A marriage of convenience; a simple food chain comprised of Lemna minor (L.) and Gammarus pulex (L.) to study the dietary transfer of zinc

Insight interactions of engineered nanoparticles with aquatic higher plants for phytoaccumulation, phytotoxicity, and phytoremediation applications: A review

With the growing age of human civilization, industrialization has paced up equally which is followed by the innovation of newer concepts of science and technology. One such example is the invention of engineered nanoparticles and their flagrant use in widespread applications. While ENPs serve their intended purposes, they also disrupt the ecological balance by contaminating pristine aquatic ecosystems. This review encompasses a comprehensive discussion about the potent toxicity of ENPs on aquatic ecosystems, with a particular focus on their impact on aquatic higher plants. The discussion extends to elucidating the fate of ENPs upon release into aquatic environments, covering aspects ranging from morphological and physiological effects to molecular-level phytotoxicity. Furthermore, this level of toxicity has been correlated with the determination of competent plants for the phytoremediation process towards the mitigation of this ecological stress. However, this review further illustrates the path of future research which is yet to be explored. Determination of the genotoxicity level of aquatic higher plants could explain the entire process comprehensively. Moreover, to make it suitable to be used in natural ecosystems phytoremediation potential of co-existing plant species along with the presence of different ENPs need to be evaluated. This literature will undoubtedly offer readers a comprehensive understanding of the stress induced by the irresponsible release of engineered nanoparticles (ENP) into aquatic environments, along with insights into the resilience characteristics of these pristine ecosystems.

Interactions of Coated-Gold Engineered Nanoparticles with Aquatic Higher Plant Salvinia minima Baker

The study investigated the interactions of coated-gold engineered nanoparticles (nAu) with the aquatic higher plant Salvinia minima Baker in 2,7, and 14 d. Herein, the nAu concentration of 1000 µg/L was used; as in lower concentrations, analytical limitations persisted but >1000 µg/L were deemed too high and unlikely to be present in the environment. Exposure of S. minima to 1000 µg/L of citrate (cit)- and branched polyethyleneimine (BPEI)-coated nAu (5, 20, and 40 nm) in 10% Hoagland’s medium (10 HM) had marginal effect on biomass and growth rate irrespective of nAu size, coating type, or exposure duration. Further, results demonstrated that nAu were adsorbed on the plants’ roots irrespective of their size or coating variant; however, no evidence of internalization was apparent, and this was attributed to high agglomeration of nAu in 10 HM. Hence, adsorption was concluded as the basic mechanism of nAu accumulation by S. minima. Overall, the long-term exposure of S. minima to nAu did not inhibit plant biomass and growth rate but agglomerates on plant roots may block cell wall pores, and, in turn, alter uptake of essential macronutrients in plants, thus potentially affecting the overall ecological function.

Contaminant metal concentrations in three species of aquatic macrophytes from the Coeur d'Alene Lake basin, USA

The Coeur d'Alene Lake basin in Northwestern USA has extensive contamination from legacy mining waste, which overlaps with aquatic macrophyte habitat. We examined concentrations of arsenic (As), cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) in three macrophytes: Elodea canadensis (submerged), Myriophyllum spicatum (submerged), and Sagittaria latifolia (emergent). We collected macrophyte tissues from five contaminated sites and one uncontaminated site. Tissue concentrations were compared to sediment quality guidelines to assess potential toxicity from metal(loid)s to macrophyte-associated biota. We used threshold and probable effect concentrations to screen for potential toxicity. For the submerged species, the highest site means ± SD (analyte mg/kg dry mass) were 96 ± 61 (As), 18 ± 1.7 (Cd), 24 ± 15 (Cu), 610 ± 392 (Pb), and 1425 ± 222 (Zn). For contaminated sites, the probable effect threshold was exceeded in 38% (As), 45% (Cd), 0% (Cu), 74% (Pb), and 67% (Zn) of submerged species concentrations. Metal concentrations in S. latifolia tubers were lower than the submerged species leaves and shoots. Tuber concentrations did not exceed the probable effect threshold for any metal. Spatial differences in concentrations were most distinct for the submerged species. Our work shows significant amounts of metals are accumulating in some macrophytes of the study area and that biota associated with this vegetation may experience toxicity based upon guideline exceedances. Additionally, managers of invasive plants (e.g., M. spicatum) should consider the ramifications of control efforts given the high metal content of some plants (e.g., disposal issue).

