The role of roots in the accumulation and removal of cadmium by the aquatic plant Hydrilla verticillata

Single and combined phytoextraction of lead and cadmium on submerged plants Potamogeton pusillus L.: removal, bioaccumulation pattern, and phytotoxicity

Under the present investigation, the submerged plant Potamogeton pusillus has been tested for the removal of lead (Pb) and cadmium (Cd). P. pusillus removal efficiency and accumulation capacity were examined in separated Pb and Cd solutions, at 0.5, 1.0, and 2 mg L-1, and in solutions where both metals were present at the same concentration (0.5, 1.0, and 2 mg L-1), under laboratory conditions for 3, 7, and 10 days. Also, we examined the removal efficiency and accumulation capacity when a set of plants were exposed to 0.5 mg L-1 of Pb (or Cd) and increasing concentrations (0.5, 1, and 2 mg L-1) of Cd (or Pb) for 10 days. The effect of Cd and Pb was assessed by measuring changes in the chlorophylls, carotenoids, and malondialdehyde contents. Results showed that P. pusillus could accumulate Cd and Pb from individual solutions. Roots and leaves accumulated the highest amount of Cd and Pb followed by the stems. Some phytotoxic effects were observed, especially at individual Cd exposures, but these effects were not observed in the two-metal system. The removal and accumulation of Pb by P. pusillus were significantly enhanced in the presence of Cd under certain conditions, presenting a good alternative for the removal of these metals from polluted aquifers. To the extent of our knowledge, this is the first report on both enhanced phytoextraction of Pb in the presence of Cd and bioaccumulation of these heavy metals by P. pusillus.

Single and combined toxicity effects of nanoplastics and bisphenol F on submerged the macrophyte Hydrilla verticillata

Nano- and microplastics pose severe risks to the ecological environment. Nanoplastics (NPs) not only directly affect aquatic organisms, but also adsorb to other pollutants, resulting in compound pollution. Bisphenol F (BPF), an endocrine-disrupting chemical, is increasingly replacing bisphenol A (BPA) and is therefore widely distributed in the environment. In this study, the toxic effects of polystyrene nanoplastics (PS-NPs) and BPF and their combined exposure on the submerged macrophytes Hydrilla verticillata (H. verticillata) and leaf biofilms, were investigated. Results showed that 10 mg/L PS-NPs and combined exposure to 10 mg/L PS-NPs and 10 mg/L BPF significantly decreased the relative growth rate and chlorophyll content of H. verticillata, whereas BPF exposure alone had no impact on the growth and the contents of photosynthetic pigments in H. verticillata. Individual and combined exposure to PS-NPs and BPF can trigger antioxidant responses such as increased activities of superoxide dismutase, peroxidase, and malondialdehyde, as well as higher levels of glutathione S-transferase and glutathione and decreased catalase activity. The results of the scanning electron microscopy (SEM) showed that the nanoplastics particles were adsorbed on the surface of plant leaves, explaining their toxic effects, whereas BPF increases the sorption of PS-NPs on the surface of H. verticillata, potentially leading to PS-NPs enrichment in the food chain. The diversity and richness of the microbial community were altered by exposure to PS-NPs and BPF individually and in combination. The current study is the first to assess the effects of PS-NPs and BPF exposure on the growth, physiological characteristics, and leaf biofilm properties of submerged macrophytes.

Phytoremediation of cadmium-contaminated sediment using Hydrilla verticillata and Elodea canadensis harbor two same keystone rhizobacteria Pedosphaeraceae and Parasegetibacter

