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To explore the effects of different water levels on competition of submerged plants, we selected the canopy submerged plant Potamogeton malaianus Miq. and the erect submerged plant Hydrilla verticillata (L. f.) Royle as study materials. Four water level gradients (25, 50, 75, 100 cm) were chosen to simulate the hydrological environment of shallow depressions in the main distribution area of submerged plants in the field. The competition between P. malaianus and H. verticillata under different water levels was investigated by measuring plant height, root length, stem thickness, number of leaves, nodal thrift, nodal spacing, above—ground biomass, below—ground biomass, and canopy whole—plant biomass. The results show that P. malaianus and H. verticillata increased in height as the water level rose. In most treatments, above—ground parts grew better than below ground parts. In the 25 cm water level, relative yields (RYs) of the above—and below—ground parts, and whole plant of P. malaianus displayed different patterns. At the water level of 25 cm, the RY values of the above—ground part, below—ground part, and whole plant of P. malaianus were greater than 1. At the low water level of 25 cm, all treatments had relative yield total (RYT) values greater than 1, and at 50 cm water level, the RYT value of P4H8 was greater than 1. RYT was less than 1 at both 75 and 100 cm at 75 and 100 cm water levels, the growth of each species was limited by the other species. In low water level environments, submerged plants switch from a competitive relationship to a facilitative relationship as the water level decreases (stress increases).

Cobalt is an essential trace metal and plays a pivotal role in the growth of all living organisms. The exposure of cobalt in the aquatic environment is ubiquitous via both natural and anthropogenic activities. The present study was carried out to investigate the accumulation response of macrophyte Hydrilla verticillata (L.f.) Royle towards the cobalt exposure at different concentrations (1, 25, 50, 75 and 100 µM) for seven days. The results showed that the cobalt accumulation increased in treated H. verticillata (L.f.) Royle with increase in concentration. The Bioconcentration factor (BCF) values were high in plants exposed to higher concentration of cobalt. Increase in growth (shoot length) and pigments (chlorophyll a, b and total chlorophyll) were detected at 25 µM cobalt concentration, whereas the growth and pigments were declined in 50, 75 and 100 µM. There was no significant difference in the carotenoid content between control and treated plant. Antioxidant enzymes (superoxide dismutase, catalase, and peroxidase) were increased at higher concentrations of cobalt. The results indicated that the decline in growth and pigments, and an increase in antioxidant enzyme activities, were triggered by the accumulation of cobalt in the plant. These observations suggest that H. verticillata (L.f.) Royle is well equipped to accumulate low concentration of cobalt in tissues to increase its growth and detoxify ROS generation. The BCF results indicate the efficiency of using H. verticillata (L.f.) Royle as a phytostabilizer.

Static-renewal ecotoxicity trials monitored growth of Hydrilla verticillata in conjunction with exposure to Cylindrospermopsis raciborskii whole-cell extracts containing the cyanotoxin cylindrospermopsin (CYN). Maximum exposure concentrations were 400 μg L-¹ CYN over 14 days. The responses of Hydrilla to the treatments were variable according to the toxin concentrations and lengths of exposure. Plant deaths, chlorosis, and necrosis were not recorded from treated plants. However, Hydrilla experienced significant growth stimulation and redistribution of plant resources in conjunction with exposure to the whole-cell extracts. Root production was particularly impacted. The results of this study imply that root production could aid in reducing C. raciborskii cell concentrations and CYN toxicity. Results of chlorophyll analyses differed, indicating that CYN in whole-cell extracts might exert complex effects on photosynthesis. This is the first study to describe the responses of an aquatic macrophyte following exposure to C. raciborskii whole-cell extracts containing the cyanotoxin cylindrospermopsin.

Background and aims Harvesting can regulate the overgrowth of submerged macrophytes and affect interspecific competition. However, the effects of harvesting on the growth and competition of submerged macrophytes with different growth forms, remains unclear. Methods Simulation experiments were conducted to study the morphological and physiological indicators, competition intensity, and received light intensity of rosette-forming Vallisneria natans ( V.natans ) under different harvesting intensities of canopy-forming Hydrilla verticillata ( H.verticillata ). Results V. natans had greater plant height and larger leaf area at a medium-intensity (harvest 30% and 45% of the plant height) harvesting than at other intensities. Medium-intensity harvesting was the most conducive to the accumulation of chlorophyll, whereas high-intensity (harvest 60% and 75% of the plant height) harvesting had the lowest chlorophyll content and highest malondialdehyde content in V. natans . Interspecific competition was observed between H. verticillata and V. natans , and the medium harvesting intensity of H. verticillata conferred a competitive advantage to V. natans . Harvesting H. verticillata affect the growth of V. natans by increasing the underwater light intensity, with a greater harvest intensity corresponding to stronger light intensity received by V. natans . Light conditions were the most suitable for the growth of V. natans under medium-intensity harvesting. Conclusions Medium-intensity harvesting not only improved the recovery of H. verticillata but also increased the competitiveness of V. natans , thus promoting the growth of V. natans . Therefore, in practice, medium-intensity harvesting can be applied to the regulation of H. verticillata and V. natans .