Beyond plastic microbeads - Short-term feeding of cellulose and polyester microfibers to the freshwater amphipod Gammarus duebeni

Monitoring studies have revealed the presence of large numbers of natural as well as anthropogenic microfibers, plastic and non-plastic, in environmental samples. However, the interaction of organisms with microfibers is largely understudied. This is the first ecotoxicological study that compares short-term feeding of anthropogenic plastic and non-plastic microfibers on a consumer (leaf-shredding detritivores) species. The freshwater amphipod Gammarus duebeni was selected for this study as it is a model ecotoxicological species. After a 96-hour exposure, 58.3% and 41.7% of the amphipods contained cellulose or polyester fibers in their digestive tracts, respectively. Microfiber ingestion was analysed per polymers in presence or absence of food. The G. duebeni group exposed to 'polyester fibers in presence of food' accumulated highest numbers of microfibers in their digestive tracts (5.2 ± 3.4 MFs/amphipod) followed by those exposed to 'cellulose in presence of food' (2.5 ± 0.9 MFs/amphipod). A significantly (Three-way ANOVA, p-value <0.05) higher number of microfibers was found in the midgut-hindgut (posterior) sections, compared to the foregut (anterior) section. Microfiber uptake had no apparent short-term negative effect on amphipod survival at 96 h. Yet, as amphipods are both predators and prey, and therefore are key species in the aquatic food web, the rapid accumulation of anthropogenic microfibers in their digestive system has potentially further ecological implications. Future studies need to consider the possible transfer of ingested anthropogenic microfibers to higher trophic levels in freshwater communities.

Feeding Behavioural Studies with Freshwater Gammarus spp.: The Importance of a Standardised Methodology

Freshwater Gammarids are common leaf-shredding detritivores, and they usually feed on naturally conditioned organic material, in other words leaf litter that is characterised by an increased palatability, due to the action and presence of microorganisms (Chaumot et al. 2015; Cummins 1974: Maltby et al. 2002). Gammarus spp. are biologically omnivorous organisms, so they are involved in shredding leaf litter and are also prone to cannibalism, predation behaviour (Kelly et al. 2002) and coprophagy when juveniles (McCahon and Pascoe 1988). Gammarus spp. is a keystone species (Woodward et al. 2008), and it plays an important role in the decomposition of organic matter (Alonso et al. 2009; Bundschuh et al. 2013) and is also a noteworthy prey for fish and birds (Andrén and Eriksson Wiklund 2013; Blarer and Burkhardt-Holm 2016). Gammarids are considered to be fairly sensitive to different contaminants (Ashauer et al. 2010; Bloor et al. 2005; Felten et al. 2008a; Lahive et al. 2015; Kunz et al. 2010); in fact Amphipods have been reported to be one of the most sensitive orders to metals and organic compounds (Wogram and Liess 2001), which makes them representative test organisms for ecotoxicological studies and valid sentinel species for assessing water quality status (Garcia-Galan et al. 2017).

Polyethylene microplastics adhere to Lemna minor (L.), yet have no effects on plant growth or feeding by Gammarus duebeni (Lillj.)

Microplastics (1-1000 μm) are ubiquitous in the marine, freshwater and terrestrial environments. These microsized plastics are considered freshwater pollutants of emerging concern, although the impacts on organisms and ecosystems are not yet clear. In particular, effects of microplastics on freshwater aquatic plants and the fate of microplastics in the freshwater trophic chain remain largely unexplored. Here we demonstrate that 10-45 μm polyethylene (PE) microplastics can strongly adsorb to all surfaces of the duckweed species Lemna minor. Despite adsorbance of up to 7 PE microplastics per mm², seven day exposure experiments showed that photosynthetic efficiency and plant growth are not affected by microplastics. Rather, dense surface coverage suggests L. minor as a potential vector for the trophic transfer of microplastics. Here we show that the freshwater amphipod Gammarus duebeni can ingest 10-45 μm PE microplastics by feeding on contaminated L. minor. In this study, ingestion of microplastics had no apparent impact on amphipod mortality or mobility after 24 or 48 h exposure. Yet, the feeding study showed that the fate of microplastics in the environment may be complex, involving both plant adsorbance and trophic transfer.

