Probing the Bioavailability of Dissolved Iron to Marine Eukaryotic Phytoplankton Using In Situ Single Cell Iron Quotas. Issue 8 (25th August 2021)

• Record Type: Journal Article
• Authors: Shaked, Yeala; Twining, Benjamin S.; Tagliabue, Alessandro; Maldonado, Maria T.
• Is Part Of: Global biogeochemical cycles. Volume 35:Issue 8(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-08-25
• Subjects: iron bioavailability -- iron limited phytoplankton -- iron uptake rates -- single cell iron quota -- standardized proxy for availability; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2021GB006979
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: We present a new approach for quantifying the bioavailability of dissolved iron (dFe) to oceanic phytoplankton. Bioavailability is defined using an uptake rate constant (kin‐app ) computed by combining data on: (a) Fe content of individual in situ phytoplankton cells; (b) concurrently determined seawater dFe concentrations; and (c) growth rates estimated from the PISCES model. We examined 930 phytoplankton cells, collected between 2002 and 2016 from 45 surface stations during 11 research cruises. This approach is only valid for cells that have upregulated their high‐affinity Fe uptake system, so data were screened, yielding 560 single cell k in‐app values from 31 low‐Fe stations. We normalized k in‐app to cell surface area (S.A.) to account for cell‐size differences. The resulting bioavailability proxy ( k in‐app /S.A.) varies among cells, but all values are within bioavailability limits predicted from defined Fe complexes. In situ dFe bioavailability is higher than model Fe‐siderophore complexes and often approaches that of highly available inorganic Fe′. Station averaged k in‐app /S.A. are also variable but show no systematic changes across location, temperature, dFe, and phytoplankton taxa. Given the relative consistency of k in‐app /S.A. among stations (ca. five‐fold variation), we computed a grand‐averaged dFe availability, which upon normalization to cell carbon (C) yields k in‐app /C of 42, 200 ± 11, 000 L mol C −1 d −1 . We utilize k in‐app /C to calculate dFe uptake rates and residence times in low Fe oceanic regions. Finally, we demonstrate the applicability of k in‐app /C for constraining Fe uptake rates in earth system models, such as those predicting climate mediated changes in net primary production in the Fe‐limited Equatorial Pacific. Plain Language Summary: In many oceanic regions, iron exerts strong control on phytoplankton growth, ecosystem structure, and carbon cycling. Yet, iron bioavailability and uptake rates by phytoplankton in the ocean are poorly constrained. Recently, Shaked et al. (2020) established a new approach for quantifying the availability of dissolved Fe (dFe) in natural seawater based on its uptake kinetics by Fe‐limited cultured phytoplankton. Here, we extend this approach to in situ phytoplankton, establishing a standardized proxy for dFe bioavailability in low‐Fe oceanic regions. Bioavailability is estimated through single cell Fe uptake constants, calculated by combining measured Fe contents of individual phytoplankton cells collected from multiple regions with concurrently measured dFe concentrations, as well as modeled growth rates. We then utilize this proxy for: (a) comparing dFe bioavailability among organisms and regions; (b) calculating dFe uptake rates and residence times in low‐Fe oceanic regions; and (c) constraining Fe uptake parameters of earth system models to better predict ocean productivity in response to climate change. Key Points: A proxy for dissolved Fe bioavailability in low‐Fe regions is established from Fe quotas, dissolved Fe concentrations, and modeled growth rates In situ phytoplankton cells record high and relatively uniform dissolved Fe bioavailability across many low‐Fe oceanic regions The new proxy is applicable for calculating in situ Fe uptake rates and biological Fe residence times and for validating global model output