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Functional redundancy is considered common in microbial systems, but recent studies have challenged this idea. The mechanism for this contradictory result is not clear. However, in this study, we hypothesize that strong environmental filtering which links to the anthropogenic activities is able to weaken microbial functional redundancy. We used metagenome and 16S rRNA gene high-throughput sequencing to investigate planktonic microbial communities in a subtropical river. We found that the weak anthropogenic activities might result in a low selection pressure in the river upstream area. Therefore, the microbial community functional attributes were stable although the community composition changed with the water temperature and NO3 -N in upstream area (this indicates functional redundancy). However, the strong anthropogenic activities in river downstream area selected pollutant-degraded functions (e.g. nitrogen metabolism, toluene, xylenes and ethylbenzene degradation) and potentially pollutant-degraded (tolerant) microbes, and therefore caused the microbial community composition synchronously changed with the variation of community functional attributes. Our results reveal that strong environmental filtering which associates with the anthropogenic activities not only has effects on microbial community composition and community functional attributes but also on their relationships. These results provide a new insight to refine the functional redundancy idea.

Cyanobacterial blooms have profound effects on the structure and function of plankton communities in inland waters, but few studies have focused on the effects of microbial-based processes in one‑carbon and nitrogen cycling on water quality improvement following the bloom. Here, we compared the structure and function of the bacterial community, focusing on microbial one‑carbon and nitrogen metabolisms during and after a cyanobacterial Microcystis bloom in a deep subtropical reservoir. Our data showed that microbial one‑carbon and nitrogen cycles were closely related to different periods of the bloom, and the changes of functional genes in microbial carbon and nitrogen cycling showed the same consistent trend as that of Methylomonas sp. With the receding of the bloom, the abundance of Methylomonas as well as the functional genes of microbial one‑carbon and nitrogen cycling reached the peak and then recovered. Our results indicate that microbial one‑carbon and nitrogen metabolisms were beneficial to the recovery of water quality from the cyanobacterial bloom. This study lays a foundation for a deep understanding of the cyanobacterial decomposition mediated by microbes in one‑carbon and nitrogen cycles in inland freshwaters.

Nutrient (nitrogen and phosphorus) removal by using bioremediation technologies in eutrophic water alters bacterial and protist community structure and function, but how it changes the stability of community remains unclear. To fill this gap, in this study, bacterial and protist communities were investigated using 16S and 18S rRNA gene high-throughput sequencing during the nutrient removal by using ecological floating beds of Canna indica L. Our results showed that both bacterial and protist community compositions in the treatment group were similar to those in the control group at the beginning of the experiment (day 1 to day 11), but then bacterial and protist community compositions became more stable with the removal of nutrients in the treatment group than those in the control group (day 12 to day 18). We further explored the mechanisms for this increased stability and found that the contribution of the stochastic process to bacterial and protist community variations was higher in the control group than that in the treatment group. This suggests that the high nutrient concentration in the control group might increase the random colonization or extinction, and therefore resulted in the high temporal variability (i.e., unstable) of bacterial and protist communities. Our findings suggest that bioremediation for eutrophication can promote the stability of aquatic communities, and therefore potentially maintain aquatic ecosystem functions and services to humanity.

The dinoflagellate Karenia mikimotoi is one of the most noxious species that can form harmful algal blooms in coastal areas worldwide, causing serious damage to aquaculture and fisheries. In the present study, twelve commonly reported candidate genes (cob, cyc, eif4e, ef2, mdh, sam, α-tub, β-tub, actin, cal, 18s, and gapdh) were selected to evaluate their expression stability under different temperatures, salinities, phosphorus and nitrogen limitation, and different time points over a diel cycle in K. mikimotoi. In addition, three algorithms (geNorm, NormFinder, and BestKeeper) were used to evaluate and select the most suitable reference genes. It was observed that α-tub and eif4e were the most suitable reference genes for all samples. In contrast, ef2 and 18s genes showed least stability. Further, a list of stable genes, including α-tub, eif4e, gapdh, and mdh was generated that can be used as reference genes in specific environmental condition in K. mikimotoi. The gene expression profile of KFcp gene was evaluated over a diel cycle using the most stable reference genes. Results revealed that the expression of KFcp with transcripts abundances was higher in light period and lower in dark period, indicating similar gene expression profile of this gene in typically reported algae. This study provides suitable reference genes for further research on the expression and regulation of genes related to important biochemical processes in K. mikimotoi, improving our understanding of the molecular mechanism of its behind toxin secretion and bloom formation

Previous studies suggested that strong positive correlations between microbial taxa destabilized the microbial communities, because high synchrony between taxa caused different taxa responded in tandem to the changes of environmental conditions. In this study, we used nutrient concentrations (nitrogen and phosphorus) as indicators for anthropogenic activities, and explored the effects of anthropogenic activities on the stability of bacterial communities in a subtropical river, Xiyuan River. We also used co-occurrence networks to indicate the synchrony between bacterial taxa. The nutrient concentrations were higher in the midstream and downstream areas than in the upstream area of Xiyuan River. As a result, the average proportion and correlation coefficient of positive correlations were higher in the midstream and downstream networks than in the upstream networks. This indicates that frequent anthropogenic activities increased the synchrony between taxa in the midstream and downstream communities, and therefore destabilized the bacterial communities. To further explore the mechanisms, we found that the changes of community stabilities associated with the changes of bacterial functions. Anthropogenic activity tolerant bacteria (e.g., nutrient removal, aromatic degradation and pathogen bacteria) and their linked bacterial taxa formed large and strong positive modules in the midstream and downstream networks, and thus destabilized the communities. Based on the perspective of microbial correlations, our results provide a new insight in the mechanisms of how anthropogenic activities alter riverine microbial communities.

