The influence of ocean acidification on phytotransferrin-mediated iron uptake rates in phytoplankton

Project Information

IRONBIND

Grant agreement ID: 844733

DOI

10.3030/844733

Project closed

EC signature date 24 April 2019

Start date 1 June 2019

End date 31 May 2020

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 87 403,20

EU contribution

€ 87 403,20

Coordinated by

ALFRED-WEGENER-INSTITUT HELMHOLTZ-ZENTRUM FUR POLAR- UND MEERESFORSCHUNG
Germany

Periodic Reporting for period 1 - IRONBIND (The influence of ocean acidification on phytotransferrin-mediated iron uptake rates in phytoplankton)

Reporting period: 2019-06-01 to 2020-05-31

Summary of the context and overall objectives of the project

The goal of Project IRONBIND is to determine whether elevated concentrations of atmospheric carbon dioxide (CO2) negatively affect the growth of marine phytoplankton by interfering with a key iron acquisition mechanism. This mechanism, phytotransferrin-mediated iron binding, requires high concentrations of carbonate ion to bind dilute concentrations of iron. At the sea surface, dissolved carbonate is inversely proportional to atmospheric CO2, thus as global CO2 concentration rises from the combustion of fossil fuels, seawater carbonate concentration decreases, potentially interfering with the ability of phytoplankton to bind and internalize iron. This interference may be critical in oceanic regions where low concentrations of iron already limit phytoplankton growth. Phytoplankton in these ‘iron limited’ regions support highly productive fisheries and marine ecosystems and function as a critical sink for planetary CO2. Changes to the ability of phytoplankton to acquire iron would likely lead to a decrease in productivity and reduce carbon drawdown rates.
Phytotransferrin was first described in 2018, and phytotransferrin-encoding gene sequences can be found in most major phytoplankton groups. However, the phytotransferrin mechanism has only been characterized in a model organism, and iron-carbonate co-limitation has never been documented in the environment. To fill these knowledge gaps, project IRONBIND set out to answer three fundamental questions: How widespread is the phytotransferrin mechanism? Can the effect of carbonate on iron uptake be measured in the environment? And if there is an effect, what are the ramifications for climate change and the global carbon cycle?

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Phytotransferrin was first described in 2018, and phytotransferrin-encoding gene sequences can be found in most major phytoplankton groups. However, the phytotransferrin mechanism has only been characterized in a model organism, and iron-carbonate co-limitation has never been documented in the environment. To fill these knowledge gaps, project IRONBIND set out to answer three fundamental questions: How widespread is the phytotransferrin mechanism? Can the effect of carbonate on iron uptake be measured in the environment? And if there is an effect, what are the ramifications for climate change and the global carbon cycle?

To determine whether carbonate co-limited iron uptake is common, IRONBIND collaborated with the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) to measure iron uptake rates in environmentally relevant species of Southern Ocean phytoplankton, including diatoms, coccolithophores, cryptophytes, chlorophytes and Phaeocystis. All tested phytoplankton showed a negative influence of low carbonate on iron uptake rates, particularly in the absence of light. The effect was consistent whether synthetic or natural iron chelators were evaluated. To determine whether this effect could be detected in the natural marine environment, IRONBIND collaborated with the Department of Environmental Affairs, Republic of South Africa, to participate in the SCALE expedition to the spring ice edge in the Southern Ocean. IRONBIND was able to confirm that carbonate co-limitation could be detected in iron uptake rates from natural populations of marine phytoplankton, and that acidification has an effect on phytoplankton growth rates. The Covid-19 pandemic of 2020 has delayed some of the analytical work; however, IRONBIND is currently preparing the dataset for modelling to provide broader ecological and biogeochemical context. These results will be archived in the PANGAEA Data Publishing portal for Earth & Environmental Science, hosted by the Alfred Wegener Institute and available to the public. Additionally, the results will be disseminated at the 2021 Ocean in a High CO2 World conference in Lima, Peru, and are being prepared for publication in a high-impact, broad interest journal.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Phytoplankton growing in iron limited regions like the Southern Ocean have an outsized role in biogeochemical cycles and the sequestration of atmospheric carbon dioxide. IRONBIND’s central hypothesis is that in iron-limited regions, the concentration of carbonate acts as a secondary control on growth, and that carbonate loss driven by ocean acidification may act to further depress phytoplankton growth. If true, this hypothesis points to the undiagnosed positive feedback loop in climate change, whereby increasing concentrations of atmospheric CO2 leads to a decrease in phytoplankton growth and therefore a decrease in carbon removal in the climate-critical Southern Ocean.

Project IRONBIND acquires seawater from the Antarctic ice edge