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Submerged plants ultimately suffer from shortage in cellular oxygen availability (hypoxia) as a result of impaired gas diffusion underwater. The gaseous plant hormone ethylene is rapidly entrapped in submerged plant tissues and is an established regulator of morphological and anatomical flood-adaptive responses. Multiple recent discoveries suggest that ethylene also plays a crucial role in hypoxia anticipation and metabolic acclimation during plant submergence. Ethylene was shown to accelerate and enhance the hypoxic response through enhanced stability of specific transcription factors (group VII ethylene response factors). Moreover, we suggest that ethylene could play an important role in the induction of autophagy and promote reactive oxygen species amelioration, thereby contributing to enhanced survival during flooding, hypoxia, and reoxygenation stress.

The microorganisms and functional predictions of leaf biofilms on submerged plants (Vallisneria natans (Val)) and in water samples (surface water (S) and bottom water (B)) in different seasons were evaluated in this study. S and B groups had 3249 identical operational taxonomic units (OTUs) (50.03%), while the Val group only had 1201 (18.49%) unique OTUs. There was significant overlap between microbial communities of S and B groups in the same season, while Val group showed the greater diversity. The dominant microbial clades were Proteobacteria (18.2-47.3%), Cyanobacteria (3.74-39.3%), Actinobacteria (1.64-29.3%), Bacteroidetes (1.31-21.7%), and Firmicutes (1.10-15.72%). Furthermore, there was a significant relationship between total organic carbon and the distribution of microbial taxa (p = 0.047), and TN may have altered the status of Cyanobacteria by affecting its biological nitrogen fixation capacity and reproductive capacity. The correlation network analysis results showed that the whole system consisted of 249 positive correlations and 111 negative correlations, indicating strong interactions between microbial communities. Functional predictions indicated that microbial functions were related to seasonal variation. These findings would guide the use of submerged plants to improve the diversity and stability of wetland microbial communities.

Vallisneria natans (Lour.) Hara is a suitable submerged plant for the phytoremediation of As-contaminated water. Rahnella aquatilis is one of the plant growth–promoting rhizobacteria. Influences of R. aquatilis on the arsenic accumulation and detoxification of V. natans were investigated. The results showed that As accumulation by V. natans could be significantly improved after R. aquatilis inoculated at the lower level of As (< 2 mg/L). At 0.5, 1, and 2 mg/L As levels, the As concentrations of V. natans with R. aquatilis were respectively 100.40%, 57.96%, and 22.62% higher than that of V. natans with no R. aquatilis . The concentration of As in V. natans was increased with the increasing the As concentration up to 1 mg/L, but it was decreased at 2 mg/L As. The correlation analysis showed that the As accumulated in the plant was positive correlated ( R 2 = 0.977, p < 0.01) with indole-3-acetic acid (IAA) produced by R. aquatilis under different As levels. IAA may be the major factor affecting the As accumulation of V. natans . The results of malondialdehyde and superoxide dismutase, hydrogen peroxidase, and ascorbate peroxidase indicated that IAA produced by R. aquatilis had alleviated the arsenic stress on V. natans . The synthesis of IAA by R. aquatilis was related to the As levels. When the As was at 2 mg/L, the IAA that produced by R. aquatilis decreased and the promotion of R. aquatilis on As accumulation by V. natans reduced. However, R. aquatilis has a positive influence on the arsenic accumulation by V. natans at the lower As levels (< 2 mg/L), and it may be a potentially useful way to improve the removal of arsenic from contaminated water using submerged plants.

Submerged plants and biofilms have significant advantages in hydro-ecology rehabilitation, but their tolerance and physiological responses to heavy metal stress have thus far been under-investigated. This study investigated the influence of lead on physiological and biochemical responses, as well as variation in bacterial communities and functional characteristics of submerged plant biofilms. The results showed that chlorophyll a content of two submerged plants decreased with increased lead concentration. The concentration of malondialdehyde of both submerged plants was higher under high lead concentrations than under low lead concentrations, and the concentrations of malondialdehyde and hydrogen peroxide in Vallisneria natans were more stable. The antioxidant enzyme systems of the two plants played protective roles against lead stress. High lead concentration can inhibit the bacterial community and lead to decreased diversity. The most abundant bacterial phyla were Proteobacteria (40.9%), Cyanobacteria (21.5%), and Bacteroidetes (14.3%). Proteobacteria abundance decreased with increased lead concentration, while Cyanobacteria abundance increased. The lead concentration in plants (19.7%, P < 0.01) and the lead concentration in aquatic environment (17.7%, P < 0.01) were significantly correlated with variation in bacterial communities. High lead concentration inhibits the activity of these bacteria related to the conversion of nitrogen and sulfur.

Both non-rooted submerged vegetation dominated by coontail (Ceratophyllum demersum) and non-rooted floating duckweed vegetation (Lemna gibba) can maintain their stable dominance in small ponds and channels. We examined the competitive interactions between them and how Ceratophyllum can sustain its stable state against floating plants in a range of nutrient concentrations. Coontail and duckweed were co-cultured in static and semi-static microcosm experiments, and their impact on the nutrients (N, P, Fe, Mn) in the water column was analysed. Coontail strongly reduced the growth of duckweed under a low nitrogen level (0.2–2 mg N L−1). This reduction seems to be due to the low availability of nutrients in the water as derived from the lower nutrient concentrations in duckweed tissue or high pH in water. High nitrogen levels in semi-static media (5–10 mg N L−1) resulted in an increasingly higher chance to overgrow C. demersum by L. gibba. Field observations revealed that C. demersum dominated over L. gibba in water bodies with total N below 3 mg L−1, while L. gibba dominance over C. demersum occurred above 5 mg L−1 total N. Ceratophyllum occurrence correlated negatively with total N in the water, while Lemna showed a positive correlation. Furthermore, the occurrence of L. gibba was negatively correlated with the frequency of C. demersum. All findings together support the theory that under a certain nutrient range, rootless submerged macrophytes have a strong potential to inhibit the dominance of floating plants in ponds, ditches and channels, and thus, they stabilize the submerged vegetation state.

