Nanoparticle-sweeping coastal bacteria found to have good bioremediation potential, but not without some caveats

Nanoparticles have been a significant research focus in ecotoxicology in recent decades, due to their increasing use in industry; they have varying effects on marine organisms (Krysanov et al., 2010). In a recent study, the potential of the coastal sediment-dwelling bacterium Pseudomonas fluorescens for bioremediation of Cadmium Selenide Quantum Dots (CaSe QDs) was investigated (Poirier et al., 2016). This bacterium survives in heavily polluted areas, making it an ideal model for ecotoxicological studies. CaSe QDs are being increasingly used in photovoltaic devices, and their toxicity, like that of many other metallic nanoparticles, is thought to derive mainly from the release of metal ions in the environment (i.e. Cd²⁺ and Se^2-). Therefore, the study aimed to investigate the effects of CaSe QDs on P. fluorescens’s physiology and proteome (see Poirier et al., 2016 for a detailed description of the parameters measured and methods), and to disentangle the impacts of NP exposure from those of metallic ion exposure, in order to assess the bioremediation potential of this species. The bacteria were exposed to nanoparticles of two sizes: 3 nm and 8 nm in diameter (named NP3 and NP8), and to free Cd²⁺ and Se^2- in concentrations equivalent to the surface atom concentrations of the NPs.
STEM analysis showed that NP3s permeated the cells, and both NP3s and NP8s were found in the cell envelope and embedded in the exopolysaccharide matrix. Growth was only negatively affected by NP8s, whereas both particle sizes caused a decrease in O₂ consumption. ROS (Reactive Oxygen Species) levels were found to be intimately linked with growth, but not majorly affected by NP exposure: in fact, the bacteria entered the exponential growing phase only after ROS levels decreased. Pyoverdine (iron-chelating compound) production was drastically reduced upon NP8 exposure, possibly indicating a slow-down of iron import into the cells. The biosorption ability of the bacteria ranged from 0.73 to 1.47 mg Cd/dry weight after exposure to NP3s and NP8s for 40 hours. The proteomic response was complex, but NP3 and NP8 exposure mainly caused an upregulation of proteins involved in redox and amino acid metabolism processes respectively, and a downregulation of translational and transport processes respectively.
This study provides a detailed report on the physiological and proteomic response of P. fluorescens to NP exposure, and represents a valid starting point in assessing the bioremediation potential of this species. However, it is difficult to interpret the findings in an ecologically realistic context: as noted by the authors, the presence of CaSe QDs has not been recorded in the marine environment yet, so any attempt to reproduce environmental concentrations would be vain. On the other hand, data on heavy metal levels in sea water is available; yet, the cadmium concentrations used in this study (0.60 and 1.5 mg/L) are two orders of magnitude higher than the top median concentration from various UK coastal sites (MEMG, 2014). Admittedly, cadmium concentrations in coastal sediments can reach higher levels, however the growth medium utilised in this study is not directly comparable. The bacteria did seem to accumulate NPs intra- and epicellularly, and they were able to sequester cadmium from the medium, which seems promising for bioremediation purposes. However, future studies should aim to use lower concentrations of nanoparticles, ideally matching environmental levels where appropriate, and extend the experiments over a longer time period. On a related note, crucially, trophic transfer of NPs would need to be assessed in situ and ex situ, in order to better understand the wider ecological implications of NP release into the marine environment.

Reviewed article:

Poirier, I., Kuhn, L., Demortière, A., Mirvaux, B., Hammann, P., Chicher, J., Caplat, C., Pallud, M. & Bertrand, M. (2016) 'Ability of the marine bacterium Pseudomonas fluorescens BA3SM1 to counteract the toxicity of CdSe nanoparticles'. Journal of Proteomics, 148 (Supplement C), pp. 213-227.

References

Krysanov, E. Y., Pavlov, D. S., Demidova, T. B. & Dgebuadze, Y. Y. (2010) 'Effect of nanoparticles on aquatic organisms'. Biology Bulletin, 37 (4), pp. 406-412.

MEMG (2014) UK National Marine Monitoring Programme - Second Report (1999-2001). CEFAS.