Reed restoration decreased nutrients in wetlands with dredged sediments: Microbial community assembly and function in rhizosphere.

Using dredged sediments as substrate for aquatic plants is a low-cost and ecological friendly way for in situ aquatic ecological restoration. However, the limited information available about how aquatic plant restoration affects the microbial ecology and nutrients in dredged sediments. In this study, nutrient contents, enzyme activities, and bacterial and archaeal communities in vertical sediment layers were determined in bulk and reed zones of wetlands constructed with dredged sediments in west Lake Taihu for three years. Reed restoration significantly decreased total nitrogen, total phosphorus, and organic carbon contents and increased alkaline phosphatase, urease, and sucrase activities compared to bulk area. Bacterial communities in vertical sediment layers had higher similarity in reed zone in comparison to bulk zone, and many bacterial and archaeal genera were only detected in reed rhizosphere zones. Compared with the bulk zone, the reed restoration area has a higher abundance of phylum Actinobacteriota, Hydrothermarchaeota, and class α-proteobacteria. The assembly process of the bacterial and archaeal communities was primarily shaped by dispersal limitation (67.03% and 32.97%, respectively), and stochastic processes were enhanced in the reed recovery area. Network analysis show that there were more complicated interactions among bacteria and archaea and low-abundance taxa were crucial in maintaining the microbial community stability in rhizosphere of reed zone. PICRUST2 analysis demonstrate that reed restoration promotes metabolic pathways related to C and N cycle in dredged sediments. These data highlight that using dredged sediments as substrates for aquatic plants can transform waste material into a valuable resource, enhancing the benefits to the environment.

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.

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.

Nitrate application decreased microbial biodiversity but stimulated denitrifiers in epiphytic biofilms on Ceratophyllum demersum.

Among nitrogen species, nitrate is more stable than ammonium and nitrite, and it is an important nitrogenous pollutant in surface water. However, little is known about the characterization of epiphytic microbial communities on submersed macrophytes under nitrate loading. In this study, we investigated the co-occurring pattern and response of bacteria and microeukaryotes in epiphytic biofilms under nitrate loading. Nitrate loading significantly affected bacterial and eukaryotic communities, and turnover played greater contribution to the total dissimilarity than nestedness by partitioning beta-diversity analysis. Cyanobacteria, α-proteobacteria, ß-proteobacteria, Actinobacteria, Planctomycetes, Bacteroidetes, and γ-proteobacteria were dominant bacterial phyla/classes. Metazoan (phylum Arthropoda, Rotifera, Gastrotricha, Annelida, and Nematoda) and algae (phylum Bacillariophyta, Chlorophyta, and Streptophyta) were dominated in eukaryotic communities. The abundances of denitrifying bacteria (Rhodobacter, Acinetobacter, Bacillus, Flavobacterium, and Pseudomonas) and genes (nirS, cnorB, and nosZ) increased with nitrate loading. The network analysis showed there were complex interactions among photosynthetic microbes, metazoan, and bacteria (including denitrifiers) that they were potentially interrelated via photosynthesis, predation or feeding. This study provides new perspectives into understanding the factors affecting nitrate removal mechanisms in wetlands with submersed macrophytes.

Bacterial succession in epiphytic biofilms and deciduous layer sediments during Hydrilla verticillata decay: A field investigation.

Submersed macrophytes decay is an important natural process and has important role in mass and energy flow in aquatic ecosystems. However, little is known about the dynamical changes in nutrients release and bacterial community during submersed macrophyte decay in natural environment. In this study, a field observation was conducted in a wetland dominated with Hydrilla verticillata for 36 days. Increase of H2O2 and malondialdehyde (MDA) content and decrease of soluble proteins concentration were detected in leaves during H. verticillata decay. Meanwhile, ammonium-N, soluble microbial products (SMP) and TOC concentration increased in overlying water. According to bacterial 16S rRNA Illumina sequencing analysis, the Shannon values were lower in epiphytic biofilms than deciduous layer sediments. The relative abundances of Proteobacteria, Cyanobacteria and Actinobacteria were higher in epiphytic biofilms than in deciduous layer sediments (P < 0.05). Co-occurrence network analyses showed that a total of 578 and 845 pairs of correlations (|r| > 0.6) were identified from 122 and 112 genera in epiphytic biofilms and deciduous layer sediments, respectively. According to co-occurrence patterns, eight hubs were mainly from phyla Proteobacteria, Acidobacteria and Parcubacteria in epiphytic biofilms; while 37 hubs from the 14 phyla (Proteobacteria, Bacteroidetes, Acidobacteria, Chloroflexi, et al.) were detected in deciduous layer sediments. Our results indicate that bacterial community in deciduous layer sediments was more susceptible than in epiphytic biofilms during decay process. These data highlight the role of microbial community in deciduous layer sediments on nutrients removal during H. verticillata decay and will provide useful information for wetland management.