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Denitrification is the most important process for nitrogen removal in eutrophic lakes and was mostly investigated in lake sediment. Denitrification could also be mediated by cyanobacterial aggregates, yet how this process impacts nitrogen (N) availability and the associated autotroph-heterotroph relationships within cyanobacterial aggregates has not been investigated. In this study, incubation experiments with nitrate amendment were conducted with Microcystis aggregates (MAs). Measurement of nitrogen contents, 16S rRNA-based microbial community profiling and metatranscriptomic sequencing were used to jointly assess nitrogen turnover dynamics, as well as changes in microbial composition and gene expression. Strong denitrification potential was revealed, and maximal N removal was achieved within two days, after which the communities entered a state of severe N limitation. Changes of active microbial communities were further promoted both with regard to taxonomic composition and transcriptive activities. Expression of transportation-related genes confirmed competition for N sources by Microcystis and phycospheric communities. Strong stress response to reactive oxygen species by Microcystis was revealed. Notably, interspecific relationships among Microcystis and phycospheric communities exhibited a shift toward antagonistic interactions, particularly evidenced by overall increased expression of genes related to cell lysis and utilization of cellular materials. Patterns of fatty acid and starch metabolism also suggested changes in carbon metabolism and cross-feeding patterns within MAs. Taken together, this study demonstrated substantial denitrification potential of MAs, which, importantly, further induced changes in both metabolic activities and autotroph-heterotroph interactions. These findings also highlight the key role of nutrient condition in shaping autotroph-heterotroph relationships. [Display omitted] • Microcystis aggregates have strong denitrification potential. • Denitrification shifted composition and gene expression of active communities. • Prolonged incubation promoted antagonistic autotroph-heterotroph relationships. • Changes in carbon metabolism and cross-feeding patterns within Microcystis aggregates also occurred. [ABSTRACT FROM AUTHOR]

The eutrophication problem now threatens many lakes and reservoirs. To avoid the occurrence of algal blooms, some cities try to increase the flow rate or directly choose lakes or reservoirs with a short water residence time (WRT) as drinking water sources. However, up to now, whether such a strategy can achieve its goal is still unclear. In this study, a newly restored lake with a WRT of approximately 3 days was chosen to investigate algal growth potential as well as its responses to external nitrogen (N) and phosphorus (P) inputs. The results suggested that although the water quality of the lake could generally meet the environmental quality standards for surface water, dissolved inorganic nitrogen reached a high level with an average value of 1.58 mg/L. Meanwhile, a considerable increase in Chl- a concentration was observed across the flow direction. Especially, in July, Chl- a concentration at the site near the outlet was 8.1 times higher than that at the inlet, and cyanobacteria became the dominant species accounting for 83% of the total cell density. Nutrient enrichment experiments showed that algae could grow rapidly within 3 days with average specific growth rates (μ) of 0.36–0.42 d−1. The addition of N and P furtherly promoted the algal growth, and μ values of the treatments with P addition were the highest at 0.67–0.83 d−1. These results indicated that even if the WRT was reduced to 3 days, the risk of the occurrence of algal blooms still exists, and this undesirable trend would be enhanced by the short-term external nutrient input. Our findings indicated that the hydrodynamic control measures may not be entirely successful in protecting the drinking water source from algal blooms, especially when its influent has already been under eutrophication. • Shortening WRT may not successfully protect the lakes from algal blooms. • Algal biomass increased across the flow direction of the reservoir with a 3-day WRT. • High N in the influent promoted the succession of dominant species to cyanobacteria. • External nutrient input promotes algal growth and enhances the risk of algal blooms. [ABSTRACT FROM AUTHOR]

Fine roots play an important role in the carbon dynamics and nutrient cycles of mangroves. Mangrove forests are threatened by sea level rise (SLR). Therefore, it is critical to understand how SLR affects dynamics of decomposition and nutrient release of dead fine roots to evaluate carbon dynamics and nutrient cycles of mangroves. Dynamics of decomposition and nutrient release of dead fine roots were compared among three sites under different intertidal elevations in a 11-year-old Kandelia obovata mangrove forest, representing tidal flooding situation of SLR values of 0, 40 and 80 cm, respectively. Mangroves at site SLR 80 cm had the highest dry weight loss of dead fine roots in the 10 and 30 cm soil layers. Mean rate of decomposition (MRD) at site SLR 80 cm was significantly higher than those at other sites, increasing by approximately 0.48 and 0.27 mg gDW−1 d−1 in the 10 cm and 30 cm soil layers due to SLR 80 cm, respectively. Site SLR 80 cm had significantly higher organic carbon (OC) losses of dead fine roots and OC loss percentages in the 10 cm soil layer than the other sites, while higher values were found at site SLR 40 cm in the 30 cm soil layer. Moreover, significantly higher nitrogen (N) losses of dead fine roots and N loss percentages were also found at site SLR 40 cm. These indicate that SLR will increase the rates of decomposition and release of nutrients of dead fine roots, which will accelerate carbon dynamics and nitrogen cycles of mangroves. • Potential sea level rise (SLR) increased decomposition of mangrove dead fine roots. • Carbon loss in mangrove dead fine roots was high under high SLR. • Potential SLR increased nitrogen release of mangrove dead fine roots. [ABSTRACT FROM AUTHOR]