Investigating the Impact of Cerium Oxide Nanoparticles Upon the Ecologically Significant Marine Cyanobacterium Prochlorococcus

Craig J Dedman,Gemma-Louise Davies,Joseph A Christie-Oleza,Marwa M I Rizk

doi:10.3389/fmars.2021.668097

Abstract

Cerium oxide nanoparticles (nCeO2) are used at an ever-increasing rate, however, their impact within the aquatic environment remains uncertain. Here, we expose the ecologically significant marine cyanobacterium Prochlorococcus sp. MED4 to nCeO2 at a wide range of concentrations (1 μg L–1 to 100 mg L–1) under simulated natural and nutrient rich growth conditions. Flow cytometric analysis of cyanobacterial populations displays the potential of nCeO2 (100 μg L–1) to significantly reduce Prochlorococcus cell density in the short-term (72 h) by up to 68.8% under environmentally relevant conditions. However, following longer exposure (240 h) cyanobacterial populations are observed to recover under simulated natural conditions. In contrast, cell-dense cultures grown under optimal conditions appear more sensitive to exposure during extended incubation, likely as a result of increased rate of encounter between cyanobacteria and nanoparticles at high cell densities. Exposure to supra-environmental nCeO2 concentrations (i.e., 100 mg L–1) resulted in significant declines in cell density up to 95.7 and 82.7% in natural oligotrophic seawater and nutrient enriched media, respectively. Observed cell decline is associated with extensive aggregation behaviour of nCeO2 upon entry into natural seawater, as observed by dynamic light scattering (DLS), and hetero-aggregation with cyanobacteria, confirmed by fluorescent microscopy. Hence, the reduction of planktonic cells is believed to result from physical removal due to co-aggregation and co-sedimentation with nCeO2 rather than by a toxicological and cell death effect. The observed recovery of the cyanobacterial population under simulated natural conditions, and likely reduction in nCeO2 bioavailability as nanoparticles aggregate and undergo sedimentation in saline media, means that the likely environmental risk of nCeO2 in the marine environment appears low.

Highlights

Cerium oxide nanoparticles represent an emerging contaminant with widespread use across a range of industries
In this study we provide new insight into the effect of nCeO2 upon marine microbial species using the ecologically significant marine cyanobacterium Prochlorococcus
We have shown that despite evidence of short-term (

Summary

Introduction

Cerium oxide nanoparticles (nCeO2) represent an emerging contaminant with widespread use across a range of industries. The use of nCeO2 as an additive for diesel fuels is a major ecological concern, and is believed to be the main source of nCeO2 particles into the natural environment (Johnson and Park, 2012). It is predicted up to 70 million metric tonnes of ceria could be released annually by road transport worldwide (Dale et al, 2017), with the highest environmental levels of nCeO2 predicted to be present in water draining from road surfaces (Johnson and Park, 2012), where nanoparticles may enter the aquatic environment. It is possible that nCeO2 may enter the ocean in this manner if used in boat fuels to enhance efficiency (Somasundaram et al, 2020), and may represent a previously understudied source of nCeO2 entry into the environment. Aside from transport, nCeO2 may enter the natural environments via release into wastewater, use of wastewater treatment sludge as fertiliser, or as a consequence of industrial production of nCeO2 or related products (Hu et al, 2006; Limbach et al, 2008)

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