Previous studies suggested that strong positive correlations between microbial taxa destabilized microbial co-occurrence network, because network members which had strong positive correlations with each other tended to decrease in abundance synchronously. Anthropogenic activities have strong influences on microbial community composition and diversity in riverine ecosystems, but how they influence the stability of microbial co-occurrence network remain unclear. In this study, we used nutrient concentrations (nitrogen and phosphorus) as an indicator for anthropogenic activities, and explored the effects of anthropogenic activities on the stability of bacterial co-occurrence networks in a subtropical river, Xiyuan River. The nutrient concentrations were higher in midstream and downstream areas than in upstream area of Xiyuan River. The average proportion and correlation coefficient of positive correlations were higher in midstream and downstream networks than in upstream networks, indicating frequent anthropogenic activities destabilized the bacterial co-occurrence networks. To further explore the mechanisms, we found that the changes of network stabilities were associated with the changes of bacterial functions. Anthropogenic activity tolerant bacteria (e.g. nutrient removal, aromatic degradation and pathogen bacteria) and their linked bacterial members formed the large and strong positive modules in the midstream and downstream networks, and thus destabilized the networks. Based on network perspective, our results provide a new insight in the mechanisms of how anthropogenic activities alter riverine microbial communities.

The availability of ambient N nutrient is often correlated with the occurrences of harmful algal bloom formed by certain dinoflagellates, making it important to understand how these species might be responding to such conditions. Here, transcriptome sequencing of Karenia mikimotoi was conducted to understand the underlying molecular mechanisms by which this dinoflagellate copes with nitrogen (N) deficiency. Transcriptomic analysis revealed 8802 unigenes (3.56%) that were differentially expressed with ≥ 2-fold change. Under N-depleted conditions, genes involved in glycolysis, fatty acid metabolism, and the tricarboxylic acid (TCA) cycle as well as lipid accumulation were significantly upregulated. The elevated expression of enzymes used in protein degradation and turnover suggests possible metabolic reconfiguration towards accelerated N recycling. Moreover, a significant increase in urea transporter was observed, indicating increased assimilation of organic nitrogen resources as an alternative in N-depleted cultures of K. mikimotoi. The down-regulated glutamate synthase genes were also identified under N deficiency, suggesting suppression of primary amino acid synthesis to save N resource. Taken together, results of this study show enhanced multiple N resource acquisition and reuse of multiple N resources constitute a comprehensive strategy to cope with N deficiency in a dinoflagellate.

The relationship between algicidal bacteria and harmful-algal-bloom-forming dinoflagellates is understudied and their action modes are largely uncharacterized. In this study, an algicidal bacterium (FDHY-03) was isolated from a bloom of Prorocentrum donghaiense and the characteristics of its action against P. donghaiense was investigated at physiological, molecular, biochemical and cytological levels. 16S rDNA sequence analysis placed this strain in the genus of Alteromonas in the subclass of γ-proteobacteria. Algicidal activity was detected in the bacterial filtrate, suggesting a secreted algicidal principle from this bacterium. Strain FDHY-03 showed algicidal activity on a broad range of HAB-forming species, but the greatest effect was found on P. donghaiense, which showed 91.7% mortality in 24 h of challenge. Scanning electron microscopic analysis indicated that the megacytic growth zone of P. donghaiense cells was the major target of the algicidal action of FDHY-03. When treated with FDHY-03 culture filtrate, P. donghaiense cell wall polysaccharides decreased steadily, suggesting that the algicidal activity occurred through the digestion of cell wall polysaccharides. To verify this proposition, the expression profile of beta-glucosidase gene in FDHY-03 cultures with or without P. donghaiense cell addition was investigated using reverse-transcription quantitative PCR. The gene expression level increased in the presence of P. donghaiense cells, indicative of beta-glucosidase induction by P. donghaiense and the enzyme’s role in this dinoflagellate’s demise. This study has isolated a new bacterial strain with a strong algicidal capability, documented its action mode and biochemical mechanism, providing a potential source of bacterial agent to control P. donghaiense blooms.

In freshwater systems, both antibiotic resistance genes (ARGs) and cyanobacterial blooms attract global public health concern. Cyanobacterial blooms can greatly impact bacterial taxonomic communities, but very little is known about the influence of the blooms on antibiotic resistance functional community. In this study, the ARGs in both free-living (FL) and particle-attached (PA) bacteria under bloom and non-bloom conditions were simultaneously investigated in a subtropical reservoir using high-throughput approaches. In total, 145 ARGs and 9 mobile genetic elements (MGEs) were detected. The most diverse and dominant of which (68.93%) were multidrug resistance genes and efflux pump mechanism. The richness of ARGs in both FL and PA bacteria was significantly lower during the bloom period compared with non-bloom period. The abundance of ARGs in FL bacteria was significantly lower under bloom condition than in the non-bloom period, but the abundance of ARGs in PA bacteria stayed constant. More importantly, the resistant functional community in PA bacteria was more strongly influenced by the cyanobacterial bloom than in the FL bacteria, although >96% ARGs were shared in both FL and PA bacteria or both bloom and non-bloom periods. We also compared the community compositions between taxonomy and function, and found antibiotic resistant communities were highly variable and exhibited lower similarity between bloom and non-bloom periods than seen in the taxonomic composition, with an exception of FL bacteria. Altogether, cyanobacterial blooms appear to have stronger inhibitory effect on ARG abundance in FL bacteria, and stronger influence on antibiotic resistant community composition in PA bacteria. Our results further suggested that both neutral and selective processes interactively affected the ARG composition dynamics of the FL and PA bacteria. However, the antibiotic resistant community of FL bacteria exhibited a higher level of temporal stochasticity following the bloom event than PA bacteria. Therefore, we emphasized the bacterial lifestyles as an important mechanism, giving rise to different responses of antibiotic resistant community to the cyanobacterial bloom. Keywords: Antibiotic resistance genes, Bacterioplankton, Community ecology, Free-living bacteria, Particle-attached bacteria, Cyanobacterial bloom, Subtropical reservoir

Although the influence of microbial community assembly processes on aquatic ecosystem function and biodiversity is well known, the processes that govern planktonic communities in human-impacted rivers remain largely unstudied. Here, we used multivariate statistics and a null model approach to test the hypothesis that environmental conditions and obstructed dispersal opportunities, dictate a deterministic community assembly for phytoplankton and bacterioplankton across contrasting hydrographic conditions in a subtropical mid-sized river (Jiulong River, southeast China). Variation partitioning analysis showed that the explanatory power of local environmental variables was larger than that of the spatial variables for both plankton communities during the dry season. During the wet season, phytoplankton community variation was mainly explained by local environmental variables, whereas the variance in bacterioplankton was explained by both environmental and spatial predictors. The null model based on Raup-Crick coefficients for both planktonic groups suggested little evidences of the stochastic processes involving dispersal and random distribution. Our results showed that hydrological change and landscape structure act together to cause divergence in communities along the river channel, thereby dictating a deterministic assembly and that selection exceeds dispersal limitation during the dry season. Therefore, to protect the ecological integrity of human-impacted rivers, watershed managers should not only consider local environmental conditions but also dispersal routes to account for the effect of regional species pool on local communities.

