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

High-nutrient, low-chlorophyll (HNLC) regions like the Equatorial Pacific are critical to the global carbon cycle, yet their productivity is often limited by the availability of iron (Fe). While it is well-established that El Niño–Southern Oscillation (ENSO) cycles modulate upwelling in this region, the specific impact of these climate-driven shifts on biological iron acquisition and recycling remains poorly understood. This study investigated the spatiotemporal relationship between Fe limitation and upwelling by comparing two research cruises: one during a strong La Niña (strong upwelling) and one during a transition toward El Niño (weaker upwelling). The researchers quantified dissolved Fe-binding ligands, siderophores, and microbial community structure to determine how these ecosystems adapt to varying Fe supply regimes.

The study reveals two distinct modes of Fe cycling in the Equatorial Pacific. During La Niña, the system was characterized by strong upwelling, high eukaryotic phytoplankton biomass, and classic signs of Fe stress, such as elevated silica uptake and increased production of excess Fe-binding ligands. In contrast, the transition to El Niño conditions, marked by weaker upwelling, favored a microbial community dominated by large heterotrophic bacteria. This community exhibited high siderophore concentrations, which the authors identify as a mechanism for efficient Fe recycling rather than a simple indicator of Fe stress. These findings suggest that upwelling does not just modulate the supply of Fe, but fundamentally reorganizes the biological pathways through which Fe is acquired and retained in the euphotic zone.

By demonstrating that ENSO-induced changes in upwelling reorganize the biological pathways of Fe cycling, this research provides a mechanistic basis for predicting how future climate-driven changes in ENSO variability will affect marine productivity and carbon sequestration. The results challenge the traditional view of siderophores as binary indicators of Fe stress, suggesting instead that their presence reflects the dominant mode of Fe cycling in a given ecosystem. This nuanced understanding is essential for accurately modeling the biogeochemical response of HNLC regions to a warming climate.