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Inorganic phosphorus (Pi) is an essential macronutrient for all organisms playing an integral role in the structure and function of key biomolecules). At low concentrations, Pi can limit primary productivity whereas at high levels, Pi can become a pollutant causing eutrophication of water bodies.  Further understanding of the biogeochemical cycling of Pi in soils is required to inform future environmental management of finite global Pi reserves and to minimise environmental impacts of over-use.

In recent years, the oxygen isotope composition of dissolved phosphates (δ18OPO4) have been used to try and track Pi sources). However, distinct δ18OPO4 values of the source material do not retain their isotope signature during biogeochemical cycling and much remains unknown about the effects of biological processing of P. In laboratory culture experiments, phosphate is completely cycled by the bacteria, resulting in complete oxygen isotope exchange between phosphate and water ). However, in the natural environment, phosphate is only completely cycled when present as a limiting nutrient. At high phosphate concentrations, δ18OPO4 values fall between the source material and equilibrium values (Figure 1). It is not known whether this is a two component mixing between source and equilibrium δ18OPO4 values or whether other processes are fractionating the source resulting in intermediate δ18OPO4 values.

We know that a) no significant O isotope fractionation occurs during adsorption-desorption and sequestration b) E. coli preferentially take up lighter isotopologues of phosphate with a fractionation factor of -3‰; this fractionation is unknown for other organisms and c) microbial dissolution of different sediment P phases result in distinct δ18OPO4 values of dissolved P. To date, however, it is not known whether any isotope fractionation occurs during partial dissolution of minerals by organic acids (although it is suggested in soils) or if this fractionation depends on the organic acid present.

The overall aim of this proposal is to examine isotope fractionation during dissolution of phosphate minerals by organic and inorganic acids, specifically:

  • Determine the isotope fractionation (if any) that occurs during partial dissolution of minerals by a range of organic and inorganic acids (recent studies suggest that phosphate release is highly dependent on the type of acid)
  • Determine the effect of microbial mediated organic acids on fractionation
  • Determine the effect of metabolically active microorganisms on fractionation
Figure 1: Processes affecting δ18OPO4 during dissolution and microbial uptake


The first part of this work will be to develop and characterise a synthetic calcium phosphate material, which will be then used for the experimental part of this project. δ18OPO4 isotope characterisation of this standard material will provide a useful resource for the wider community. Following the calcium phosphate development, a range of batch experiments will be used to determine the effect of a range of inorganic and organic acids (none microbially mediated) on the phosphate material. Environmental parameters, such as temperature and pH will be consistent between experiments and measured at each sampling stage.

Stage 2 - will progress the complexity of the experiments by investigating how microbially mediated organic acids effect the isotope composition by extracting the organic acids from  a range of phosphate solubilizing bacteria and determining the phosphate fractionation effects resulting from this.

Stage 3 - will involve isolation of microorganisms from soil samples. Further batch experiments will introduce these bacteria to stimulate biotic production of organic acids and monitoring of phosphate dissolution as in stage 2. From the fundamental understanding gained from initial experiments, the study will develop into ground truthing results in experimental soils. Thus the overall aim of a greater understanding of environmental cycling P in field-based scenarios can be gained.

Training and Skills

NERC CENTA students are required to complete 45 days training throughout their PhD including a 10 day work placement. In the first year, students will be trained as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to CENTA research themes.

The project will be divided between experimental microbiology work at the Open University and isotope geochemistry at the NERC Isotope Geoscience Facility at the British Geological Survey headquarters in Nottingham. Thus the student will gain world class training in both microbiology methods including isolation and culturing at the OU and training in isotope ratio mass spectrometry techniques at the NERC national facility. This combination of training  will give the student a unique portfolio of skills in geomicrobiology and an opportunity to increase knowledge of the mechanisms of the P biogeochemical cycle.


Year 1:  Initial induction, literature review and familiarisation with the OU sample collection. Fieldwork in India (3 weeks).  Sample preparation, initial data collection. Laboratory trip to Perth (2 weeks)

Year 2: Interpretation of first data set. Presentation at national conference. Second laboratory trip to Perth (2 weeks). Work placement (2 weeks).

Year 3:  Interpretation of second data set. Preparation of papers. Presentation at an international conference. Writing and submission of thesis.

Partners and collaboration (including CASE)

The student will be based at the Open University in Milton Keynes and join a vibrant microbiology group that has extensive experience in weathering experiments. The CASE partner will be the world leading NERC Isotope Geoscience Facility at the British Geological Survey headquarters in Nottingham. As the CASE partner, the student will receive isotope geochemistry training here for a significant proportion of the project.

Further Details

Please contact Karen Olsson-Francis (k.olsson-francis@open.ac.uk) for further information.

Applications should include:


Applications should be sent to


by 5 pm on Monday 22nd January 2018