Microeukaryotic plankton is an abundant and diverse component of marine environments and plays an important role in microbial food webs. However, few studies have been conducted on the genetic diversity of microeukaryotes in the subtropical bays of China. In the present study, we investigated the microeukaryotic plankton in the Shenhu Bay by using denaturing gradient gel electrophoresis (DGGE) and sequencing of prominent bands. Our results indicated that Copepoda and Dinophyceae were the most diverse groups, and that the microeukaryotic communities varied significantly between summer and autumn, with the autumn communities exhibited a higher diversity than summer communities. Furthermore, the community composition and diversity from both surface and bottom waters showed more significant differences in summer than in autumn. Environmental parameters also displayed obvious seasonal patterns. Redundancy analysis (RDA) showed that temperature was the most significant environmental factor shaping the seasonal patterns of the microplanktonic members in the Shenhu Bay. Community-level molecular techniques such as DGGE appear as useful tools to detect the presence of red tide causing species and to guide the management of coastal water mariculture.

Lustrin A is the first nacre protein with specific structure and amino acid residue content that was identified in abalone; since its identification, homologs have been found in several abalone species. In this study, we isolated and cloned the complete cDNA of Lustrin A from Haliotis discus hannai, which was named Hdh-Lustrin A. Hdh-Lustrin A has characteristic cysteine- and proline-rich domains, glycine- and serine-rich domains, and a whey acidic protein (WAP)-like C-terminus. The cysteine- and proline-rich domains showed internal similarity repeats that arrayed in gene coding region, and the phylogenetic tree of these repeats indicated that the similarity of structural repetitive unit components in different abalone species, reflecting their evolutionary distance. A tissue distribution analysis showed that the mRNA level of Hdh-Lustrin A has tissue-specific expression in mantle. Under lipopolysaccharide (LPS) challenge, Hdh-Lustrin A showed a significantly increase, while it showed a more complex pattern with two peaks in the process of shell regeneration. Moreover, acidification and warming raised the expression level of Hdh-Lustrin A in shell regeneration in two different manners; acidification raised the gene expression in quick response, in contrast the long run in warming treatment. Similar pattern also has been detected in immune reaction and the thermal treatments. These results suggest that the Hdh-Lustrin A is a nacre protein, which can be distinguished by its cysteine- and proline-rich domain. It involves in shell regeneration and innate immunity in abalone, and its expression pattern during shell regeneration can be disrupted by physicochemical properties of the environment.

External environments to microbial eukaryotic communities often change gradually with time. However, whether the responses of microbial eukaryotic communities to these gradually changed environments are continuous or hysteretic and the mechanisms underlying these responses are largely unknown. Here, we used a microcosm to investigate the temporal variation of microbial eukaryotic communities with the gradually decreased nutrient concentrations (nitrogen and phosphorus). We found the differences of microbial eukaryotic community composition and species richness between the control and treatment groups were low during the days 0 to 12, although the nutrient concentrations decreased rapidly during this period in treatment group. However, these differences were clear during the days 14 to 18, although the nutrient concentrations decreased slowly during this period in treatment group. The mechanisms for these results are that the strong homogenous selection (perhaps due to the biotic factors) during the days 8 to 10 in treatment group might enhance the stability of microbial eukaryotic communities. However, the continuously decreased nutrient concentrations weakened the homogenous selection and promoted the strength of environmental filtering, and therefore resulted in the distinct change of microbial eukaryotic communities during the days 14 to 18 in treatment group. Fungi, Chlorophyta and Chrysophyta which associated with the nutrient removal played important roles in this hysteretic change of microbial eukaryotic communities. Overall, our findings suggest that disentangling the non-linear response of communities to gradual environmental changes is essential for understanding ecosystem restoration and degradation in future.

Bacterial antibiotic resistance genes (ARGs), a kind of emerging environmental pollutants, greatly threat human health through pathogenic bacteria. High-throughput quantitative PCR (HT-qPCR) and metagenomic approaches are two popular tools applied in aquatic environmental ARGs monitoring. However, current poor knowledge of different ARG profiling approaches' impacts on the understanding of the ARGs in aquatic environments greatly limit the further field application of these two approaches. For filling such knowledge gap, this study simultaneously employed these two approaches to examine and compare the ARGs in a freshwater reservoir across space and time. We found metagenomic approach detected more ARG subtypes and much higher bacitracin resistance genes' abundances than HT-qPCR. In general, HT-qPCR and metagenomics analyses both revealed similar ARG dynamic patterns and co-occurrence patterns between ARGs and bacterial taxa as well as the relationships between ARGs and environmental factors. Our results indicated the impacts of different ARG profiling approaches on the understanding of bacterial ARGs might be minor or negligible. HT-qPCR approach has the superiorities of time-saving, absolute quantification, low requirement for bioinformatics skills but also has some drawbacks including higher PCR amplification & primer bias, higher primer dependency and relative lower ARG subtype quantification capability compared to metagenomic approach. We suggest HT-qPCR approach can be employed for routine aquatic environmental monitoring, and metagenomic approach could be applied in comprehensive surveys for getting more ARG subtype information. Our data can be a useful reference for choosing right ARG profiling approaches for bacterial ARGs monitoring and risk assessment in aquatic environments.

