Ecological role of reed belts in lakeside zone: Impacts on nutrient retention and bacterial community assembly during Hydrilla verticillata decomposition.

Reed belts acting as basic nutrient filters are important parts of lake buffer riparian zones. However, little is known about their impacts on nutrient release and bacterial community during plant litter decomposition. In this study, a field experiment was conducted in west-lake Taihu to monitor the changes in nutrients, bacterial enzymatic activities, and bacterial community in plant debris during Hydrilla verticillata (H. verticillata) decomposition in open water (HvC) and reed belts (HvL) area for 126 days. We found that there was lower temperature but higher nutrient concentrations in overlying water in HvL than HvC. Partial least squares path modeling revealed that environmental parameters in overlying water had important impacts on bacterial activities and nutrient release (such as alkaline phosphatase, cellulase, and soluble sugar) and therefore affected dissolved organic matter components in plant debris. According to Illumina sequencing, 46,003 OTUs from 10 dominant phyla were obtained and Shannon index was higher in HvL than HvC at the same sampling time. Neutral community model explained 49% of bacterial community variance and immigration rate by the estimate of dispersal in HvC (Nm: 27,154) and HvL (Nm: 25,765), respectively. Null model showed stochastic factors governed the bacterial community assembly in HvC (66.67%) and HvL (87.28%). TP and pH were key factors affecting the bacterial community structure at the phylum level. More hubs and complex interactions among bacteria were observed in HvL than HvC. Function analysis showed bacterial community had important role in carbon, organic phosphorus, and nitrogen removal but phosphorus-starvation was detected in debris of H. verticillata. This study provides useful information for understanding the changes in nutrients and bacterial community in litter during H. verticillata decomposition and highlights the role of reed belts on retained plant litter to protect lake from pollution.

Potamogeton crispus restoration increased the epiphytic microbial diversity and improved water quality in a micro-polluted urban river.

Special characterization and assembly of epiphytic microbial communities remain unclear in micro-polluted water column during submersed macrophytes restoration. In this study, an in-situ enclosure area sowing with turions of Potamogeton crispus (P. crispus) was conducted in a micro-polluted urban river to investigate the characterization of P. crispus and epiphytic microbial communities and their response to water environment under different water depths. Turions completely germinated in water column with <90 cm water depth and the germination speed decreased with increasing water depth within 18 days. There were obvious differences in morphological characteristics of P. crispus between deep and shallow water layers. P. crispus restoration decreased by 12-32%, 13-36%, 9-43% and 5-36% of COD, NH4+-N, TN and TP concentration, respectively, in enclosed overlying water compared to the river (P < 0.05) during 5 months of experiment. Illumina sequencing was employed to explore the epiphytic bacterial and microeukayotic communities at water depth 25-35 cm (shallow area) and 80-90 cm (deep area). A total of 9 bacterial and 12 microeukayotic dominant phyla were obtained in eight samples. It should be noted that the algae abundances were higher in shallow area than deep area but a reverse trend was observed for methanotrophs. Null model analysis revealed that dispersal limitation and undominated process was the most important assembly process, whereas stochastic processes gained more importance in shallow area than deep one. According to cooccurrence analysis (|r| > 0.6, P < 0.05), there were more strongly correlated edges in shallow area (456 edges) than deep area (340 edges). These results highlight that submerged macrophytes restoration can increase microbial diversity and improve water quality, and provide a "summer disease cured in winter" way by using could-resistant P. crispus for water purification in micro-polluted rivers in low-temperature season.

Vallisneria natans decreased CH4 fluxes in wetlands: Interactions among plant physiological status, nutrients and epiphytic bacterial community.

