Microbial iron limitation in the ocean's twilight zone

Area of Science:

• Oceanography
• Marine microbial ecology
• Biogeochemistry

Background:

• Ocean primary production relies on nutrients like nitrate, phosphate, and iron (Fe).
• Microbial metabolism in the mesopelagic 'twilight zone' (200-500m) is assumed to be limited by organic carbon, not nutrients.
• Nutrient's role in mesopelagic microbial production remains understudied.

Purpose of the Study:

• Investigate the distribution and uptake of siderophores, indicators of microbial iron deficiency.
• Assess the prevalence of iron limitation in the mesopelagic twilight zone of the eastern Pacific Ocean.

Main Methods:

• Collected water samples along a meridional transect in the eastern Pacific Ocean.
• Measured siderophore concentrations as biomarkers for microbial iron deficiency.
• Analyzed siderophore uptake to infer microbial iron limitation.

Main Results:

• High siderophore concentrations were observed in both surface waters and the twilight zone of the North and South Pacific subtropical gyres.
• These findings indicate widespread microbial iron deficiency in the mesopelagic twilight zone.
• This suggests iron availability, not just organic carbon, limits microbial metabolism in this region.

Conclusions:

• Bacterial iron deficiency is likely common in the mesopelagic twilight zone across major ocean basins.
• This expands the known extent of nutrient-limited microbial metabolism in the ocean.
• The findings have significant implications for understanding the ocean's biological carbon cycle and carbon storage capacity.