Aquatic macrophytes have been widely employed for in-situ phytoremediation of cadmium (Cd) polluted sediments. But, little is known about the responses of rhizosphere bacteria and their interspecific interactions to phytoremediation. In this study, the α-diversity, community composition, co-occurrence network and keystone species of sediment bacteria in rhizosphere zones of two typical macrophytes, Hydrilla verticillata and Elodea canadensis, were investigated using 16S rRNA gene high-throughput sequencing. The results showed that after fifty days of phytoremediation, a group of specialized sediment bacteria were assembled in the rhizosphere zones closely associated with different host macrophytes. Rhizosphere micro-environments, i.e., the increases of redox potential and organic matter and the decreases of pH, nitrogen and phosphorus, reduced bacterial α-diversity through niche-based species-sorting process, which in turn reduced interspecific mutualistic relationships. But meanwhile, benefiting from the nutrients supplied from macrophyte roots, more bacterial species survived in the highly Cd-contaminated sediments (50 mg kg^-1). In addition, the co-occurrence network revealed that both macrophytes harbored two same keystone bacteria with the high betweenness centrality values, including the family Pedosphaeraceae (genus_unclassified) and genus Parasegetibacter. Their relative abundances were up to 28-fold and 25-fold higher than other keystone species, respectively. Furthermore, these two keystone bacteria were metabolic generalists with vital ecological functions, which posed significant potentials for promoting plant growth and tolerating Cd bio-toxicity. Therefore, the identified keystone rhizobacteria, Pedosphaeraceae and Parasegetibacter, would be potential microbial modulations applied for the future optimization of phytoremediation in Cd-contaminated sediment.

Phytotoxicity and accumulation of Cu in mature and young leaves of submerged macrophyte Hydrilla verticillata (L.f.) Royle

Phytotoxicity and accumulation of Cu in mature and young leaves of submerged macrophyte Hydrilla verticillata (L.f.) Royle were investigated by analyzing the chlorophyll contents, chloroplast ultrastructure and leaf surface structure under different Cu treatments (0, 0.01, 0.05 and 0.1 f mg L^-1). The results showed that 0.05 and 0.1 mg L^-1 Cu treatment decreased the contents of Chl a and Chl b, and caused damage on leaf surface structure and chloroplast ultrastructure compared with control (0 mg L^-1 Cu treatment). Higher concentration of Cu induced Chlorophyll decreases and the damages on the leaf surface structure and chloroplast ultrastructure were more pronounced in mature than in young leaves. It was observed that leaf Cu concentration increased almost linearly with exposure time and majority of the Cu accumulated in the cell walls. Among different cell wall fractions, the majority of Cu accumulated in cell walls was bound to the hemicellulose 1 and cellulose, followed by the pectin, hemicellulose 2. Mature leaves had significantly higher the concentrations of total Cu and bound-Cu in cell walls due to higher uronic acid content in their cell wall fractions (pectin, hemicellulose 1 and cellulose) than young leaves. Distinct cell wall composition might partially contribute to the different Cu toxicity and accumulation between mature and young leaves of submerged macrophyte H. verticillata. Our results show that mature leaves are more efficient in the uptake and accumulation of Cu than young leaves, which might explain why mature leaves sustain more severe damage.

Effects of excess ammoniacal nitrogen (NH4+-N) on pigments, photosynthetic rates, chloroplast ultrastructure, proteomics, formation of reactive oxygen species and enzymatic activity in submerged plant Hydrilla verticillata (L.f.) Royle

Although excess ammoniacal-nitrogen (NH₄⁺-N) results in the disturbance of various important biochemical and physiological processes, a detailed study on the effects of NH₄⁺-N stress on the photosynthesis and global changes in protein levels in submerged macrophytes is still lacking. Here, the changes of excess NH₄⁺-N on physiological parameters in Hydrilla verticillata (L.f.) Royle, a submerged macrophyte were investigated, including the contents of photosynthetic pigments, soluble sugars, net photosynthesis and respiration, glutamine synthetase (GS) and glutamate synthase (GOGAT) activities, chloroplast ultrastructure, chloroplast reactive oxygen species (ROS) accumulation and protein levels. Our results showed that the net photosynthetic rate and pigment content reached maximum values when the plants were treated with 1 and 2 mg L^-1 NH₄⁺-N, respectively, and decreased at NH₄⁺-N concentrations at 5, 10, 15 and 20 mg L^-1. This decrease might be caused by ROS accumulation. Compared that in 0.02 mg L^-1 NH₄⁺-N as a control, ROS generation in chloroplasts significantly increased in the presence of more than 2 mg L^-1 NH₄⁺-N. Consistently, the damages caused by over-accumulated ROS were observed in chloroplast ultrastructure, showing a loose thylakoid membranes and swollen grana/stroma lamellae. Furthermore, through proteomic analysis, we identified 91 differentially expressed protein spots. Among them, six proteins involved in photosynthesis decreased in abundance in response to excess NH₄⁺-N. Surprisingly, the abundance of all the identified proteins that were involved in nitrogen assimilation and amino acid metabolism tended to increase under excess NH₄⁺-N compared with the control, suggestive of the imbalanced carbon and nitrogen (C-N) metabolisms. In support, activated GS and GOGAT cycle was observed, evidenced by higher activities of GS and GOGAT enzymes. To our knowledge, this work is the first description that excess NH₄⁺-N results in chloroplast ultrastructural damages and the first proteomic evidence to support that excess NH₄⁺-N can lead to a decline in photosynthesis and imbalance of C-N metabolism in submerged macrophytes.

