Physical and chemical forcing of diazotrophy in the (sub)-tropical Atlantic Ocean

Lead Research Organisation: University of Liverpool
Department Name: Earth, Ocean and Ecological Sciences


The oceans play a central role in the global carbon cycle, and have taken up ca. 30-40% of the anthropogenically produced CO2. It has long been known that ocean biota play a major role in sequestering CO2 on very long time scales (>1000 y). Recent evidence also suggests that the ocean biota play an important role on shorter time scales (10-100 y). The balance between phytoplankton photosynthesis and community respiration determines the ability of the oceans to take up CO2. Nitrogen (N) is generally considered to be the nutrient that limits phytoplankton photosynthesis. However, it is unclear what controls the amount of N in the ocean. Unlike most phytoplankton, which are N-limited, N2 fixing cyanobacteria (diazotrophs) have an unlimited supply of N in the form of N2 gas. N2-fixers play a significant role in ocean nutrient and biogeochemical cycles as they are a major source of N, providing N for up to 50% of primary productivity in nutrient poor oceanic regions. N2 fixation is a key process that modulates the ability of the oceans to sequester CO2 on time scales of 10 to 10,000 y. Limitation of N2 fixation results in lowered N availability for other primary producers reducing the potential of oceans to sequester carbon. Whilst the colony forming Trichodesmium is considered the most important oceanic diazotroph, recently a range of new diazotrophs have been discovered in the ocean. This brings us to the questions of 'what constrains the amount of N2 fixation in the ocean?', and 'what determines the species distribution of diazotrophs in the ocean?' Iron appears to be the key environmental factors constraining N2 fixation based on a recently observed direct link between Fe and N2 fixation in the Atlantic, with Fe determining surface ocean P cycling. The goal of this project is to investigate quantitatively the link between iron supply and N2 fixation in the Atlantic, and for this it is essential to understand the importance and strengths of various iron sources. The iron sources are considered to be atmospheric dust deposition and low oxygen shelf sediments in the NW African upwelling region. The strengths of these sources are expected to change in future with changes in dust deposition and expansion of the oxygen minimum zones in the oceans. Identification and quantification of the sources is hence key to undertake model estimates of N2 fixation under future climate scenarios. This proposal will relate the supply and biogeochemical cycling of Fe and P to N2 fixation and the community structure of diazotrophs in the (sub)-tropical Atlantic Ocean. We will undertake this research using a combination of ship-board observations and radiotracer uptake experiments, and modeling activities involving nutrients and Fe. We will quantify sources of these elements for the diazotroph community from the atmosphere and ocean circulation, and by use of chemical source tracers. We will link the supply of nutrients and Fe to the activity and species distribution of diazotrophs. Molecular techniques will be used to determine the different diazotrophs in the study region. We will undertake the work on a dedicated cruise in the (sub)-tropical Atlantic which involves east-west transect from the African shelf to characterise the trace element gradient in the oxygen minimum zone and thus the potential for lateral advection from the shelf. The cruise will also traverse the dust/redox plumes in the study region and characterize the horizontal trace element gradients along the edges of the dust/redox plumes. We will sample the common diazotroph Trichodesmium and study its uptake of Fe and P using radiotracers. We will use a circulation model to provide a large scale context for the programme, with sources and cycling of nutrients and Fe adapted according to the observational studies. This research will ultimately assist with oceanographic studies on nutrient cycling and modeling with a view on the future importance of the oceans as C sink.
Description 1. We have provided the first simultaneous measurements of phosphate uptake, alkaline phosphatase activity and DOP production in the tropical Atlantic.

2. Model output predicts that there is net production of DOP in the upwelling and shelf region of the eastern tropical Atlantic. Our observations support this view.

3. We have written a review of our current understanding of the hydrolytic enzyme, alkaline phosphatase, and how organisms use this enzyme to access DOP. Experiments conducted on the research cruise linked to this grant (specifically, nutrient, dust and zinc additions) have provided novel and timely results on the impact of trace metals on phosphorus biogeochemistry. This will be taken forward in a new standard grant proposal to be submitted in January 2015.

4. We submitted a NERC standard grant building on work performed during this grant and it was awarded a 9 and received funding in June 2015.

5. Work from this grant also lead to the lead PI being invited to the Gordon Research Conference to take about the research.
Exploitation Route The findings from this project could be used to demonstrate how we verify model output using observations. We have published the results in Global Biogeochemical Cycles.
Sectors Environment

Description NERC standard grant
Amount £1,100,000 (GBP)
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 01/2017 
End 01/2020
Description Invited to European Nitrogen Fixation Conference in Stockholm in August 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Invited to speak in the first Marine Workshop at the European Conference on Nitrogen fixation. The ENFC is attended by mostly terrestrial scientists and this was the first time that there was a special session on marine nitrogen fixation.
Year(s) Of Engagement Activity 2018