Ecological concrete (eco-concrete) can reduce excess nutrients (nitrogen and phosphorus) in water, but its effectiveness in removing nutrients in marine coastal sediments and the response of sediment microbial communities to its use are largely unknown. In this study, eco-concrete planted with Bruguiera gymnorrhiza was used to remove nutrients in marine coastal sediment. We found that the mean removal efficiencies of sediment total nitrogen and total phosphorus by using planted eco-concrete were 11.50% and 30.31% on day 60, and were higher than those obtained by only using B. gymnorrhiza (7.14% and 7.36%). the Diatoms and bacterial genera Fusibacter and Anoxynatronum (which belong to Firmicutes) increased and became the abundant microbes by day 60 when using planted eco-concrete, indicating their potential roles in nutrient removal. Moreover, the eco-concrete did not endanger the core microbes in sediment suggesting its environment-friendly character. Our results suggest a potential method to control marine coastal eutrophication.

Lutein is a metabolite produced by plants or microalgae, and offers various health benefits. Chlorella sorokiniana FZU60 is considered a hyper-lutein producing microalga. In this study, the effects of temperature and light intensity on cell growth and lutein accumulation by FZU60 under mixotrophic conditions were investigated. Maximal lutein productivity and production were obtained at 33 °C, and high light intensity enhanced cell concentration but decreased lutein content when nitrate was replete. Further, two fed-batch strategies with gradient (fed-batch strategy I) or constant (fed-batch strategy II) NaAc feeding concentrations were compared. Results showed that the second strategy was more desirable for lutein production. Moreover, two types of mixotrophy/photoautotrophy two-stage strategies with a light intensity of 750 μmol/m2/s (two-stage strategy I) or 150 μmol/m2/s (two-stage strategy II) in stage 2 were explored. Under the two-stage strategy I, high lutein content (11.22 mg/g), productivity (8.25 mg/L/d), and production (65.96 mg/L) were achieved. The lutein content, productivity, and production obtained in this study are higher than those reported in most related studies, indicating that the proposed strategy can be used in commercial production.

Ocean warming and acidification caused by global climate change interferes with the shell growth of mollusks. In abalone Haliotis discus hannai, the microstructural changes in the shell under stress are unclear, and the effect of thermal stress on biomineralization is unknown. The lack of gene information has also hampered the study of abalone biomineralization mechanisms. In this study, the microstructure of reconstructed shell in H. discus hannai was observed to determine the effects of thermal and acidification stress on shell growth. Three nacre protein genes, Hdh-AP7, Hdh-AP24, and Hdh-perlustrin, were characterized, and their expression pattern during shell repair was measured under thermal and acidification stress and compared with those of two known biomineralization-related genes, Hdh-AP-1 and Hdh-defensin. The stress resulted in aragonite plates with corroded or irregular microstructures. The gene expression of two nacre proteins (Hdh-AP7 and Hdh-AP24), which directly induce crystal formation, were more sensitive to thermal stress than to acidification, but the expression of the regulatory nacre protein (Hdh-perlustrin) and the two known genes (Hdh-AP-1 and Hdh-defensin), which are also related to immunity, showed an interlinked, complex pattern change. We concluded that high temperature and acidification damages the shell microstructure by disturbing the expression pattern of biomineralization-related genes.

Chlorella sorokiniana FZU60, a lutein-enriching microalga, was cultivated in 50 L column photobioreactor to evaluate its potential for lutein production. Initial cell concentration, phosphate concentration and aeration rate were optimized, and results showed that optimal conditions of these three parameters were 0.10 g/L, 0.06 g/L and 0.02 vvm (2.5% CO2), respectively. In addition, a novel two-stage strategy was successfully developed, in which algae were firstly cultivated under fed-batch mixotrophic condition to achieve high biomass concentration, and then shifted to photoautotrophic condition for enhancing lutein accumulation. Moreover, dissolved oxygen was found to be an efficient indicator of acetate depletion in fed-batch stage. The obtained lutein content, production and productivity reached 9.51 mg/g, 33.55 mg/L and 4.67 mg/L/d, respectively, which were greater than those reported in other pilot-scale studies. This proposed strategy provided a cost-effective approach for high-efficient microalgae-based lutein production at pilot-scale, indicating great potential for commercial production.

Temperature has been regarded as an important factor for microalgal growth and the accumulation of lutein and fatty acid. In this study, the lutein and fatty acid producing microalga Chlamydomonas sp. JSC4 was cultivated at various temperatures (20, 25, 30, 35, and 40 °C). Maximum biomass productivity (1271 mg/L/d) and lutein productivity (3.27 mg/L/d) were achieved at 35 °C, whereas the highest lutein content (3.82 mg/g) was obtained at 20 °C. Similarly, maximum total fatty acid (TFA) productivity (177.31 mg/L/d) was obtained at 35 °C, yet TFA content (202.56 mg/g) was highest at 20 °C with increased unsaturated fatty acid proportion. It was observed that carotenoid, fatty acid, and protein contents increased, but carbohydrate content decreased at low temperature, which could be due to the modulation of metabolic flux from carbohydrate to carotenoid, fatty acid and protein. Inspection of gene expression profile showed that the lutein biosynthesis genes (psy, pds, and lut1) and fatty acid biosynthesis genes (bccp and bc) were upregulated at 20 °C when nitrate was replete and at the beginning of nitrate depletion. This study will help to develop temperature manipulation strategy and provide molecular basis of genetic engineering for improvement of the co-production of lutein and fatty acid in strain JSC4.

Ecological floating beds can rapidly remove nutrients (nitrogen and phosphorus) from eutrophic water, but we still know little about whether this process can simultaneously recover microbial eukaryotic communities. To fill this gap, planktonic microbial eukaryotic communities were investigated using 18S rRNA high-throughput gene sequencing during nutrient removal by floating beds of Canna indica L. We found that nutrient concentrations were high in both the control and treatment groups during period 1 (days 0-5) but rapidly decreased in the treatment group during period 2 (days 6-9) and period 3 (days 10-18). However, the microbial eukaryotic species richness and community compositions were similar between the control and treatment groups during periods 1 and 2 but showed small differences during period 3. The microbial eukaryotic co-occurrence networks between the control and treatment groups also showed similar degree centrality and interconnected eukaryotic members. We found that some abundant fungi species significantly responded to nutrient variations, but a large number of abundant ciliates were insensitive to nutrient removal. Our findings suggest that ecological floating beds can rapidly remove nutrients in eutrophic waters but that it is difficult to quickly and simultaneously improve microbial eukaryotic communities. This result reveals the critical influence of nutrient pollution on aquatic ecosystems and therefore on long-term and comprehensive aquatic habitat restoration, as aquatic macrophyte recoveries should be conducted after nutrient controls have been implemented.

