Siderophore cycling in the North Pacific Subtropical Gyre

Across large regions of the surface ocean, the essential micronutrient iron (Fe) limits the growth of microbes, including phytoplankton and heterotrophic bacteria. In order to adapt to chronic low Fe conditions, some heterotrophic bacteria are able to produce siderophores, which are small organic ligands with extraordinarily high stability constant for Fe(III), to facilitate Fe uptake. The recently developed mass spectrometry based techniques allow for the characterization and quantification of siderophores in seawater for the first time, and the use of siderophores as biomarker of Fe limitation on heterotrophic bacteria. However, there has been no depth resolved transect data on siderophores distribution, and the prevalence, turnover, and of controlling factor of siderophores in the ocean remain unknown. In this thesis, I report the distribution of siderophores in the upper 1000 m of the US GEOTRACES GP15 Pacific Meridional Transect. Siderophore concentration ranges from 0-70 pM, and there is no correlation between the concentration of siderophores and dissolved Fe (dFe, <0.2 μm). In contrast, most siderophore hotspots, defined by concentration that is higher than 20 pM, are associated with a dFe:NO3- ratio that is lower than 50 μmol/mol, suggesting that the distribution of siderophores which are N rich molecules might be controlled by dFe:NO3- ratio. In the North Pacific Subtropical Gyre, hotspots of siderophores are found at 200-400 m. In the upwelling regions where nitracline is lifted to shallower depths, the hotspots are also lifted, to up to 20 m. The transect is dominated by marinobactins, a suite of non polar siderophores with a peptidic head group for Fe binding and a fatty acid chain. In addition, we found that the same siderophore could bind either Fe or Al, which is the first time Al-siderophores are reported in seawater samples. The hotspots of siderophore could be explained by either a dynamic turnover, or slow turnover and accumulation. To investigate the turnover of siderophores, I report the measurement of 57Fe uptake from siderophores by heterotrophic bacteria at Station ALOHA, which is representative for the North Pacific Subtropical Gyre - using enclosed bottle incubations. Between 200 and 400 m, we found almost complete consumption of 57Fe-siderophores for all of the siderophores over a period of 5 days. At the end of the incubation, approximately 60% of the added siderophores were present as nonmetallated apo siderophores. The presence of apo siderophores demonstrates that the drawdown of 57Fe-siderophore was largely due to Fe uptake from siderophores. Therefore, heterotrophic bacteria are Fe limited at 200-400 m, which is in accordance with the high concentration of siderophores in the North Pacific Subtropical Gyre. Our results reveal that the zone of potential Fe limitation in the ocean extends well below the euphotic zone and includes oligotrophic gyres that are not classically Fe limited.

Grantor dc:publisher

Massachusetts Institute of Technology and Woods Hole Oceanographic Institution

Year dc:date.issued

2023

Author dc:creator

• Li, Jingxuan

Advisor dc:contributor.advisor

• Repeta, Daniel J.

• Siderophore
• Metal
• North Pacific Subtropical Gyre

Statement dc:rights

• © 2023 Jingxuan Li All rights reserved. The author hereby grants to MIT and WHOI permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created.

Language dc:language.iso

en_US

DOI

10.1575/1912/66364

OAI identifier oai:identifier

oai:darchive.mblwhoilibrary.org:1912/66364