Optical Imaging of Uranium Biotransformations by Microorganisms (OPTIUM)

Lead Research Organisation: University of Manchester
Department Name: Chemistry

Abstract

One of the most pressing problems facing society today is the management of existing and future waste forms arising from nuclear energy production. Although radioactivity is naturally occurring in the environment, 60+ years of anthropogenic activities including mining, industrial nuclear power production, accidental release and military use of nuclear materials has led to greatly increased levels of radionuclides in the natural environment. Although, in many cases, the contamination is concentrated and not widespread, the impact of these radionuclides pose to the wider ecosystems is intricately linked to the bioavailability of the radionuclide in question, which is dictated by their concentration and chemical form (oxidation state and speciation). Given that the heavy metal uranium comprises the majority waste by mass, the chemical transformation of uranium from its water soluble, and therefore mobile form (uranyl(VI)) to essentially an insoluble, and therefore immobile form (uranium(IV) mineral forms) is an important strategy in managing safe disposal to prevent leaching.
Various microbial processes, often involving bacterially mediated redox transformations, have been suggested as viable bioremediation techniques. Typically these reactions are studied on the bulk level by X-ray absorption techniques, using purely quantitative techniques or on fixed (dead) cells by electron microscopy. There is currently a lack of techniques that are capable of quantitatively probing the distribution and micro- environment of radionuclides, particularly in living cells.
Here we propose to introduce the powerful technique of two-photon fluorescence microscopy using the intrinsic emissive signals of the uranyl(VI) cation to follow and unravel these microbial processes at the sub-micron level in vivo in order to gain a full understanding of the proposed bioremediation process in situ at high spatial resolution. Two-photon microscopy is currently widely used in biology to visualise cellular processes in three dimensions, but has not yet been used to image cellular processes that involve uranium. The fundamental photophysical properties of the uranyl cation will enable two-photon excitation in the less damaging near infra-red region of the electromagnetic spectrum compared to UV/visible excitation which is damaging to cells in a one photon process. The long-lived uranyl emission itself (cf. dyes) and inherent spatial control of two-photon excitation allow high-resolution visualisation of uranyl-containing biological material, while fluorescence lifetime mapping demonstrates the ability to visualise the microscopic redox conditions over the surface of U(VI)-reducing bacterial cells.
The first ever use of non-destructive 3D multi-photon optical imaging techniques combined with state of the art spectroscopy will be developed as a new technology in this research field and used as tools to address the challenge of understanding uranium speciation and reactivity in a range of biogeochemical systems, here, bacteria and fungi. We aim to exploit the intrinsic optical properties of the uranium ions as direct visible emissive probes as they interact with these microorganisms on chemical to more geologically relevant timescales. Our overall vision is to implement 3D optical imaging to both identify and image uranium ions and their speciation at a previously unseen level of detail (sub micron and sub ns timescale) and augment this with X-ray and electron microscopy approaches to create a new toolbox for understanding microbial and fungal systems that bioaccumulate, biotransform and biomineralise radiotoxic and environmentally hazardous actinide ions into less mobile forms. Working with a range of key stakeholders (e.g. Radioactive Waste Management Ltd., National Nuclear Laboratory), we can use this optical imaging technique to better predict radionuclide mobility at contaminated sites and inform disposal and land management in the UK and wider afield.

Planned Impact

This proposal is firmly grounded in the "environmental nuclear fission" area, but will also make important contributions across a range of other areas including photon science and nanotechnology, and analytical science, specifically the translation and adaptation of the technology of two-photon imaging and lifetime mapping at sub micron resolution in 3D. This is a new and rapidly emerging area of rapid sensing, identification and characterisation of biogeochemical transformations of uranium in line with the need for a sustainable future nuclear fuel cycle involving legacy clean-up operations, decommissioning and new ways to deal with future nuclear wastes.
Our primary dissemination routes will be through high impact peer reviewed journal articles and conference/workshop presentations that will introduce this optical imaging technique as part of a tool box of techniques to the nuclear community. Here, our key stakeholders from the National Nuclear Laboratories and Radioactive Waste Management Ltd. will be the source of primary industrial advice and they will advise on the best routes to follow and help the team disseminate their results and seek further advice/consultations from other nuclear companies that are potential users of this technology. These include the Nuclear Decommissioning Authority (NDA), Radioactive Waste Management Ltd. (RWM), Sellafield Ltd., Areva Mining and the Atomic Weapons Establishment (AWE), who have been previous past project partners on consortia grants. These are all potential end users of the proposed technology and we will ensure that we seek advice and showcase our results as appropriate.
Work will be disseminated at major international and national conferences, through online press releases and promotional material by the University of Manchester's Press Office when appropriate. We will aim to publish in leading chemistry and environmental science journals (e.g. Science, Nature Publishing Group, Journal of the American Chemical Society, Angewandte Chemie, Chemical Science, Environmental Science and Technology), as well as in specialist journals, to maximise impact. Future collaborations will be facilitated by links established through the PI and Co-Is active involvement with the EPSRCs Next Generation Nuclear Centre for Doctoral Training (NGN CDT) and the STFC Environmental Radiation Network in addition to the COST-CM1006 (European f-Element Network), the EU-Actinide NMR network and the EU Actinide TALISMAN network that the PI was heavily involved in. These networks give the PI and Co-Is opportunities to build collaborations with spectroscopists, nuclear scientists engineers and theoreticians. Mechanisms for communicating this work to the public and exploiting potential commercialisation opportunities and industrial applications are in place (see pathways to impact document).

Publications

10 25 50