Climate-driven shifts in upwelling reorganize biological iron cycling in the Equatorial Pacific HNLC, revealed by siderophores

This study investigates how the El Niño–Southern Oscillation (ENSO) modulates iron (Fe) cycling in the Equatorial Pacific, a high-nutrient, low-chlorophyll (HNLC) region. The researchers sought to determine whether changes in upwelling intensity—which alters both nutrient supply and the residence time of surface waters—shift the biological strategies used by microbial communities to acquire and recycle iron.

The authors analyzed seawater samples from two meridional transects across the Equatorial Pacific during contrasting climate states: a La Niña period (strong upwelling) and a transition toward El Niño (weaker upwelling). They quantified dissolved Fe-binding ligands, specifically siderophores, using liquid chromatography-inductively coupled plasma mass spectrometry (LC-ICP-MS). These chemical data were integrated with flow cytometry for microbial community structure, shipboard Fe-addition experiments, and historical nutrient datasets to characterize the Fe-limitation state of the ecosystem.

The study reveals two distinct modes of Fe cycling. During La Niña, strong upwelling fueled a diatom-dominated community that exhibited classic signs of Fe stress, such as elevated silica uptake and lower Fe-to-nitrogen ratios. In contrast, during the transition to El Niño, weaker upwelling favored a community rich in large heterotrophic bacteria. This community utilized high concentrations of siderophores, which facilitated efficient Fe recycling and resulted in less pronounced Fe stress. The authors conclude that siderophores are not merely binary indicators of Fe stress but reflect the dominant mode of Fe cycling in the system.

Understanding these mechanisms is critical for predicting how HNLC regions will respond to climate change. As global warming potentially alters the frequency and intensity of ENSO events, this research provides a mechanistic basis for anticipating shifts in primary productivity, microbial community composition, and the biological carbon pump in the world's most productive ocean regions.