The composition of microbial communities can vary at the microspatial scale between free-living (FL) and particle-attached (PA) niches. However, it remains unclear how FL and PA bacterial communities respond to cyanobacterial blooms across water depths. Here, we examined the community dynamics of the FL (0.2–3 μm) and PA (>3 μm) bacterioplankton based on 16S rRNA gene high-throughput sequencing in a subtropical stratified reservoir under Microcystis aeruginosa bloom and non-bloom conditions. Both FL and PA bacterioplankton communities showed different responses in alpha- and beta-diversities to the bloom, suggesting the idea that the responses of bacterial community could depend on lifestyle. Specifically, abundant PA subcommunities showed a greater variation between bloom and non-bloom groups than abundant FL ones. In contrast, rare FL subcommunities exhibited a stronger response to water depth than rare PA ones. Furthermore, the rare taxa exhibited a preference for PA status, shaped and stimulated by the M. aeruginosa bloom. Our analyses also showed that PA bacterial communities were generally more diverse and appeared to be more responsive to routinely measured environmental variables than FL bacteria. Microcystis blooms had a facilitative influence on specific bacteria by mediating the transitions from free-living to particle-attached lifestyles. Altogether, these findings highlight the importance of bacterial lifestyle and abundance in understanding the dynamics of microbial community in cyanobacterial bloom aquatic ecosystem.

Although it is widely recognized that cyanobacterial blooms have substantial influence on the plankton community in general, their correlations with the whole community of eukaryotic plankton at longer time scales remain largely unknown. Here, we investigated the temporal dynamics of eukaryotic plankton communities in two subtropical reservoirs over a 6-year period (2010-2015) following one cyanobacterial biomass cycle-the cyanobacterial bloom (middle 2010), cyanobacteria decrease (late 2010-early 2011), non-bloom (2011-2014), cyanobacteria increase, and second bloom (late 2014-2015). The eukaryotic community succession that strongly correlated with this cyanobacterial biomass cycle was divided into four periods, and each period had distinct characteristics in cyanobacterial biomass and environments in both reservoirs. Integrated co-occurrence networks of eukaryotic plankton based on the whole study period revealed that the cyanobacterial biomass had remarkably high network centralities, and the eukaryotic OTUs that had stronger correlations with the cyanobacterial biomass exhibited higher centralities. The integrated networks were also modularly responded to different eukaryotic succession periods, and therefore correlated with the cyanobacterial biomass cycle. Moreover, sub-networks based on the different eukaryotic succession periods indicated that the eukaryotic co-occurrence patterns were not constant but varied largely associating with the cyanobacterial biomass. Based on these long-term observations, our results reveal that the cyanobacterial biomass cycle created distinct niches between persistent bloom, non-bloom, decrease and increase of cyanobacteria, and therefore associated with distinct eukaryotic plankton patterns. Our results have important implications for understanding how complex aquatic plankton communities respond to cyanobacterial blooms under the changing environments.

Free-living amoebae are widespread in freshwater ecosystems. Although many studies have investigated changes in their communities across space, the temporal variability and the drivers of community changes across different habitat types are poorly understood. A total of 108 surface water samples were collected on a seasonal basis from four reservoirs and two rivers in Xiamen city, subtropical China. We used high throughput sequencing and qPCR methods to explore the occurrence and abundance of free-living amoebae. In total, 335 amoeba OTUs were detected, and only 32 OTUs were shared by reservoir and river habitats. The reservoirs and rivers harbored unique amoebae communities and exhibited distinct seasonal patterns in community composition. High abundance of the 18S rRNA gene of Acanthamoeba was observed in spring and summer, whereas the abundance was low in autumn and winter. In addition, the abundance of Hartmannella was significantly higher when isolated from reservoirs in summer/autumn and from river in spring/summer. Moreover, the temporal patterns of amoebae communities were significantly associated with water temperature, indicating that temperature is an important variable controlling the ecological dynamics of amoebae populations. However, our comparative analysis indicated that both environmental selection, and neutral processes, significantly contributed to amoeba community assembly. The genera detected here include pathogenic species and species that can act as vectors for microbial pathogens, which can cause human infections.

Microeukaryotic plankton (0.2–200 μm) are critical components of aquatic ecosystems and key players in global ecological processes. High-throughput sequencing is currently revolutionizing their study on an unprecedented scale. However, it is currently unclear whether we can accurately, effectively and quantitatively depict the microeukaryotic plankton communities using traditional size-fractionated filtering combined with molecular methods. To address this, we analysed the eukaryotic plankton communities both with, and without, prefiltering with a 200 μm pore-size sieve –by using SSU rDNA-based high-throughput sequencing on 16 samples with three replicates in each sample from two subtropical reservoirs sampled from January to October in 2013. We found that ~25% reads were classified as metazoan in both size groups. The species richness, alpha and beta diversity of plankton community and relative abundance of reads in 99.2% eukaryotic OTUs showed no significant changes after prefiltering with a 200 μm pore-size sieve. We further found that both >0.2 μm and 0.2–200 μm eukaryotic plankton communities, especially the abundant plankton subcommunities, exhibited very similar, and synchronous, spatiotemporal patterns and processes associated with almost identical environmental drivers. The lack of an effect on community structure from prefiltering suggests that environmental DNA from larger metazoa is introduced into the smaller size class. Therefore, size-fractionated filtering with 200 μm is insufficient to discriminate between the eukaryotic plankton size groups in metabarcoding approaches. Our results also highlight the importance of sequencing depth, and strict quality filtering of reads, when designing studies to characterize microeukaryotic plankton communities.

