School of Civil, Environmental and Mining Engineering

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Eric Raes

Phone: (+61 8) 6488 2446


Start date

Jun 2012

Submission date

Jun 2016


Eric Raes

Eric Raes profile photo


Is nitrogen fixation the key driver of planktonic CO2 uptake in the eastern Indian Ocean ?


Deep-water nitrate (NO3), brought to the oligotrophic surface waters through upwelling and convection has long been considered the key source of new nitrogen supporting phytoplankton growth. However, increasing knowledge on the phylogenetic diversity of N2 fixing organisms (or diazotrophs) in the oceans as well as their global distribution, has revised our understanding of the global N cycle. Biological N2 fixation has been shown to be a key source of new nitrogen for primary production and carbon export in tropical and sub-tropical neritic and open ocean environments. The diazotrophic organisms have become the focus of global interest and marine nitrogen fixation has become recognized a core component in ocean and atmospheric CO2 coupling.

Our primary hypothesis is to test:

Whether nitrogen fixation is the key process facilitating planktonic CO2 uptake in the eastern Indian Ocean.

The study aims to:

1. Measure N2-fixation rates in the eastern Indian Ocean and compare these rates with vertical nitrate fluxes from the deep sea.

2. Identify phylogenetic N2 fixing groups, with molecular techniques. A microarray will be used to determine the presence of most genes in used in the nitrogen cycle.

3. Test how sub µmolar cycling of fixed N impacts phytoplankton and heterotrophic microbial community structures. Quantify the impact how the diazotrophic community can pass the low δ15N signature of fixed nitrogen up the food chain and/or into the deep sea.

4. Use new satellite algorithms incorporating data from the Integrated Marine Observing System (IMOS) to determine offshore Trichodesmium blooms that fix atmospheric nitrogen thereby incorporating it into the marine ecosystems.

The data following from this study will contribute as a major piece of new information in global N fixation rates and provide new insights that will allows us to better understand, predict, manage the impacts of human and climate-induced changes.

Why my research is important

The Indian Ocean has the third-largest water mass globally and covers, from an anthropological point of view, vital sea lanes that sustain Asia’s largest economies. The Indian Ocean is home to ~40% of the world’s population and boarders over one-third of Australia’s coastline.

The International Geosphere-Biosphere Program and the UN Scientific Committee on Ocean Research have highlighted the importance to understand the biogeochemical role of the Indian Ocean in a global oceanic context (CLIVAR Pub. Series No.100, 2006). A Science Plan for Indian Ocean research where N-fixation by Trichodesmium and its potential iron limitation are identified as key unknowns has therefore been developed by the Sustained Indian Ocean Biogeochemistry and Ecosystem Research program (SIBER Science Plan, 2011 Theme 4).

This study will contribute regional knowledge of important global fluxes of key elements from a largely unstudied region. These are important in understanding phytoplankton ecology and biogeochemical cycles in one of the largest water masses off Australia.

The questions nowadays will be to assess how the accelerated anthropogenic climate change will impact the tight linkage between N2 fixation and the world’s ocean productivity and what factors will influence N2 fixation as a marine feedback system on a global scale for C fixation.


  • Australian Postgraduate scholarship from the University of Western Australia
  • CSIRO Wealth from Oceans postgraduate top-up scholarship.

Trichodesmium slicks, take aboard the Southern Surveyor, in the eastern Indian Ocean

School of Civil, Environmental and Mining Engineering

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Last updated:
Thursday, 19 September, 2013 11:39 AM