Submerged macrophytes provide niches for epiphytic microbes (including aerobic methanotrophs) growth. However, little is known about the impacts of submerged macrophytes growth status and nutrients loadings on methanotroph community and methane release in wetlands. In the present study, methane fluxes, bacterial and methanotroph community in epiphytic biofilm, and environmental parameters were investigated during Vallisneria natans senescence in wetlands under low (VnL) and high (VnH) nutrients for seven weeks. Relative conductivity and concentration of H2O2, total chlorophyll and malondialdehyde were higher in leaves of V. natans in VnH than VnL at the same sampling time. Nutrients loading increased methane fluxes in treatments with or without (Control) macrophytes, while healthy V. natans plants reduced the methane flux and nutrients concentration in water columns. CH4 fluxes were positively correlated to temperature and COD (p < 0.05). Methane oxidation rates were 3.04-31.68 µmol methane mg-1 fresh weight of V. natans leaves - epiphytic biofilm within 1 h. Proteobacteria, Cyanobacteria, Bacteroidetes, Verrucomicrobia, Planctomycetes, Actinobacteria and Acidobacteria were dominant phylum in all epiphytic biofilms. The mean abundances of pmoA/16S rRNA were higher in VnL than VnH. According to Illumina sequencing results of pmoA gene, γ-proteobacteria and α-proteobacteria were the dominant methanotroph class in epiphytic biofilm from VnH and VnL, respectively. Among seven detected methanotrophic genera, Methylomonas was significantly higher in VnH than VnL. Network analysis revealed that there were much closer relationships between the environmental parameters and epiphytic bacterial community in VnH than in VnL. COD and MDA were negatively correlated with Methyloglobulus, Methylosarcina, Methylobacter and Methylocystis, but positively correlated with Methylomonas and Methylosinus. This study highlights that methanotrophs in epiphytic biofilm play important roles in methane-oxidizing, which can be affected by plant physiological status and environmental parameters.

Eichhornia crassipes-rhizospheric biofilms contribute to nutrients removal and methane oxidization in wastewater stabilization ponds receiving simulative sewage treatment plants effluents.

Wastewater stabilization ponds (WSPs) have been used in treating sewage treatment plants (STPs) effluents. However, little is known about the role of rhizospheric biofilms on methane release in WSPs with floating plants. In the present study, the nutrient removal, CH4 fluxes, CH4 oxidization potential and rhizospheric bacterial community were investigated in WSPs with Eichhornia crassipes under simulate STPs effluents for 31 days. At the end of the experiment, E. crassipes biomass was 5.60-8.81 times of initial weight and increased with increasing nutrients concentration. E. crassipes effectively reduced methane release and nutrients. Compared to control, E. crassipes reduced 52.30%-83.21% of CH4 fluxes at water-atmosphere interface and had better inhibition effect on CH4 fluxes in treatments with high nutrients. However, methane oxidization rates of E. crassipes roots were higher in low nutrients (0.83 ± 0.046 mg CH4 (kg fresh plant)-1 day-1) than high nutrients (0.12 ± 0.04 mg CH4 (kg fresh plant)-1 day-1). Structural equation modeling revealed that biomass of E. crassipes has negative effect on CH4 fluxes (-0.453, p = 0.000). Proteobacteria, Bacteroidetes, Planctomycetes, Chloroflexi and Actinobacteria were the predominant phyla in the rhizospheric biofilm of E. crassipes and contributed to nutrients removal. Aerobic methanotrophs and pomA abundances were higher in rhizospheric biofilm exposed to high nutrients than low nutrients and aerobic methanotrophs had close interactions with other microorganisms and participated in the carbon and nitrogen cycle, demonstrating that many bacteria harboring pmoA gene did not fully involve in methane oxidization. These data highlight plants E. crassipes have an important role in both reducing methane release and nutrients removal.

Interactions between suspended sediments and submerged macrophytes-epiphytic biofilms under water flow in shallow lakes.

Suspended sediments (SS) pollution is one of the factors affecting the transfer from turbid water state to clear water state in shallow lakes. However, the interactions between suspended sediments and submerged plants are far from clear. In this study, we investigated the settlement laws of SS in overlying water and its impact on the epiphytic biofilm of Myriophyllum verticillatum and Vallisneria natans under water flow. At least 90% of turbidity can be removed from overlying water, and the decreasing trend of water turbidity fitted the logarithmic decay model in all treatments. The size distribution of SS fit the log-normal distribution model in the first 240 min after SS addition. It should be noted that the main peak particle sizes were lower in treatments with submerged macrophytes (8.71-13.18 µm) than without plants (15.14-19.95 µm). Water flow and SS addition significantly increased the thickness of biofilms attached to M. verticillatum (p < 0.05), but they together significantly reduced the biofilm thickness on V. natans (p < 0.05). SS increased the bacterial α-diversity but decreased eukaryotic one in epiphytic biofilms. However, water flow had a more significant impact on microbial communities (especially eukaryotes) than SS and plant species. The relative abundances of dominant phylum Proteobacteria, class Alphaproteobacteria and Betaproteobacteria, and class Verrucomicrobiae increased in epiphytic biofilms after SS addition. Co-occurrence networks reveal that photosynthetic microbes in epiphytic biofilms played an important role in microbial communities under water flow and SS, and many hub microbes were increased by SS addition but reduced by water flow. These data highlight that SS decline can be predicted by the logarithmic decay model and, SS and water flow can affect the epiphytic-biofilm on submerged macrophytes.

