The footprint of iron-manganese limitation of the biological carbon cycle in a changing climate

The importance of iron in driving net primary production (NPP) and the biological carbon pump across the Southern Ocean has been explored in numerous studies. However, the potential role for manganese, essential to oxygen production and combating oxidative stress, has not received the same attention despite the noted physiological inter-dependencies between iron-manganese and that both are strongly depleted in Southern Ocean. In the sixth climate model intercomparison project, earth system models (ESMs)  project increasing NPP in the Southern Ocean due to supply of additional iron, while the global trend shows a decline. Similar mechanisms also describe the role of the ocean carbon cycle during the last glacial maximum. However, under increasing iron supply, more manganese is required to fulfil phytoplankton growth, and the neglect of manganese limitation in ESMs can further increase the uncertainty of future NPP in the Southern Ocean under future or past climate change.

Here we use a hierarchy of experiments with the state-of-the-art global ocean biogeochemical model PISCES-QUOTA, including explicit manganese limitation, to explore how the physiological traits govern iron and manganese stress in response to a changing climate. Our results show that manganese is deficient throughout much of the Southern Ocean, but iron is generally the limiting resource. Explicitly representing iron and Mn co-limitation through oxidative stress enhances the extent of manganese deficiency, especially for diatoms. Traits associated with photophysiological adaptation and management of oxidative stress may be unique in Antarctic plankton and are critical in modulating the footprint of both iron and manganese stress and hence the impacts on the carbon cycle in a changing climate. Overall, our results indicate that both iron and manganese are key determinants of the impact of climate change on the Southern Ocean, with a notable role for region-specific adaptive and acclimatory responses that require further constraint.