Are iron nanoparticles in wet deposition a potential source of bioavailable Fe to marine algae?

Summary

Mineral dust provides nutrients such as iron and phosphorus to marine ecosystem, which is a major sink for atmospheric carbon dioxide. These nutrients have been shown to promote the growth of CO2-fixing marine microorganisms such as phytoplankton, and therefore indirectly affect the climate system. A quantitative understanding of this effect is essential to model the Earth's climatic systems and therefore to predict climate change. However, this effect is currently inadequately modeled because the bioavailable iron flux from atmospheric depositions (both dry and wet), a key parameter to link two major components of earth system models (i.e., AEROSOLS and ECOSYSTEMS components in HadGEM2), is poorly quantified. The bioavailable iron flux to the ocean is dependent on (1) the total dust deposition flux, (2) total iron content in the dust and (3) bioavailability of iron in the dust. In these three parameters, iron bioavailability is the most uncertain one.

As a significant and sometimes predominant source of iron to the ocean, we choose to focus on wet deposition in this work. Currently, models often assumed that all the measured dissolved Fe in the wet deposition is bioavailable. However, a large proportion of Fe in the 'dissolved' fraction of rainwater is present as nanoparticles, which have been identified (and visualized) in one of our recent studies. Therefore, to improve the model estimates of the bioavailable iron flux to the global ocean, it is essential to quantify the bioavailability of iron nanoparticles in the rainwater.

The goal of this work is to determine how bioavailable are the Fe nanoparticles in natural rainwater to a model organism Thalassiosira pseudonana, a typical marine phytoplankton (centric diatom). This will contribute to a more accurate estimate of the bioavailable iron flux to the global ocean for earth system models. The specific objectives of this project are:
1) To demonstrate that laboratory synthesized Fe nanoparticles with organic coatings can provide bioavailable Fe to Thalassiosira pseudonana
2) To quantify the bioavailability Fe nanoparticles in laboratory-processed mineral dust (after simulated acid and then cloud processing) to Thalassiosira pseudonana
3) To quantify the bioavailability of Fe nanoparticles in natural rainwater to Thalassiosira pseudonana

These objectives are designed to answer questions of three levels of complexity. The experiment to realize each objective will provide data to interpret the more complex and more environmentally representative later experiments. These objectives will be met by laboratory nanoparticle synthesis, nanoparticle separation and characterization as well as laboratory marine algal culturing. State-of-the-art techniques such as Scanning Transmission Electron Microscopy, nanoparticle tracking analysis, Flow Field Flow Fractionation and Cross Flow Ultrafiltration available at the NERC Facility for Environmental Nanoscience Analysis and Characterisation (FENAC) will be used for nanoparticle separation and characterization. The geochemical and microbiology laboratories in the University of Birmingham will be used for nanoparticle synthesis and marine algal culture. The expertise of the investigator in atmospheric chemistry and global biogeochemical cycles is complemented by that of Named Researcher, Dr. Michala Pettitt, in microbiology. Our expertise is further enhanced by those of Prof. Jamie Lead and Prof. Roy Harrison in nanoscience and environmental sciences at the University of Birmingham.

The scientific results will be published in international peer-reviewed journals and fed to global models such as UK Chemistry and Aerosol Model (UKCA) to improve the estimation of atmospheric bioavailable iron deposition flux, which can be readily fed to and Met Office Earth system model (HadGEM3) to predict the impact of atmospheric nutrient input on ocean productivity and climate in the present and the future.

The proposed work is entirely consistent with NERC's Strategy 2007-2012 for developing Earth System Science and Climate System. It is in line with Meeting the Challenge in Earth System Science focusing on the iron biogeochemical cycle. It is also consistent with Meeting the Challenge in Climate System Science by improving the estimate of an important parameter in earth system models, the bioavailable Fe deposition flux to the ocea.

Potential beneficiaries of this work include (1) Global climate and earth system modellers; (2) General public and third parties; (3) Wider science community.

1. Global climate and earth system modellers: One of the key aims of the NERC and Met Office Earth System Model is to quantify the long term trends in dust deposition, ocean biological activity, and carbon dioxide draw down. However, an important uncertainty in these models is the bioavailable Fe flux to the global ocean, which is the key parameter that links the two major compoents of the earth system models (Aerosols/Atmospheric Chemistry and Ecosystems). The proposed research would provide the experimental results that can be used to improve the estimation of this important parameter. This would be an important step for increasing the capability of global climate and earth system models to understand the present climate system and predict climate change. Therefore, in the long-run, the proposed research will contribute to science based policy-making in national and international level including IPCC, and therefore this research will have indirect impact in the regulatory arena.

2. General public, particularly young people: Iron deficiency in people results in anemia. Iron decifiency in the ocean limit the growth of marine microorganims (e.g., diatoms and nitrogen fixing bacteria). How these marine microorganisms respond to the additon of nutrients from rainwater is important in understanding how nature developed different mechanisms in releiving the "iron deficiency" in the ocean. The beauty of the diatoms and their potential impact on the global climate would inspire the interests of young people on the ocean. Therefore, the relevant knowledge in this project is useful for science education. Timescale of the impact is during and after the project.

3. Wider science community: The proposed research directly address the bioavailability of iron in dust (SOLAS Science and Implementation Plan, Activity 1.4; GEOTRACES Science Plan, Theme 1). The results of this proposal will benefit these communities by reducing uncertainties in the estimation of bioavailable nutrient fluxes at ocean interfaces, which have direct implications for studying air/sea gas exchange, nutrient cycling in the ocean and modelling. Timescale of the impact is during and after the project.