In this work, a field test was conducted to investigate the effects of heavy metal stabilizer addition on brown rice and microbial variables in a cadmium (Cd)-contaminated farmland from April to October in 2016. Compared with the control, red mud-based stabilizer (RMDL) effectively reduced the concentration of Cd in brown rice (with the removal rate of 48.14% in early rice, 20.24 and 47.62% in late rice). The results showed that adding 0.3 kg m−2 RDML in early rice soil or soil for both early and late rice increased the microbial biomass carbon (MBC), the number of culturable heterotrophic bacteria and fungi, and the catalase activity in soil at different stages of paddy rice growth. Furthermore, there was no notable difference in the diversity of the bacterial species, community composition, and relative abundance at phylum (or class) or operational taxonomic unit (OTU) levels between the control and treatment (RMDL addition) groups. In a word, RMDL could be highly recommended as an effective remediation stabilizer for Cd-contaminated farmland, since its continuous application in paddy soil cultivating two seasons rice soil could effectively decrease the Cd content in brown rice and had no negative impact on soil microorganisms.

The degradation of freshwater quality induced by cyanobacterial blooms is a major global environmental concern. Microbially driven nitrogen removal could alleviate eutrophication to some degree in freshwater ecosystems. However, the response of anaerobic ammonium oxidizing (anammox) bacterial communities to cyanobacterial blooms remains poorly understood, especially in reservoir ecosystems. Here we compared the dynamics of anammox bacterial communities during and after a cyanobacterial bloom in a subtropical reservoir. Our data showed that a cyanobacterial bloom triggered a significant increase in bottom anammox bacterial abundance. During the bloom period, anammox bacterial abundance in bottom waters was 9-fold and 52-fold higher compared with non-bloom stratification and mixing periods, respectively. The community composition of anammox bacteria in surface waters changed substantially accompanied by the disappearance of the cyanobacterial bloom, and a shift of dominance from unidentified anammox genera to Ca. Brocadia was observed. Although Ca. Brocadia was always predominant in both middle and bottom waters, the non-bloom period had more unique taxa than the bloom period. Cyanobacterial bloom-related changes in environmental conditions (e.g. NH4-N and total organic carbon) and water stratification together influenced the distribution and dynamics of anammox bacteria. Altogether, our study lays the basis for a better understanding of the breakdown of cyanobacterial blooms in a stratified reservoir.

Freshwater microbial diversity is subject to multiple stressors in the Anthropocene epoch. However, the effects of climate changes and human activities on freshwater protozoa remain poorly understood. In this study, the diversity and distribution of testate amoebae from the surface sediments were investigated in 51 Chinese lakes and reservoirs along two gradients, latitude and trophic status. A total of 169 taxa belonging to 24 genera were identified, and the most diverse and dominant genera were Difflugia (78 taxa), Centropyxis (26 taxa) and Arcella (12 taxa). Our analysis revealed that biomass of testate amoebae decreased significantly along the latitudinal gradient, while Shannon-Wiener indices and species richness presented an opposite trend (P

Difflugia is a morphologically diverse genus of testate amoebae, which are common components of freshwater ecosystems. We observed a new morphotype similar to Difflugia tuberspinifera but without spine in four Xiamen reservoirs, Fujian Province, southeast China. We investigated its morphology and biometry using light and scanning electron microscopy. The linear discriminant analysis and principal component analysis of biometric characters revealed that the spiny and spineless forms of D. tuberspinifera differed only in the presence or absence of spine. Shell height, shell diameter, aperture diameter and collar height did not differ significantly between the two morphotypes. The number of conical spines varies from 0 to 8. However, the distribution of spine numbers showed two main modes at 0 (spineless form 45.1% of individuals) and 4-6 (38.9%), suggesting the possible existence of two genetically distinct lineages. Spines may have ecological and evolutionary significance. Our results suggested that the spiny and spineless morphotypes of D. tuberspinifera represent either a single variable taxon with different ecotypes or sibling species. Further morphological studies on clonal variations and molecular approaches are needed to clarify if the spineless morph represents an independent species or not.

Nitrogen-fixing microorganisms play important roles in the structure and function of aquatic ecosystems. However, the diversity and distribution of diazotrophic bacteria along the lake depth continuum are so far poorly understood. In this study, we investigated the dynamic variations of diazotrophs in a subtropical deep reservoir during the stratified period. We applied an in-depth biomolecular approach (DGGE, clone libraries, and quantitative real-time PCR) to explore the nitrogenase (nifH) gene diversity and abundance. The diazotrophic community shifted between the oxic/anoxic interface and the nifH diversity increased with depth. The Cyanobacteria, affiliated to the toxic bloom-forming Cylindrospermopsis raciborskii, were the dominant diazotrophic cluster in the surface waters, whereas diazotrophic Alphaproteobacteria were dominant in the bottom waters. The relationships between microbial and environmental factors clearly demonstrated that the temperature gradient and the oxygen concentration affect the heterogeneity of the diazotrophic community, thereby influencing the entire aquatic nitrogen cycle.

This study isolated and identified the lutein-enriching microalga Chlorella sorokiniana FZU60. Different types of media and concentrations of sodium acetate and nitrate were evaluated to improve mixotrophic growth and lutein production. Highest lutein content, production, and productivity were obtained in BG11 medium with 1 g/L acetate and 0.75 g/L nitrate. Additionally, pulse feeding with 1 g/L acetate every 48 h led to the alternation between mixotrophy and photoinduction, resulting in a lutein production of 33.6 mg/L. Most notably, excellent lutein content (9.57 mg/g) and productivity (11.57 mg/L/d) were obtained using a new multi-operation integrated strategy, and the achieved levels exceed those reported in most related studies. This work demonstrates the synergistic integration of simple and effective strategies for the enhancement of lutein production in the indigenous microalga C. sorokiniana FZU60 and provides new insight into the highly efficient microalgae-based lutein production.