Orthophosphate modulates the phytotoxicity of nano-ZnO to Lemna minor (L.)

Because of their applications in large numbers of products, Zinc Oxide nanoparticles (nano-ZnO) will inevitably enter into the environment. Nano-ZnO released into the environment will be present in a complex matrix which can cause various chemical and physical transformations and modulate the biological reactivity of these particles. Due to their rapid growth and small size, Lemna minor is recommended by OECD for toxicological testing. Here, we tested how nano-ZnO reactivity is modulated by the suite of macro- and micronutrients that are present in Lemna minor growth media. Specifically, we measured ex situ Reactive Oxygen Species (ROS) formation by nano-ZnO, and subsequent in planta toxicity. The data show how orthophosphate can modulate both ex situ ROS formation, and in planta toxicity. This has ramifications for phytotoxicity testing, which is commonly performed under controlled conditions and on media containing orthophosphate.

Application of common duckweed (Lemna minor) in phytoremediation of chemicals in the environment: State and future perspective

Over the past 50 years, different strategies have been developed for the remediation of polluted air, land and water. Driven by public opinion and regulatory bottlenecks, ecological based strategies are preferable than conventional methods in the treatments of chemical effluents. Ecological systems with the application of microbes, fungi, earthworms, plants, enzymes, electrode and nanoparticles have been applied to varying degrees in different media for the remediation of various categories of pollutants. Aquatic macrophytes have been used extensively for the remediation of pollutants in wastewater effluents and aquatic environment over the past 30 years with the common duckweed (L. minor) as one of the most effective macrophytes that have been applied for remediation studies. Duckweed has shown strong potentials for the phytoremediation of organic pollutants, heavy metals, agrochemicals, pharmaceuticals and personal care products, radioactive waste, nanomaterials, petroleum hydrocarbons, dyes, toxins, and related pollutants. This review covers the state of duckweed application for the remediation of diverse aquatic pollutants and identifies gaps that are necessary for further studies as we find pragmatic and sound ecological solutions for the remediation of polluted environment for sustainable development.

Interactions of metal-based engineered nanoparticles with aquatic higher plants: A review of the state of current knowledge

The rising potential for the release of engineered nanoparticles (ENPs) into aquatic environments requires evaluation of risks to protect ecological health. The present review examines knowledge pertaining to the interactions of metal-based ENPs with aquatic higher plants, identifies information gaps, and raises considerations for future research to advance knowledge on the subject. The discussion focuses on ENPs' bioaccessibility; uptake, adsorption, translocation, and bioaccumulation; and toxicity effects on aquatic higher plants. An information deficit surrounds the uptake of ENPs and associated dynamics, because the influence of ENP characteristics and water quality conditions has not been well documented. Dissolution appears to be a key mechanism driving bioaccumulation of ENPs, whereas nanoparticulates often adsorb to plant surfaces with minimal internalization. However, few reports document the internalization of ENPs by plants; thus, the role of nanoparticulates' internalization in bioaccumulation and toxicity remains unclear, requiring further investigation. The toxicities of metal-based ENPs mainly have been associated with dissolution as a predominant mechanism, although nano toxicity has also been reported. To advance knowledge in this domain, future investigations need to integrate the influence of ENP characteristics and water physicochemical parameters, as their interplay determines ENP bioaccessibility and influences their risk to health of aquatic higher plants. Furthermore, harmonization of test protocols is recommended for fast tracking the generation of comparable data. Environ Toxicol Chem 2016;35:1677-1694. © 2016 SETAC.

