorococcus and SAR11 maintain large extracellular phosphate buffers (5-40 times chromosome replication needs).
• Observed that buffer size inversely correlates with cellular phosphate uptake rates.
• Found that phosphate uptake becomes marginal in phosphate-replete tropical waters due to buffer saturation.

Conclusions:

• Extracellular buffer stocking is a key adaptation for SAR11 and Prochlorococcus, ensuring growth by reducing dependency on fluctuating ambient phosphate.
• This strategy allows these bacteria to thrive in oligotrophic environments and manage phosphate acquisition effectively.
• Buffer saturation explains counter-intuitive lower phosphate uptake in more productive tropical waters.