[Impounding Impacts of the Three Gorges Reservoir on Phytoplankton Function Groups and Its Relationship with Resource Use Efficiency].

In order to investigate the relationship between phytoplankton community functional group compositions and resource use efficiency in important tributaries of the Three Gorges Reservoir, phytoplankton and environment parameters were sampled from five tributaries, the Xiangxi River, Daning River, Meixi River, Pengxi River, and Huangjin River, in August and November, 2020. There were 119 species (variants) belonging to 62 genera and 7 phyla identified in summer, whereas 118 species (variants) belonging to 7 divisions of 58 genera were found in winter. According to Padisak's theory, all phytoplankton were divided into 25 functional groups, of which there were six important functional groups in both summer and winter:L0, H1, D, Y, MP, and P in summer and L0, H1, A, M, MP, and Y in winter. The α-diversity of the phytoplankton functional group in summer was higher than that in winter. Moreover, a higher α-diversity was also found in downstream samples relative to that in upstream samples, indicating that the community structure was more complex, and the community stability was relatively better in downstream regions of the rivers. Redundancy analysis (RDA) showed that the environment factors, i.e., ν, pH, permanganate index, WT, and RUETN, significantly affected phytoplankton functional groups (P<0.05). Variance partitioning analysis (VPA) indicated that environmental factors had a higher explanatory degree for the change in functional group composition in summer (45.23%); on the contrary, resource use efficiency had a higher explanatory degree in winter (42.33%). The linear fitted model showed that functional groups L0, H1, D, and Y showed a significant positive correlation relationship with RUETN and RUETP in summer, whereas only four functional groups (M, MP, Y, and A) had a linear relationship with RUETP, and all function groups had a good linear relationship with RUETN in winter. These results indicated that the functional groups belonging to cyanobacteria, dinoflagellates, and cryptophyta were more efficient at using limited resources in summer, whereas the diatoms had a good linear relationship with resource use efficiency and formed a dominant group in the low temperature environment of winter. These results suggest that the impounding of the Three Gorges Reservoir area can significantly change the resource use efficiency of phytoplankton, resulting in changes in the phytoplankton functional group composition and community structure.

Temporal heterogeneity of bacterial communities and their responses to Raphidiopsis raciborskii blooms.

To elucidate the interspecies connectivity between cyanobacteria and other bacteria (noncyanobacteria), microbial diversity and composition were investigated through high-throughput sequencing (HTS) in a drinking water reservoir in Chongqing city, Southwest China, during Raphidiopsis raciborskii blooms. Significant temporal changes were observed in microbial community composition during the sampling period, primarily reflected by variations in relative bacterial abundance. The modularity analysis of the network demonstrated that the bacterial community forms co-occurrence/exclusion patterns in response to variations in environmental factors. Moreover, five modules involved in the dynamic phases of the R. raciborskii bloom were categorized into the Pre-Bloom, Bloom, Post-Bloom, and Non-Bloom Groups. The reservoir was eutrophic (i.e., the average concentrations of total nitrogen (TN) and total phosphorus (TP) were 2.32 and 0.07 mg L-1, respectively) during the investigation; however, Pearson's correlation coefficient showed that R. raciborskii was not significantly correlated with nitrogen and phosphorus. However, other environmental factors, such as water temperature, pH, and the permanganate index, were positively correlated with R. raciborskii. Importantly, Proteobacteria (α-, γ-Proteobacteria), Acidobacteria, Chloroflexi, and Firmicutes were preferentially associated with increased R. raciborskii blooms. These results suggested that the transition of R. raciborskii bloom-related microbial modules and their keystone species could be crucial in the development and collapse of R. raciborskii blooms and could provide a fundamental basis for understanding the linkage between the structure and function of the microbial community during bloom dynamics.

Transcriptomic responses to phosphorus in an invasive cyanobacterium, Raphidiopsis raciborskii: Implications for nutrient management.

Phosphorus (P) is a vital macronutrient associated with the growth and proliferation of Raphidiopsis raciborskii, an invasive and notorious bloom-forming cyanobacterium. However, the molecular mechanisms involved in P acclimation remain largely unexplored for Raphidiopsis raciborskii. Here, transcriptome sequencing of Raphidiopsis raciborskii was conducted to reveal multifaceted mechanisms involved in mimicking dipotassium phosphate (DIP), ß-glycerol phosphate (Gly), 2-aminoethylphosphonic acid (AEP), and P-free conditions (NP). Chlorophyll a fluorescence parameters showed significant differences in the NP and AEP groups compared with the DIP and Gly-groups. Expression levels of genes related to phosphate transportation and uptake, organic P utilization, nitrogen metabolism, urea cycling, carbon fixation, amino acid metabolism, environmental information, the ATP-synthesis process in glycolysis, the tricarboxylic acid (TCA) cycle, and the pentose phosphate pathway were remarkably upregulated, while those related to photosynthesis, phycobiliproteins, respiration, oxidative phosphorylation, sulfur metabolism, and genetic information were markedly downregulated in the NP group relative to the DIP group. However, the expression of genes involved in organic P utilization, the urea cycle, and genetic information in the Gly-group, and carbon-phosphorus lyase, genetic information and environmental information in the AEP group were significantly increased compared to the DIP group. Together, these results indicate that Raphidiopsis raciborskii exhibits the evolution of coordination of multiple metabolic pathways and certain key genes to adapt to ambient P changes, which implies that if P is reduced to control Raphidiopsis raciborskii bloom, there is a risk that external nutrients (such as nitrogen, amino acids, and urea) will stimulate the growth or metabolism of Raphidiopsis.

[Analysis of Phytoplankton Community Stability and Influencing Factors in a Tributary of the Three Gorges Reservoir].