Subcellular accumulation and source of O2- and H2O2 in submerged plant Hydrilla verticillata (L.f.) Royle under NH4+-N stress condition

In this study, the effects of excess NH₄⁺-N on the subcellular accumulation of O₂^- and H₂O₂ in submerged plant Hydrilla verticillata (L.f.) Royle were investigated using both histochemical and cytochemical methods. Treatments with ≥ 2.00 and ≥ 5.00 mg L^-1 NH₄⁺-N for 5 d significantly increased production of O₂^- and H₂O₂, respectively. The activities of plasma membrane-bound NADPH (nicotinamide adenine dinucleotide phosphate) oxidases and antioxidant enzymes (superoxide dismutase, peroxidase, ascorbate peroxidase, catalase, dehydroascorbate reductase and glutathione reductase) were also increased correspondingly. This study also provides the first cytochemical evidence of subcellular accumulation of O₂^- and H₂O₂ in the submerged plants. In the leaves of H. verticillata treated with 20.0 mg L^-1 NH₄⁺-N, O₂^- dependent DAB precipitates were found primarily on the inner side of the plasma membrane, extracellular space and chloroplasts. H₂O₂-CeCl₃ precipitates were mainly localized on the inner side of the plasma membrane and extracellular space of the mesophyll cells. Treatments with the inhibitors of NADPH oxidase (diphenylene iodonium and imidazole) indicate that NH₄⁺-N-induced production of O₂^- and H₂O₂ in H. verticillata leaves may involve plasma membrane-bound NADPH oxidase. Moreover, low-light treatment decreased NH₄⁺-induced O₂^- production, suggesting that alterations in the photosynthetic electron transfer chain due to NH₄⁺ toxicity could lead to O₂^- production.

High-potential accumulation and tolerance in the submerged hydrophyte Hydrilla verticillata (L.f.) Royle for nickel-contaminated water

Water contamination by nickel (Ni) has become an increasing concern in recent decades. Hydrilla verticillata (L.f.) Royle has been recognized as a promising accumulator of several potentially toxic elements (PTEs) in phytoremediation, but its Ni-accumulation characteristics and its mechanisms of tolerance to Ni remain largely unknown. This research investigated the biochemical responses of leaves and stems of H. verticillata to various concentrations of Ni (5, 10, 15, 20, and 40 μM) over periods of 7, 14, or 21 days. Plants accumulated considerable Ni to a maximum amount of 1080 mg kg^-1 dry weight (DW) with a maximum bioconcentration factor of 1100; thus, high Ni accumulation was detected in H. verticillata. Low concentrations (5-15 μM) or short durations (less than 14 days) of Ni exposure might promote plant growth without adversely affecting normal metabolism. After peaking at day 14, a decline in bioaccumulation was unexpectedly observed as a long-term effect of Ni toxicity. Malondialdehyde content and the activities of defense-related enzymes changed in a similar pattern after treatment with Ni, increasing with both Ni concentration and exposure time to a peak (often at 5-15 μM on day 14), followed by a decline. Through a comprehensive analysis of all the test parameters, the tolerance thresholds were determined to be > 40.0 μM, 24.0 μM, and 15.8 μM at days 7, 14, and 21, respectively. Hydrilla verticillata could be a "high-potential accumulator" capable of decontaminating aquatic bodies polluted by Ni within the threshold range.