The interplay between iron supply and demand shapes the future iron limitation of ocean microbes

The future evolution of iron limitation of ocean microbes will be a crucial component shaping the response of the carbon and other biogeochemical cycles, as well as impacts on upper trophic levels, by regulating net primary production (NPP). However, the cycling of iron in the ocean is distinct from the major nutrients nitrate and phosphate, and significant uncertainties remain regarding the representation of the internal cycling linked to biological activity and scavenging and their representation in models. This is important because microbes will be responding to the interactive influences of changing iron, light and temperature. Here we use the state-of-the-art ocean biogeochemical model PISCES to explore how the ocean iron cycle evolves to modulate ocean NPP over the coming century under the RCP8.5 scenario. There is a clear mosaic in the response of the upper ocean iron cycle to climate change, driven by regional variations in resource limitation and the sign of projected changes in NPP. In particular, the future ocean will experience changes in both the availability of iron and the complex cellular demand for iron driven by the modified ocean environment. Moreover, modifying the model assumptions regarding cellular iron stoichiometry and the extent of iron storage, as well as the iron cost associated to key cellular processes can cause significant changes in the projected NPP change at regional scales. The magnitude and sign of projected NPP changes may be dampened, amplified or even reversed depending on the assumptions varied, with implications for regional projections of carbon cycling and ecosystem impacts. Improved observational constraints on cellular iron stoichiometry of different microbes, alongside models that account for emerging feedbacks linked to iron uptake pathways and climate will improve our understanding of how environmental change will impact ocean microbes.