Organic acids released by submerged macrophytes with damaged leaves alter the denitrification microbial community in rhizosphere.

Submerged macrophytes have important impacts on the denitrification and anaerobic ammonia-oxidizing (anammox) processes. Leaf damage in these plants probably changes the rhizosphere environment, affecting organic acid release and denitrifying bacteria. However, there is a lack of comprehensive understanding of the specific changes. This study investigated these changes in the rhizosphere of Potamogeton crispus with four degrees of leaf excision. When 0 %, 30 %, 50 % and 70 % of leaves were excised, the concentrations of total organic acid were 31.45, 32.67, 38.26, and 35.16 mg/L, respectively. The abundances of nirS-type denitrifying bacteria were 2.10 × 1010, 1.59 × 1010, 2.54 × 1010, and 4.67 × 1010 copies/g dry sediment, respectively. The abundances of anammox bacteria were 7.58 × 109, 4.59 × 109, 3.81 × 109, and 3.90 × 109 copies/g dry sediment, respectively. The concentration of total organic acids and the abundance of two denitrification microorganisms in the rhizosphere zone were higher than those in the root zone and non-rhizosphere zone. With increasing leaf damage, the number of OTUs in the Pseudomonas genus of nirS-type denitrifying bacteria first increased and then decreased, while that of the Thauera genus was relatively stable. The overall increase in the OTU number of anammox bacteria indicated that leaf damage promotes root exudates release, thereby leading to an increase in their diversity. The co-occurrence network revealed that the two denitrification microorganisms had about 60.52 % positive connections in rhizosphere while 64.73 % negative connections in non-rhizosphere. The abundance and community composition of both denitrification microorganisms were positively correlated with the concentrations of various substances such as oxalic acid, succinic acid, total organic acids and NO2--N. These findings demonstrate that submerged plant damage has significantly impacts on the structure of denitrification microbial community in the rhizosphere, which may alter the nitrogen cycling process in the deposit sediment. SYNOPSIS: This study reveals leaf damage of macrophyte changed the rhizosphere denitrification microbial community, which is helpful to further understand the process of nitrogen cycle in water.

Effects of illumination on nirS denitrifying and anammox bacteria in the rhizosphere of submerged macrophytes.

Visibility in lakes can decrease due to increases in the amounts of suspended solids and algae, which inhibits the growth of submerged macrophytes. However, the understanding about whether illumination reduction affects the nitrogen-cycling microorganisms in the rhizosphere of submerged macrophytes, is limited. The abundance and biodiversity of nirS denitrifying and anammox bacteria in the rhizosphere of Potamogeton crispus were studied under 0% (natural light), 20%, 40%, and 60% shading treatments. The abundance of the nirS gene was highest under 60% shading treatment, while the anammox 16S rRNA gene was highest under 40% shading treatment. Moreover, the abundance of the two genes were lower under natural light than under shading conditions during most sampling periods. The quantitative ratio of the two gene (anammox 16S rRNA to nirS gene) abundance fluctuated wildly with the distance away from the roots, under natural light and 20% shading treatment. However, the ratio varied relatively little under 40% and 60% shading treatments. The diversity of nirS denitrifying bacteria was high in the rhizosphere, while the diversity of anammox bacteria was low, and Candidatus Brocadia fulgida was dominant. This study revealed that illumination reduction not only facilitated the growth of nirS denitrifying and anammox bacteria in the rhizosphere, but also weakened the competition between the two bacteria.

The abundance of nirS-type denitrifiers and anammox bacteria in rhizospheres was affected by the organic acids secreted from roots of submerged macrophytes.

Excessive nitrogen has been a global concern to cause lake eutrophication. The denitrification and anammox processes are considered to be effective biological pathways for nitrogen removal. Submerged macrophytes also play a key role in the nitrogen cycle of lakes. However, the mechanism of submerged macrophytes on regulating biological nitrogen removal pathways has not been well quantified. Therefore, this study investigated the impacts of submerged macrophytes on the community structures and abundance of the nirS-type denitrifiers and anammox bacteria in the rhizospheres. The qPCR results indicated that the abundance of two bacteria in the near-rhizospheres of submerged macrophytes was significantly lower than the root compartments and non-rhizospheres, while the concentrations of organic acids in the near-rhizospheres were higher than those of the root compartments and non-rhizospheres. Redundancy analysis results illustrated that concentrations of NO3--N, NO2--N, citric acid and oxalic acid were the key environmental indicators which had the significant impact on the microbial community. The concentrations of citric acid and oxalic acid were negatively correlated with the nirS-type denitrifiers abundance, and the oxalic acid concentrations were negatively correlated with the anammox bacteria abundance. These results indicated that submerged macrophytes could reduce the abundance of nirS-type denitrifiers and anammox bacteria by releasing organic acids. In addition, the highest diversity of denitrifier community were found in the rhizosphere of the Hydrilla verticillata, while the highest diversity of anammox community were found in the Potamogeton maackianus rhizosphere. These results indicate that the impacts of submerged macrophytes on the biological nitrogen removal pathways were species-dependent.

Microbial community of nitrogen cycle-related genes in aquatic plant rhizospheres of Lake Liangzi in winter.

This study investigated the community structure of ammonia-oxidizing bacteria /archaea (AOB and AOA), as well as the effects of four aquatic plants (namely Ceratophyllum demersum, Hydrilla verticillata, Potamogeton crispus, and Nymphaea tetragona) rhizospheres on the abundance of AOB amoA, AOA amoA, anammox 16S rRNA, nirK, and nirS in Lake Liangzi, China. Phylogenetic analysis revealed that most AOB groups were Nitrosospira and Nitrosomonas, in which Nitrosospira was dominant. The AOA amoA were affiliated with two branches of classical sequences which belonging to Thaumarchaeota: water/sediments branch and soil/sediments branch. The abundance of AOA amoA in the rhizospheres of aquatic plants were higher than in the non-rhizosphere (p < 0.05), indicating that aquatic plants may promote the growth of AOA. However, the anammox 16S rRNA showed the opposite trend relative to AOA amoA (p < 0.05). Redundancy analysis (RDA) showed that the differences in abundance of AOB, AOA, anammox bacteria, and denitrifying bacteria are very likely related to the different contents of ammonia nitrogen (NH4 + -N), pH and dissolved oxygen (DO) and thus to the rhizosphere states of aquatic plants.