Over the last decades, invasion by the exotic cordgrass (Spartina alterniflora) has become one of the most serious and challenging environmental issues threatening mangrove wetland ecosystems in China. The purpose of the study was to investigate the microeukaryotic diversity and community composition after S. alterniflora invasion in a mangrove nature reserve and to elucidate the factors that are driving succession or shifts in microeukaryotic communities. In this study, the spatiotemporal distributions of microeukaryotic communities in coastal wetland sediments of the Jiulong River Estuary, southeast China were investigated. The microeukaryotic communities from the wetland were distributed within six major groups (i.e. Alveolata, Stramenopiles, Rhizaria, Viridiplantae, Fungi and Metazoa). Our results indicated that vegetation changes were the primary factor driving the shift in microeukaryotic community composition, rather than seasonal separation. Total nitrogen (TN) and oxidized nitrogen (nitrite and nitrate) were significant environmental factors in explaining the largest portion of the variation in microeukaryotic composition. The exotic cordgrass invasions affected the microeukaryotic communities and played a key role in shaping community structure. It appears that microeukaryotic community could be used as a bioindicator in assessing the recovery process of mangrove ecosystems.

Phytoplankton species are dominant components in reservoir ecosystems, yet little is known about their variability and dynamics, especially along the depth continuum. This study examined vertical and horizontal differences in phytoplankton communities in a typical subtropical deep reservoir (i.e. Dongzhen Reservoir) from 2011 to 2012. Phytoplankton communities separated into four groups based on the biomass data, indicating that temporal differences in the community structure were greater than spatial differences. Autumn communities had the highest diversity measured by the Shannon-Wiener index (2.47±0.07), while summer communities had the lowest diversity (0.46±0.09). Both winter and spring communities were dominated by diatoms (90.4±1.7%), while cyanobacteria dominated in summer communities during our sampling period. In spring and summer, however, the three surface communities characterized by high biomass were most similar to each other, indicating that vertical variation was significantly higher than horizontal differences. Furthermore, Cylindrospermopsis raciborskii accounted for over 96% of the total phytoplankton biomass in the summer surface water. The redundancy analysis (RDA) illustrated that the temporal factor (summer), the spatial factor (depth), and nutrients (nitrite and nitrate nitrogen, ammonium nitrogen, phosphate phosphorus) were significant variables affecting the dynamics of phytoplankton communities. High temperature with stable thermal stratification might have been the cause of C. raciborskii dominance in Dongzhen Reservoir in summer. Regular and long-term monitoring of dominant species is urgently needed for water quality protection and sustainable reservoir management. Copyright© of Dept. of Oceanography and Geography, University of Gdańsk, Poland

Few studies of plankton communities address anthropogenic influences on lotic ecosystems using multiple methods. Here we used both morphological and molecular techniques (denaturing gradient gel electrophoresis) to explore the spatial diversity of the plankton communities in a subtropical urban river during the month of March 2011. We identified 64 morphological taxa and 36 molecular taxa. Our results indicated that plankton communities in the Houxi River might be strongly impacted by anthropogenic disturbance. Nutrient concentrations (nitrogen and phosphorus), water temperature, conductivity, and suspended solids increased whereas dissolved oxygen decreased dramatically from upstream to downstream, and these environmental variables were found to be significantly related (p

Many countries in the world still suffer from high toxic cyanobacterial blooms in inland waters used for human consumption. Regional climate change and human activities within watersheds exert a complex and diverse influence on aquatic ecosystem structure and function across space and time. However, the degree to which these factors may contribute to the long-term dynamics of plankton communities is still not well understood. Here, we explore the impacts of multiple disturbance events (e.g. human-resettlement, temperature change, rainfall, water level fluctuations), including six combined disturbances, on phytoplankton and cyanobacteria in two subtropical reservoirs over six years. Our data showed that combined environmental disturbances triggered two apparent and abrupt switches between cyanobacteria-dominated state and non-cyanobacterial taxa-dominated state. In late 2010, the combined effect of human-resettlement (emigration) and natural disturbances (e.g. cooling, rainfall, water level fluctuations) lead to a 60–90% decrease in cyanobacteria biomass accompanied by the disappearance of cyanobacterial blooms, in tandem with an abrupt and persistent shift in phytoplankton community. After summer 2014, however, combined weather and hydrological disturbances (e.g. warming, rainfall, water level fluctuations) occurred leading to an abrupt and marked increase of cyanobacteria biomass, associated with a return to cyanobacteria dominance. These changes in phytoplankton community were strongly related to the nutrient concentrations and water level fluctuations, as well as water temperature and rainfall. As both extreme weather events and human disturbances are predicted to become more frequent and severe during the twenty-first century, prudent sustainable management will require consideration of the background limnologic conditions and the frequency of disturbance events when assessing the potential impacts on reservoir biodiversity and ecosystem functioning and services.

Fujian reservoirs in southeast China are important water resources for economic and social sustainable development, although few have been studied previously. In recent years, growing population and increasing demands for water shifted the focus of many reservoirs from flood control and irrigation water to drinking water. However, most of them showed a rapid increase in the level of eutrophication, which is one of the most serious and challenging environmental problems. In this study, we investigated the algae community characteristics, trophic state, and eutrophication control strategies for typical subtropical reservoirs in southeast Fujian.Surface water samples were collected using polyvinyl chloride (PVC) plastic bottles from 11 Fujian reservoirs in summer 2010. Planktonic algae were investigated by optical microscopy. Water properties were determined according to the national standard methods.Shallow reservoirs generally have higher values of trophic state index (TSI) and appear to be more susceptible to anthropogenic disturbance than deeper reservoirs. A total of 129 taxa belonging to eight phyla (i.e., Bacillariophyta, Chlorophyta, Chrysophyta, Cryptophyta, Cyanophyta, Euglenophyta, Pyrrophyta, Xanthophyta) were observed and the most diverse groups were Chlorophyta (52 taxa), Cyanophyta (20 taxa), Euglenophyta (17 taxa), Chrysophyta (14 taxa). The dominant groups were Chlorophyta (40.58%), Cyanophyta (22.91%), Bacillariophyta (21.61%), Chrysophyta (6.91%). The species richness, abundance, diversity, and evenness of algae varied significantly between reservoirs. TSI results indicated that all 11 reservoirs were eutrophic, three of them were hypereutrophic, six were middle eutrophic, and two were light eutrophic. There was a strong positive correlation between algal diversity and TSI at P0.05. Our canonical correspondence analysis (CCA) results illustrated that temperature, transparency, conductivity, DO, TC, NH(4)-N, NO(x)-N, TP, and chlorophyll a were significant environmental variables affecting the distribution of algae communities. The transparency and chlorophyll a were the strongest environmental factors in explaining the community data. Furthermore, the degradation of water quality associated with excess levels of nitrogen and phosphorus in Fujian reservoirs may be impacted by interactions among agriculture and urban factors. A watershed-based management strategy, especially phosphorus control, should be developed for drinking water source protection and sustainable reservoirs in the future.All investigated reservoirs were eutrophicated based on the comprehensive TSI values; thus, our results provided an early warning of water degradation in Fujian reservoirs. Furthermore, the trophic state plays an important role in shaping community structure and in determining species diversity of algae. Therefore, long-term and regular monitoring of Euglenophyta, Cyanophyta, TN, TP and chlorophyll a in reservoirs is urgently needed to further understand the future trend of eutrophication and to develop a watershed-based strategy to manage the Cyanophyta bloom hazards.