Changes in the community stability of freshwater phytoplankton not only induce a series of ecological environment problems but also influence freshwater ecosystem service functions. To understand the changes in community stability and its driving factors, phytoplankton and environmental parameters were analyzed at 11 sample sites in Huaxi River, a tributary of the Three Gorges Reservoir, in spring, summer, autumn, and winter. Moreover, the resource use efficiency (RUEPP), phytoplankton richness (S), phytoplankton evenness (J), and community turnover (BC) were also determined. Results showed that a total of 8 phyla, including 103 genera and 380 species, were identified in Huaxi River throughout the year. Among them, 264 species were collected in spring, 181 in summer, 197 in autumn, and 183 in winter. The number of Chlorophyta was the largest, followed by Bacillariophyta, Euglenophyta, and Cyanophyta. The number of species and cell density in S0 site were the smallest, while those in S2 site were the largest. The RUEPP was fluctuated in four seasons, with the maximum in summer and the minimum in autumn. BC was significantly negatively correlated with RUEPP, phytoplankton richness, total phosphorus (TP), orthophosphate (PO43--P), total nitrogen (TN), nitrate (NO3--N), permanganate index, and conductivity (Spc); however, it was significantly positively correlated with phytoplankton evenness and dissolved oxygen (DO). These results suggest that water level regulation in the Three Gorges Reservoir has a significant impact on the structure of phytoplankton community in Huaxi River, which leads to the instability of phytoplankton community and easy replacement, and the degree of community turnover is affected by the combined effect of biological and abiotic factors.

[Changes in Epilithic Algae Community and Its Relationship with Environmental Factors in the Meixi River, a Tributary of the Three Gorges Reservoir].

To explore the relationship between the community of epilithic algae and environmental factors in tributaries of the Three Gorges Reservoir region, the epilithic algae and related environment factors were investigated from 26 sampling sites in Meixi River, a tributary of the Three Gorges Reservoir region, during flood period (August 2016), drought period (November 2016), and normal water period (March 2017). Results showed that 106 species (including varieties) belonging to 47 genera and 5 families were identified from 26 sampling sites during the three periods. Among these, 73 species belonged to 38 genera and 5 families in the flood period, 67 species belonged to 36 genera and 4 families in the drought period, and 63 species belonged to 33 genera and 4 families in the normal water period. Nineteen, 17, and 18 dominant species were identified during the flood period, drought period, and normal water period, respectively. The main dominant species were Achnanthes sp., Aphanizomenon sp., and Phormidium sp. in the flood period; Achnanthes sp., Gomphonema sp., and Microcystis sp. in the drought period; and Gomphonema sp., Nitzschia sp., and Chroococcus sp. in the normal water period. Moreover, species such as Cocconeis placentula, Achnanthes sp., Gomphonema subclavatum, G. parvulum, Navicula parva, and G. constrictum were determined to be the common dominant species during the three periods. Redundancy analysis revealed that the changes in the relative abundance of dominant species in the backwater sections were significantly related to the electrical conductivity, dissolved oxygen, pH, total phosphorus, and total nitrogen, and the relative abundance of dominant species in the natural sections was mainly related to the electrical conductivity, pH, total phosphorus, temperature, and velocity. Moreover, a different relationship with the environmental factors was determined among the three periods. However, a negative correlation between total phosphorus and the relative abundances in G. parvulum, G. constrictum, and Achnanthes sp. and a positive correlation between pH and the relative abundances in G. parvulum, G. constrictum, and Achnanthes sp. were found during the three periods. These results indicated that the differences in the hydrological regime and environmental factors in the Three Gorges Reservoir played an important role on the composition of epilithic algae community and led to a significant change in the community structure of epilithic algae.

[Dynamics of Epilithic Algae Communities and Their Relationship with Environmental Factors During Storage and Non-storage Periods in the Three Gorges Reservoir].

In order to explore the composition of epiphytic algae and its related environmental factors, 12 sampling sites in the natural reaches and the backwater reaches (including perennial backwater sections and fluctuating backwater sections) were investigated among tributaries of the Caotang River, the Meixi River, and the Daxi River in the Fengjie district of the Chongqing section of the Three Gorges Reservoir under different hydrological regimes (i.e., storage and non-storage periods). Results showed that 103 species of epilithic algae belonging to 45 genera and 4 families are found in the 3 tributaries. This included 67 species belonging to 34 genus in the natural sections and 82 species (64 species in perennial backwater sections and 41 species in fluctuating backwater sections) belonging to 34 genera in the backwater sections. During the storage period, the dominant species in the natural sections were Melosira varians, Cocconeis placentula, Diatoma vulgure, Gyrosigma scalproides, and Oscillatoria tenuis, while the dominant species in the backwater sections were M. varians, Cymbella affinis,D. vulgure, Eucapsis alpina, and M. granulata. During the non-storage period, the dominant species in the natural sections were M. varians, C. affinis, and C. placentula, whereas the dominant species in the backwater sections were O. princeps, O. rupicola,O. formosa, Synedra acus, Ulothrix sp., Merismopedia elegans, and O. tenuis. These results suggested that the compositions of dominant species showed significant differences during the non-storage period, while little difference was found during the storage period. In addition, the dominant species did not show a significant change in the natural sections, but a marked difference was observed in the backwater sections. Similar dominant species were observed in both perennial and fluctuating backwater sections during the non-storage period, but significantly different dominant species were found during the storage period. Redundancy analysis suggested that the composition of epilithic algae was influenced by different environmental factors, such as temperature, electrical conductivity, pH, total nitrogen, and total phosphorus. Indeed, changes in the cell densities of dominant algae at the different sites were mainly affected by temperature and the concentrations of nitrogen and phosphorus. These results suggest that the different hydrological regimes had an important role not only on the reservoir water environment, but also the dynamics of epilithic algal communities.