Biomineralization of Sr by the Cyanobacterium Pseudanabaena catenata Under Alkaline Conditions

Lynn Foster,Gianni Vettese,Jonathan R Lloyd,Katherine Morris,Heath Bagshaw,Jon K Pittman,Kejing Zhang,Kurt F Smith,Adrian Cleary,David Sigee

doi:10.3389/feart.2020.556244

Abstract

A non-axenic culture of Pseudanabaena catenata, a close relative of the bloom-forming cyanobacterium found in the high pH First Generation Magnox Storage Pond (FGMSP) at the Sellafield Nuclear Facility, was supplemented with 1 mM of SrCl2, to determine its effect on the fate of Sr. The addition of 1 mM Sr to the P. catenata culture resulted in ~16 % reduction in the overall growth of the culture (OD600nm) and a 21 % reduction in the concentration of chlorophyll-a (Chl-a) compared to those without Sr. The fate of Sr was assessed using a multi-technique approach. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) showed that virtually all of the Sr was removed from solution, while extracellular biomineral precipitates were analysed using TEM analysis, and were shown to contain Sr, Ca and S using energy-dispersive X-ray spectroscopy (EDS) analysis. In addition, intracellular P-containing electron-dense features, likely to be polyphosphate bodies, were associated with the P. catenata cells and contained Sr. Bulk analysis of the cultures by X-ray diffraction (XRD) showed the presence of Ca-containing strontianite whilst EXAFS analysis showed the presence of strontium phosphate minerals. The presence of Sr associated with intracellular polyphosphate was unexpected, and contrasts with other model photosynthetic systems in the literature that have highlighted carbonate biominerals as the dominant sink for Sr. Understanding the fate of Sr within microorganisms associated with Spent Nuclear Fuel Ponds (SNFPs) is crucial to understanding the fate of radioactive 90Sr in such extreme environments, and could also suggest a potential remediation strategy for treatment of 90Sr contaminated waters from SNFPs and in contaminated aquatic systems.

Highlights

The generation of energy and the development of weapons by the nuclear industry has resulted in the production of significant levels of radioactive material (Wilson, 1996; Crossland, 2012)
The First Generation Magnox Storage Pond (FGMSP) situated on the Sellafield site contains a significant inventory of radioactive fission products, including 90Sr
This current study investigated the fate of Sr when incubated with a non-axenic culture of P. catenata, representative of the microbial community detected in a microbial bloom in the pond in August 2016 (Foster et al, 2020a; Foster et al, 2020b)

Summary

Introduction

The generation of energy and the development of weapons by the nuclear industry has resulted in the production of significant levels of radioactive material (Wilson, 1996; Crossland, 2012). A wide range of microorganisms, including bacteria, cyanobacteria, eukaryotic algae and fungi are known to influence the speciation and fate of radionuclides by either metabolism-independent or dependent mechanisms (Gadd, 1990). The majority of carbonate mineral formation is thought to occur as extracellular precipitates that can adsorb to a variety of surfaces including clays and extracellular features of microorganisms (Gadd, 1990; Schultze-Lam and Beveridge, 1994; Chiang et al, 2010). Strontium carbonate formation has recently been shown to occur intracellularly in a small number of microorganisms including the desmid green alga Closterium moniliferum, and more recently some cyanobacterial species, e.g., Gloeomargarita lithophora (Krejci et al, 2011; Couradeau et al, 2012; Cam et al, 2015; Cam et al.,2016). The number of microbial species which are identified as being capable of forming such intracellular carbonate minerals may increase with improvements in preparation techniques

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