The toxicity of zinc oxide nanoparticles to Lemna minor (L.) is predominantly caused by dissolved Zn

Nano-ZnO particles have been reported to be toxic to many aquatic organisms, although it is debated whether this is caused by nanoparticles per sé, or rather dissolved Zn. This study investigated the role of dissolved Zn in nano-ZnO toxicity to Lemna minor. The technical approach was based on modulating nano-ZnO dissolution by either modifying the pH of the growth medium and/or surface coating of nano-ZnO, and measuring resulting impacts on L. minor growth and physiology. Results show rapid and total dissolution of nano-ZnO in the medium (pH 4.5). Quantitatively similar toxic effects were found when L. minor was exposed to nano-ZnO or the "dissolved Zn equivalent of dissolved nano-ZnO". The conclusion that nano-ZnO toxicity is primarily caused by dissolved Zn was further supported by the observation that phytotoxicity was absent on medium with higher pH-values (>7), where dissolution of nano-ZnO almost ceased. Similarly, the reduced toxicity of coated nano-ZnO, which displays a slower Zn dissolution, is also consistent with a major role for dissolved Zn in nano-ZnO toxicity.

The first draft genome of the aquatic model plant Lemna minor opens the route for future stress physiology research and biotechnological applications

BACKGROUND

Freshwater duckweed, comprising the smallest, fastest growing and simplest macrophytes has various applications in agriculture, phytoremediation and energy production. Lemna minor, the so-called common duckweed, is a model system of these aquatic plants for ecotoxicological bioassays, genetic transformation tools and industrial applications. Given the ecotoxic relevance and high potential for biomass production, whole-genome information of this cosmopolitan duckweed is needed.

RESULTS

The 472 Mbp assembly of the L. minor genome (2n = 40; estimated 481 Mbp; 98.1 %) contains 22,382 protein-coding genes and 61.5 % repetitive sequences. The repeat content explains 94.5 % of the genome size difference in comparison with the greater duckweed, Spirodela polyrhiza (2n = 40; 158 Mbp; 19,623 protein-coding genes; and 15.79 % repetitive sequences). Comparison of proteins from other monocot plants, protein ortholog identification, OrthoMCL, suggests 1356 duckweed-specific groups (3367 proteins, 15.0 % total L. minor proteins) and 795 Lemna-specific groups (2897 proteins, 12.9 % total L. minor proteins). Interestingly, proteins involved in biosynthetic processes in response to various stimuli and hydrolase activities are enriched in the Lemna proteome in comparison with the Spirodela proteome.

CONCLUSIONS

The genome sequence and annotation of L. minor protein-coding genes provide new insights in biological understanding and biomass production applications of Lemna species.

Do particle size and surface functionality affect uptake and depuration of gold nanoparticles by aquatic invertebrates?

Because of the widespread use of engineered nanoparticles (ENPs) in consumer and industrial products, it is inevitable that these materials will enter the environment. It is often stated that the uptake of ENPs into organisms in the environment is related to the particle size and surface functionality. To test this assumption, the present study investigated the uptake and depuration of gold nanoparticle (Au NPs) coated with either citrate (Au-citrate NPs), mercaptoundecanoic acid (Au-MUDA NPs), amino polyethylene glycol (PEG) thiol (Au-NH2 NPs), or PEG (Au-PEG NP) by the aquatic invertebrate Gammarus pulex. The studies were performed using a range of standard ecotoxicity media and natural waters, resulting in varying degrees of aggregation of the different NPs. Uptake of gold by G. pulex varied depending on the surface coatings, with Au-MUDA and Au-citrate NPs being taken up to a greater extent than Au-NH2 and Au-PEG NPs in all test media and natural waters. In all test media evaluated, higher amounts of amino and PEG-coated ENPs were eliminated compared with MUDA- and citrate-coated ENPs. No obvious relationships were seen between the aggregation state of the different Au NPs in treatment and uptake, suggesting that the widely accepted assumption that Au NP uptake is related to particle size does not hold for the range of aggregation states studied (67.1-178.8 nm). Positive correlations between particle number concentration in the media and uptake were observed, indicating that this factor might partly explain the differences in uptake of a particle from different media types.