Acclimation of Phytoplankton Fe:C Ratios Dampens the Biogeochemical Response to Varying Atmospheric Deposition of Soluble Iron

Abstract

Dissolved iron (dFe) plays an important role in regulating marine productivity. In high nutrient, low chlorophyll regions (>33% of the global ocean), iron is the primary growth limiting nutrient, and elsewhere iron can regulate nitrogen fixation by diazotrophs. The link between iron availability and carbon export is strongly dependent on the phytoplankton iron quotas or cellular Fe:C ratios. This ratio varies by more than an order of magnitude in the open ocean and is positively correlated with ambient dFe concentrations in field observations. Representing Fe:C ratios within models is necessary to investigate how ocean carbon cycling will interact with perturbations to iron cycling in a changing climate. The Community Earth System Model ocean component was modified to simulate dynamic, group‐specific, phytoplankton Fe:C that varies as a function of ambient iron concentration. The simulated Fe:C ratios improve the representation of the spatial trends in the observed Fe:C ratios. The acclimation of phytoplankton Fe:C ratios dampens the biogeochemical response to varying atmospheric deposition of soluble iron, compared to a fixed Fe:C ratio. However, varying atmospheric soluble iron supply has first order impacts on global carbon and nitrogen fluxes and on nutrient limitation spatial patterns. Our results suggest that pyrogenic Fe is a significant dFe source that rivals mineral dust inputs in some regions. Changes in dust flux and iron combustion sources (anthropogenic and wildfires) will modify atmospheric Fe inputs in the future. Accounting for dynamic phytoplankton iron quotas is critical for understanding ocean biogeochemistry and projecting its response to variations in atmospheric deposition.

Biodiversity and Stoichiometric Plasticity Increase Pico‐Phytoplankton Contributions to Marine Net Primary Productivity and the Biological Pump

“…The diazotroph group is assigned the highest maximum iron quotas based on the high iron requirement of the nitrogen-fixing enzyme nitrogenase (Berman-Frank et al, 2007). Phytoplankton iron quotas are reduced linearly when in situ dissolved Fe concentrations fall below a prescribed threshold (FeOpt), set to 1.75 nM for diatoms, 1.6 nM for nano, 1.5 nM for diaz, 1.0 nM for peuk, and 0.8 nM for pro and syn (Wiseman et al, 2023).…”

On the Variability of Equatorial Pacific Nitrate and Iron Utilization

“…While the exact mechanisms of this preferential iron cycling are still being understood (Barbeau et al, 1996;Boyd et al, 2015Boyd et al, , 2017Bundy et al, 2024;Pham et al, 2022), there are clear signs that HNLC microbial communities work to retain iron in the system. For example, iron-stressed ecosystems can lower their Fe requirements (see Wiseman, 2023, andWiseman et al, 2023, andreferences therein). Additionally, retention of muchneeded iron may occur by siderophore production (Boiteau et al, 2016;Bundy et al, 2018;Manck et al, 2022), grazing, and viral lysates (Poorvin et al, 2004;Sato et al, 2007).…”