Response of bacteria and meiofauna to iron oxide colloids in sediments of freshwater microcosms

Long-term exposure of a free-living freshwater micro- and meiobenthos community to microplastic mixtures in microcosms

Microplastics in a wide range of shapes and polymer types (MPs; <5 mm) accumulate in freshwater sediments, where they may pose an environmental threat to sediment-dwelling micro- and meiobenthos. To date, the effects of MPs on those organisms have mostly been studied in single-species experiments exposed to high particle concentrations. By contrast, there have been few investigations of the effects resulting from the long-term exposure of natural communities to environmental relevant MPs. This research gap was addressed in the present study. A microcosm experiment was conducted to examine the impact of a mixture of MPs of varying polymer composition, shape, and size (50% polystyrene (PS) beads: 1-μm diameter; 37% polyethylene terephthalate (PET) fragments: 32 × 21 μm in size, and 13% polyamide (PA) fibers 104 × 15 μm in size; % based on the total particle number) provided at two concentrations (low: 4.11 × 10⁵ MPs/kg sediment dw and high: 4.11 × 10⁷ MPs/kg sediment dw) and two exposure durations (4 and 12 weeks) on a micro- and meiobenthic community collected from a freshwater sediment. MPs exposure did not alter the abundance of protozoa (ciliates and flagellates) as well as the abundance and biomass of meiobenthic organisms (nematodes, rotifers, oligochaetes, gastrotrichs, nauplii), whereas the abundance and biomass of harpacticoid copepods was affected. Neither nematode species diversity (species richness, Shannon-Wiener index, and evenness) nor the NemaSPEAR[%]-index (pollution-sensitive index based on freshwater nematodes) changed in response to the MPs. However, changes in the structure of the meiobenthic and nematode community in the presence of environmentally relevant MPs mixtures cannot be excluded, such that microcosms experiments may be of value in detecting subtle, indirect effects of MPs.

Remediation of zinc-contaminated groundwater by iron oxide in situ adsorption barriers - From lab to the field

Heavy metals such as zinc cannot be degraded by microorganisms and form long contaminant plumes in groundwater. Conventional methods for remediating heavy metal-contaminated sites are for example excavation and pump-and-treat, which is expensive and requires very long operation times. This induced interest in new technologies such as in situ adsorption barriers for immobilization of heavy metal contamination. In this study, we present steps and criteria from laboratory tests to field studies, which are necessary for a successful implementation of an in situ adsorption barrier for immobilizing zinc. Groundwater and sediment samples from a contaminated site were brought to the lab, where the adsorption of zinc to Goethite nanoparticles was studied in batch and in flow-through systems mimicking field conditions. The Goethite nanoparticles revealed an in situ adsorption capacity of approximately 23 mg Zn per g Goethite. Transport experiments in sediment columns indicated an expected radius of influence of at least 2.8 m for the injection of Goethite nanoparticles. These findings were validated in a pilot-scale field study, where an in situ adsorption barrier of ca. 11 m × 6 m × 4 m was implemented in a zinc-contaminated aquifer. The injected nanoparticles were irreversibly deposited at the desired location within <24 h, and were not dislocated with the groundwater flow. Despite a constantly increasing inflow of zinc to the barrier and the short contact time between Goethite and zinc in the barrier, the dissolved zinc was effectively immobilized for ca. 90 days. Then, the zinc concentrations increased slowly downstream of the barrier, but the barrier still retained most of the zinc from the inflowing groundwater. The study demonstrated the applicability of Goethite nanoparticles to immobilize heavy metals in situ and highlights the criteria for upscaling laboratory-based determinants to field-scale.

Linking ecological health to co-occurring organic and inorganic chemical stressors in a groundwater-fed stream system

Freshwaters are among the most endangered ecosystems worldwide, due predominantly to excessive anthropogenic practices compromising the future provisioning of ecosystem services. Despite increased awareness of the role of multiple stressors in accounting for ecological degradation in mixed land-use stream systems, risk assessment approaches applicable in field settings are still required. This study provides a first indication for ecological consequences of the interaction of organic and inorganic chemical stressors, not typically evaluated together, which may provide a missing link enabling the reconnection of chemical and ecological findings. Specifically, impaired ecological conditions - represented by lower abundance of meiobenthic individuals - were observed in the hyporheic zone where a contaminant groundwater plume discharged to the stream. These zones were characterized by high xenobiotic organic concentrations, and strongly reduced groundwater (e.g. elevated dissolved iron and arsenic) linked to the dissolution of iron hydroxides (iron reduction) caused by the degradation of xenobiotic compounds in the plume. Further research is still needed to separate whether impact is driven by a combined effect of organic and inorganic stressors impacting the ecological communities, or whether the conditions - when present simultaneously - are responsible for enabling a specific chemical stressor's availability (e.g. trace metals), and thus toxicity, along the study stream. Regardless, these findings suggest that benthic meioinvertebrates are promising indicators for supporting biological assessments of stream systems to sufficiently represent impacts resulting from the co-occurrence of stressors in different stream compartments. Importantly, identification of the governing circumstances is crucial for revealing key patterns and impact drivers that may be needed in correctly prioritizing stressor impacts in these systems. This study further highlights the importance of stream-aquifer interfaces for investigating chemical stressor effects in multiple stressor systems. This will require holistic approaches for linking contaminant hydrogeology and eco(toxico)logy in order to positively influence the sustainable management of water resources globally.

Agglomeration of Escherichia coli with Positively Charged Nanoparticles Can Lead to Artifacts in a Standard Caenorhabditis elegans Toxicity Assay

The increased use and incorporation of engineered nanoparticles (ENPs) in consumer products requires a robust assessment of their potential environmental implications. However, a lack of standardized methods for nanotoxicity testing has yielded results that are sometimes contradictory. Standard ecotoxicity assays may work appropriately for some ENPs with minimal modification but produce artifactual results for others. Therefore, understanding the robustness of assays for a range of ENPs is critical. In this study, we evaluated the performance of a standard Caenorhabditis elegans ( C. elegans) toxicity assay containing an Escherichia coli ( E. coli) food supply with silicon, polystyrene, and gold ENPs with different charged coatings and sizes. Of all the ENPs tested, only those with a positively charged coating caused growth inhibition. However, the positively charged ENPs were observed to heteroagglomerate with E. coli cells, suggesting that the ENPs impacted the ability of nematodes to feed, leading to a false positive toxic effect on C. elegans growth and reproduction. When the ENPs were tested in two alternate C. elegans assays that did not contain E. coli, we found greatly reduced toxicity of ENPs. This study illustrates a key unexpected artifact that may occur during nanotoxicity assays.