Eutrophication of water bodies causes the disappearance of submerged macrophytes and frequent blooms of phytoplankton, which leads to changes in the underwater light environment in the aspect of light intensity and light quality. However, compared with underwater light intensity, only few studies have been concentrated in the effect of light quality on submerged macrophytes. The effect of light quality on plants is heterogeneous, and there are two absorption peaks in red and blue wavelengths. Thus, we carried out a mesocosm experiment to study the effects of a series of red and blue light ratios (red/blue light ratio = 1/8, 1/4, 1/1, 4/1, and 8/1) on two submerged macrophyte species, Hydrilla verticillata and Vallisneria natans . We hypothesized that functional traits (growth strategy, morphological, photosynthetic, and nutritional traits) of submerged macrophytes will be modified by different red/blue light ratios. With the increase of red/blue light ratio, plant height of the two submerged macrophytes decreased, while tillers number increased. We could not completely verify our hypothesis, but we found species-specific differences. The leaf area of H. verticillata under 8/1 red/blue light ratio was significantly higher than that under 1/8 red/blue light ratio, whereas the leaf area of V. natans under 4/1 red/blue light ratio was lower than that under 1/8 red/blue light ratio. Our results are helpful to understand the disappearance mechanism of submerged macrophytes in eutrophic lakes and devise more appropriate measures for the recovery of submerged macrophytes.

The current hydroponic experiment investigated differences in the uptake, physiological response and defence mechanisms of Hydrilla verticillata (L.f.) Royle in response to three representative toxic heavy metals. The results revealed the following: as an excellent heavy metal accumulator, H. verticillata showed an accumulation pattern of Ni > Cd > Pb within experimental scope. Fourteen days (Ni and Cd) and 21 days (Pb) were the time thresholds under the same heavy metal concentration toxicity, while 33.06 μM (Ni) and 40 μM (Cd and Pb) were the concentration thresholds under the same 21-day duration treatment, to accumulate the most amount of metals. Hence, Pb might be accumulated more if it expands the experimental duration and concentration, for it continuously increases throughout the experimental period. Reasons for these uptake differences are that plant physiological response, tolerance and resistance vary under different heavy metal stress. First, the biomass and protein content of H. verticillata were both the highest under Pb stress, indicating the plant largest tolerance to Pb stress. Second, the tolerance thresholds of three antioxidant enzymes (SOD, CAT and POD) were the highest under Pb stress. Third, the three antioxidant enzymes and two other related resistance-causing enzymes (PPO and PAL) revealed that plant resistance was strongest at low Cd concentrations (0–20 μM) and at high Pb stress levels (40 μM). Furthermore, CAT is the most important antioxidant enzyme to combat three metal stresses (average relevance: CAT(0.89) > POD(0.48) > SOD(0.42)), while PAL is more important than PPO (average relevance: PAL (0.77) > PPO(0.72)). In conclusion, Pb-polluted water is best treated with H. verticillata because of the latter’s high uptake potential and strong defence capacity. These results provide an accurate, efficient and economical reference for phytoremediation. Graphical abstract

This research aims to determine the capacity of Hydrilla Verticillata activated carbon adsorbent in absorbing the heavy metal chromium. The method used in making Hydrilla Verticillata activated carbon is by carbonization at a temperature of 3000C and chemically activated. Activated carbon was contacted with a solution of the heavy metal chromium with a concentration of 100 ppm with contact times of 15, 30, 45, 60, 90 and 120 minutes. The structure of hydrilla verticillata active carbon from the XRD results is graphite and BET BJH characterization data shows that activated carbon has a surface area of 53.143 m2/g, an average pore size of 0.2764 nm and a pore volume of 0.0789 cc/g. The SEM- Edx characterization results show that the size distribution is not homogeneous but forms agglomerations like plate particles consisting of slitted pores. Test results using AAS show that the ability of hydrilla verticillata activated carbon to remediate the heavy metal chromium based on contact time is around 47.35%, 46.77%, 49.08%, 54.85%, 61.30% and 69.69% and the adsorption capacity is around 13.94 mg/g.