Prokaryotic and eukaryotic microbes are key organisms in aquatic ecosystems and play pivotal roles in the biogeochemical cycles, but little is known about genetic diversity of these communities in subtropical rivers. In this study, microbial planktonic communities were determined by using denaturing gradient gel electrophoresis (DGGE) analysis from the Jiulong River, southeast China, and their relationships with local environmental factors were studied. The Betaproteobacteria (26%) and Dinophyceae (26%) were the most dominant taxa in prokaryotic and eukaryotic clones derived from DGGE bands, respectively. Further, both cluster and ordination analyses of prokaryotic and eukaryotic DGGE fingerprinting resulted in three identical groups from the 15 sites, which were closely related with the environmental factors. Partial redundancy analysis (partial RDA) revealed that agricultural pollution (phosphorus and nitrogen) and saltwater intrusion (conductivity and salinity) were the main factors impacting microbial community composition, by explaining more than two-thirds of the total variation in both prokaryotic (67.0%) and eukaryotic (70.5%) communities. Moreover, the robust and quantifiable relationship between DGGE results and environmental variables indicated that the community-level molecular fingerprinting techniques could support the physicochemical assessment of riverine water quality and ecosystem health.

Globally aquatic ecosystems are likely to become more vulnerable to extreme water fluctuation rates due to the combined effects of climate change and human activity. However, relatively little is known about the importance of water level fluctuations (WLF) as a predictor of phytoplankton community shifts in subtropical reservoirs. In this study, we used one year of data (2010-2011) from four subtropical reservoirs of southeast China to quantify the effects of WLF and other environmental variables on phytoplankton and cyanobacteria dynamics. The reservoirs showed an apparent switch between a turbid state dominated by cyanobacteria and a clear state dominated by other non-cyanobacterial taxa (e.g., diatoms, green algae). Cyanobacterial dominance decreased, or increased, following marked changes in water level. Multiple regression analysis demonstrated that pH, euphotic depth, WLF, and total phosphorus provided the best model and explained 30.8% of the variance in cyanobacteria biomass. Path analysis showed that positive WLF (i.e. an increase in water level) can reduce the cyanobacteria biomass either directly by a dilution effect or indirectly by modifying the limnological conditions of the reservoirs in complex pathways. To control the risk of cyanobacterial dominance or blooms, WLF should be targeted to be above +2m/month; that is an increase in water level of 2m or more. Given that WLF is likely to be of more frequent occurrence under future predicted conditions of climate variability and human activity, water level management can be widely used in small and medium-sized reservoirs to prevent the toxic cyanobacterial blooms and to protect the ecosystem integrity or functions.

The simultaneous analysis of multiple components of ecosystems is crucial for comprehensive studies of environmental changes in aquatic ecosystems, but such studies are rare. In this study, we analyzed simultaneously the bacterioplankton and phytoplankton communities in three Chinese subtropical reservoirs and compared the response of these two components to seasonal environmental changes. Time-lag analysis indicated that the temporal community dynamics of both bacterioplankton and phytoplankton showed significant directional changes, and variance partitioning suggested that the major reason was the gradual improvement of reservoir water quality from middle eutrophic to oligo-mesotrophic levels during the course of our study. In addition, we found a higher level of temporal stability or stochasticity in the bacterioplankton community than in the phytoplankton community. Potential explanations are that traits associated with bacteria, such as high abundance, widespread dispersal, potential for rapid growth rates, and rapid evolutionary adaptation, may underlie the different stability or stochasticity of bacterioplankton and phytoplankton communities to the environmental changes. In addition, the indirect response of bacterioplankton to nitrogen and phosphorus may result in the fact that environmental deterministic selection was stronger for the phytoplankton than for the bacterioplankton communities.

The Southern Ocean is among the least explored marine environments on Earth and still little is known about regional and vertical variability in the diversity of Antarctic marine prokaryotes. In this study, the bacterioplankton community in both epipelagic and mesopelagic waters was assessed at two adjacent stations by high-throughput sequencing and quantitative PCR. Water temperature was significantly higher in the superficial photic zone, while higher salinity and dissolved oxygen were recorded in the deeper water layers. The highest abundance of the bacterioplankton was found at a depth of 75 m, corresponding to the deep chlorophyll maximum layer. Both Alphaproteobacteria and Gammaproteobacteria were the most abundant taxa throughout the water column, while more sequences affiliated to Cyanobacteria and unclassified bacteria were identified from surface and the deepest waters, respectively. Temperature was the most significant environmental variable affecting the bacterial community structure. The bacterial community composition displayed significant differences at the epipelagic layers between two stations, whereas those in the mesopelagic waters were more similar to each other. Our results indicated that the epipelagic bacterioplankton might be dominated by short-term environmental variable conditions, whereas the mesopelagic communities appeared to be structured by longer water-mass residence time and relative stable environmental factors.