Impacts of nano-titanium dioxide toward Vallisneria natans and epiphytic microbes.

In this study, Vallisneria natans plants were exposed to 5 and 20 nm of titanium dioxide nanoparticles (TiO2 NPs) anatase and 600-1000 nm of bulk at 5 and 20 mg/L for 30 days. SEM images and EDX spectra revealed that epiphytic biofilms were more prone to TiO2 NPs adhesion than bare plant leaves. TiO2 NPs injured plant leaf cells, ruptured epiphytic diatoms membranes and increased the ratio of free-living microbes. The TN, NH4⁺-N and NO3--N concentrations significantly decreased, respectively, by 44.9%, 33.6%, and 23.6% compared to bulk treatments after 30 days due to macrophyte damage and a decline in diversity of epiphytic bacterial community and abundance of nitrogen cycle bacteria. TiO2 NPs size-dependent decrease in bacterial relative abundance was detected, including phylum Cyanobacteria, Planctomycetes, and Verrucomicrobia. Although TiO2 NPs increased eukaryotic diversity and abundance, abundances of Bacillariophyceae and Vampyrellidae classes and Gastrotricha and Phragmoplastophyta phylum decreased significantly under TiO2 NPs exposure compared to bulk and control. TiO2 NPs reduced intensities of interaction relationships among epiphytic microbial genera. This study shed new light on the potential effects of TiO2 NPs toxicity toward aquatic plants and epiphytic microbial communities and its impacts on nitrogen species removal in wetlands.

The fate of tetracycline in vegetated mesocosmic wetlands and its impact on the water quality and epiphytic microbes.

The fate of antibiotics and their impact on antibiotic resistance genes (ARGs) and microbial communities are far from clear in wetlands. The fate and impact of tetracycline (TC) on the nutrient degradation of wetlands and epiphytic microbes were investigated. This study showed that after TC spiking, 99.7% of TC were removed from the surface water of wetlands containing Vallisneria spiralis within 4 days post-treatment. TC spiking impaired the nutrient removal capacity and disrupted epiphytic microbial community structure while enhancing the abundance of 11 ARGs subtypes, including tetracycline resistance genes, tetX, tetM, tetO, tetQ, tetS, and tet36. TC decreased bacterial biodiversity but amplified the relative abundance of Proteobacteria and Firmicutes by 4% and 61%, respectively, and increased eukaryotic diversity. 16 metabolic pathways including Carbohydrate, Energy, Amino acid, 'cofactor and vitamins' metabolisms were significantly (p < 0.01) increased in TC treatment. Phylogenetic, functional prediction analysis indicated that Flavobacterium was positively related with xenobiotics, cell motility, 'terpenoids and polyketides' metabolism but negatively related to nucleotide metabolism, while Rhodobacter showed a reverse trend but positively related with nucleotide and 'glycan biosynthesis' and metabolism. These data highlighted that TC has negative impacts on epiphytic microbial community and nutrients removal in wetlands.

Ciprofloxacin increased abundance of antibiotic resistance genes and shaped microbial community in epiphytic biofilm on Vallisneria spiralis in mesocosmic wetland.

This study investigated the fate of ciprofloxacin (CIP) in wetlands dominated by Vallisneria spiralis. About 99% of CIP was degraded from overlaying water within 4 days of treatment but significantly inhibited the nutrient removal capacity (TN, TP, and COD) by causing a drastic reduction in microbial aggregation in epiphytic biofilm and bacterial biodiversity. CIP triggered resistance mechanisms among dominant bacteria phyla such as Proteobacteria, Actinobacteria, and Planctomycetes causing their increased relative abundance. Additionally, the relative abundances of eukaryotic microorganisms (including; Chloroplastida, Metazoa, and Rhizaria) and 13 ARGs subtypes (including; Efflux pump, Tetracycline, Multi-drug, Rifampin, Beta-lactam, Peptide, Trimethoprim) were significantly increased. While dominant metabolic pathways such as Carbohydrate, amino acid, energy and nucleotide metabolism were inhibited. This study revealed that V. spiralis has great sorption capacity for CIP than sediment and though CIP was effectively removed from the overlying water, it caused a prolonged effect on the epiphytic biofilm microbial communities.