Epiphytic bacteria constitute a vital component of aquatic ecosystems, pivotal in regulating elemental cycling. Despite their significance, the diversity and functions of epiphytic bacterial communities adhering to various submerged macrophytes remain largely unexplored. In this study, we employed a metagenomic approach to investigate the diversity and function of epiphytic bacterial communities associated with six submerged macrophytes: Ceratophyllum demersum , Hydrilla verticillata , Myriophyllum verticillatum , Potamogeton lucens , Stuckenia pectinata , and Najas marina . The results revealed that the predominant epiphytic bacterial species for each plant type included Pseudomonas spp., Microbacterium spp., and Stenotrophomonas rhizophila . Multiple comparisons and linear discriminant analysis effect size indicated a significant divergence in the community composition of epiphytic bacteria among the six submerged macrophytes, with 0.3–1% of species uniquely identified. Epiphytic bacterial richness associated with S. pectinata significantly differed from that of both C. demersum and H. verticillata , although no significant differences were observed in diversity and evenness. Functionally, notable variations were observed in the relative abundances of genes associated with carbon, nitrogen, and phosphorus cycling within epiphytic bacterial communities on the submerged macrophyte hosts. Among these communities, H. verticillata exhibited enrichment in genes related to the 3-hydroxypropionate bicycle and nitrogen assimilation, translocation, and denitrification. Conversely, M. verticillatum showcased enrichment in genes linked to the reductive citric acid cycle (Arnon-Buchanan cycle), reductive pentose phosphate cycle (Calvin cycle), polyphosphate degradation, and organic nitrogen metabolism. In summary, our findings offer valuable insights into the diversity and function of epiphytic bacteria on submerged macrophyte leaves, shedding light on their roles in lake ecosystems.

The inorganic phosphate–solubilizing bacteria (IPB) in the rhizosphere of Hydrilla verticillata can convert insoluble inorganic phosphorus in the environment into soluble phosphorus that can be directly absorbed and utilized by Hydrilla verticillata . In this research, the roots and rhizosphere sediments of Hydrilla verticillata were collected from high–organic matter urban landscape water. The National Botanical Research Institute’s Phosphate growth medium (NBRIP medium) was used to screen for efficient cultivable IPB. The 16S rRNA gene sequence analysis was used to determine the taxonomic affiliation of the strains, and ammonium molybdate spectrophotometry was used to detect the phosphate-solubilizing ability of the strains. The results show that a total of 28 IPB strains with good phosphate-solubilizing effect are obtained from the roots and rhizosphere sediments of Hydrilla verticillata . These IPB strains belong to two phyla, four orders, seven classes, nine families, and nine genera. Among these, Bacillus and Acinetobacter are the dominant genera, and the strains SWIH-7, SWIP-6, SWIP-7, SWIP-13, SWIP-15 and SWIP-16 are potential new species. The IPB strains isolated and screened in this research are rich in diversity, with potential new species and stable phosphate-solubilizing characteristic. These IPB strains are suitable for further development as microbial bacterial agents, which can be applied to promote the recovery of submerged plants in polluted water with high–organic matter, treatment of polluted water and ecological restoration of water.

Marine and freshwater macrophytes are known to filter off microplastics from the water column; however, the effects of microplastic size and type on their retention by different macrophytes species have yet to be investigated. Here we tested the retention of different sizes and types of microplastics, introduced under two flow regimes (vertical deposition in still water and lateral deposition in a unidirectional current), by two submerged macrophyte species, Hydrilla verticillata and Mayaca fluviatilis, using ex situ experiments. Microplastics entrapment efficiency in macrophytes was determined by calibrating the dry weight (DW) of the plant and analysing the characteristics of each macrophyte species via a comparison of their leaf outer-edge perimeter-to-area ratio (P:A). The entrapment efficiency of macrophytes was higher in still water than in moving water. Hydrilla verticillata had greater average leaf surface area and retained the most polyethylene terephthalate (PET) of size 800-1000 μm under both lateral deposition in moving water (1.75±0.11 g) and vertical deposition in still water (2.85±0.24 g). Conversely, M. fluviatilis had greater P:A, surface area, and high surface cellulose and retained the most PET of size 600-800 μm in both moving (0.73±0.07 g) and still (0.92±0.159 g) water. Our findings highlight the influence of microplastic size and material type, macrophyte morphology and surface area, and water flow conditions in determining the entrapment rate of microplastics by macrophytes.