The increase of nitrogen and phosphorus causes eutrophication in water bodies. Using submerged plants to decrease the pollution from water bodies is an effective way. In this research, three common submerged plants (Vallisneria natans, Hydrilla verticillata, and Ceratophyllum demersum) and their combinations were used to purify eutrophic water. The control treatment did not contain any plants. The removal effects and dynamic regulations of the three plants with their combinations of nutrients (such as nitrogen and phosphorus) in water were analyzed. All three species and their combinations above could grow in the eutrophic water and efficiently remove aquatic nutrients. All the treatment groups had a higher pollutant removal rate for total nitrogen (TN) and total phosphorus (TP) than that of the blank control. In these treatment groups, treatment F (50 g Vallisneria natans plus 50 g Ceratophyllum demersum) had the highest removal rate of TP at 57.53%; treatment B (100 g Vallisneria natans) had the best removal rate of TN at 92.04 %. Among these plants and their combinations, Vallisneria natans and Ceratophyllum demersum showed better purification ability; the combination of these two submerged plants and the combination of three submerged plants were more applicable for the restoration of eutrophic water.

In shallow macrophytic lakes, epiphytic biofilms are formed on the surface of submerged plant stems and leaves because of algae and bacterial accumulation. Epiphytic biofilms significantly impact the health of the host vegetation and the biogeochemical cycling of lake elements. However, community diversity, species interactions, and community assembly mechanisms in epiphytic bacterial communities (EBCs) of plants during different growth periods are not well understood. We investigated the successional dynamics, co-occurrence patterns, and community assembly processes of epiphytic biofilm bacterial communities of submerged plants, Najas marina and Potamogeton lucens, from July to November 2020. The results showed a significant seasonal variation in EBC diversity and richness. Community diversity and richness increased from July to November, and the temperature was the most important driving factor for predicting seasonal changes in EBC community structure. Co-occurrence network analysis revealed that the average degree and graph density of the network increased from July to November, indicating that the complexity of the EBC network increased. The bacterial community co-occurrence network was limited by temperature, pH, and transparency. The phylogeny-based null model analysis showed that deterministic processes dominated the microbial community assembly in different periods, increasing their contribution. In addition, we found that as the dominance of deterministic processes increased, the microbial co-occurrence links increased, and the potential interrelationships between species became stronger. Thus, the findings provide insights into the seasonal variability of EBC assemblage and co-occurrence patterns in lacustrine ecosystems.

Some environmental factors, such as brownification and eutrophication, may influence the successful invasions of non-native submerged macrophytes. However, few studies have focused on how interactions between these factors influence the performance of exotic submerged plants. Here, we conducted an experiment in 60 indoor containers (170 l) over 68 days using the native species Hydrilla verticillata (L. f.) Royle and the non-native species Elodea nuttallii (Planch.) St. John to test the effects of brownification, eutrophication and their interactions on the growth and competition of native and non-native aquatic plants. Our results showed that the biomass of both H. verticillata and E. nuttallii increased in the brown water treatment and that eutrophication and water brownification did not lead to a shift from a native species-dominated system to a non-native-dominated system. However, brown water treatment decreased the relative competitive ability of E. nuttallii, and this decrease was exacerbated when brown water and nutrient treatments were combined. Our results indicated that some environmental factors, such as water brownification, eutrophication and their interactions, may not benefit the competition of some non-native submerged macrophytes. Further studies with more species are needed to corroborate these conclusions.

Biomanipulation via fish regulation combined with submerged plant introduction is an effective measure to restore eutrophic shallow lakes. Improved water quality and clarity promote growth of benthic algae, which with submerged plants may limit sediment phosphorus (P) release, thereby reinforce lake recovery. Our study sought to evaluate the effect of such a biomanipulation on water quality, benthic algal development and sediment P release in a shallow, tropical lake by (1) comparing porewater and lake water quality, light intensity and benthic algal development in restored and unrestored sections; (2) conducting a 32 P radiotracer experiment to track P release from sediment cores sampled from both sections. The biomanipulation led to lower total P, total dissolved P, and soluble reactive P concentrations in lake water, lower phytoplankton biomass, and increased light intensity at sediment surface, stimulating benthic algal development. Moreover, sediment 32 P release was lower in the restored than unrestored section. Concurrently, dissolved oxygen levels in upper layers of the sediment cores were higher in the restored section. Our study indicates that the biomanipulation improved water quality and enhanced growth of benthic algae, thereby reducing sediment P release, which may be one of the main mechanisms to create successful restoration.

Sustainable management of coastal and inland water areas requires knowledge of how tourism and recreation affects the ecosystems. Here, we present the first systematic review and meta-analysis to quantify to what extent recreational boat traffic and infrastructure for mooring affect the abundance of submerged vegetation on soft bottoms. Our systematic search yielded 25 studies containing data on effects of boat traffic, docks and mooring buoys on vegetation abundance. The abundance below docks was on average 18% of that in controls, and areas with boat traffic had on average 42% of the abundance in control areas. Mooring buoys often created scour areas without vegetation. However, the effects were variable and there were too few studies to test the reasons for this variability. We conclude that boating can cause significant declines in submerged vegetation but that informed management of boat traffic and improved design of docks and buoys can reduce negative impacts.