The roles of rare and abundant microbial species in the primary succession of biological soil crusts are differentiated in metal tailings ponds with different states.

Tailings ponds formed by long-term accumulation of mineral processing waste have become a global environmental problem. Even worse, tailings ponds are often simply abandoned or landfilled after they cease to be used. This allows pollution to persist and continue to spread in the environment. The significance of primary succession mediated by biological soil crusts for tailings pond remediation has been illustrated by previous studies. However, the process of primary succession may not be the same at different stages during the lifetime of tailings ponds. Therefore, we investigated the environmental differences and the successional characteristics of microbial communities in the primary successional stage of tailings ponds at three different states. The results showed that the primary succession process positively changed the environment of tailings ponds in any state of tailings ponds. The primary successional stage determined the environmental quality more than the state of the tailings pond. In the recently abandoned tailings ponds, abundant species were more subjected to heavy metal stress, while rare species were mainly limited by nutrient content. We found that as the succession progressed, rare species gradually acquired their own community space and became more responsive to environmental stresses. Rare species played an important role in microbial keystone species groups.

Nitrogen-fixing cyanobacteria enhance microbial carbon utilization by modulating the microbial community composition in paddy soils of the Mollisols region.

Nitrogen-fixing cyanobacteria (NFC) are photosynthetic prokaryotic microorganisms capable of nitrogen fixation. They can be used as biofertilizers in paddy fields, thereby improving the rice tillering capacity and yield. To reveal the microbiological mechanisms by which nitrogen-fixing cyanobacteria alter soil carbon storage, we conducted a field experiment using NFC as a partial substitute for nitrogen fertilizer in paddy fields in the Sanjiang Plain of Northeast China's Mollisols region. Using metagenomic sequencing technology and Biolog Ecoplate™ carbon matrix metabolism measurements, we explored the changes in the soil microbial community structure and carbon utilization in paddy fields. The results indicated that the replacement of nitrogen fertilizer with NFC predisposed the soil microbial community to host a great number of copiotrophic bacterial taxa, and Proteobacteria and Actinobacteria were closely associated with the metabolism of soil carbon sources. Moreover, through co-occurrence network analysis, we found that copiotrophic bacteria clustered in modules that were positively correlated with the metabolic level of carbon sources. The addition of NFC promoted the growth of copiotrophic bacteria, which increased the carbon utilization level of soil microorganisms, improved the diversity of the microbial communities, and had a potential impact on the soil carbon stock. The findings of this study are helpful for assessing the impact of NFC on the ecological function of soil microbial communities in paddy fields in the black soil area of Northeast China, which is highly important for promoting sustainable agricultural development and providing scientific reference for promoting the use of algal-derived nitrogen fertilizers.

Integrated evaluation of the impact of water diversion on water quality index and phytoplankton assemblages of eutrophic lake: A case study of Yilong Lake.

Water diversion has been widely utilized to enhance lake water quality and mitigate cyanobacterial blooms. However, previous studies have mainly focused on investigating the effects of water diversion on water quality or aquatic ecological health. Consequently, there is limited research investigating the combined impact of water diversion on the water quality and the ecological health of eutrophic lakes, and whether the WQI and phytoplankton assemblages demonstrate similar patterns following water diversion. In this study, the effects of water diversion on the ecosystem health of eutrophic lakes were comprehensively evaluated based on the WQI indices and phytoplankton assemblages during the NWDP-21 and WDP-22. The results showed that the annual mean of WQI increased from 52.02 to 54.36 after water diversion, which improved the water quality of the lake, especially NH3-N and TN decreased by 58.6% and 15.2%, respectively. The phytoplankton assemblages changed significantly before and after water diversion, and we observed that the total biomass of phytoplankton decreased by 12.3% and phytoplankton diversity indices (Shannon-Wiener diversity, Pielou evenness, and Simpson index) increased by 8.6%-8.9% after water diversion, with an improvement in the connectivity and stability of the phytoplankton. Notably, enhanced adaptations of rare sub-communities for resource use in water diversion environments, and water diversion inhibited the dispersal ability of dominant functional groups, and the effects of hydrological disturbances on the structure of phytoplankton assemblage favored the ecological health of eutrophic lakes. VPA analysis further reveals that water diversion alters the drivers of phytoplankton functional group biomass and phytoplankton diversity. The results of the PLS-PM analysis clarify that water diversion indirectly impacts the total phytoplankton biomass and phytoplankton diversity primarily by modifying light availability. Significant correlations are observed between the dominant functional groups biomass and diversity indices of WQI. The trends in changes observed in water quality indices and phytoplankton following water diversion align with the evaluation of water ecological health. This study provides valuable guidance for the ecological management of the diversion project in Yilong Lake and serves as a reference for similar projects in other lakes.

Soil microbe-mediated carbon and nitrogen cycling during primary succession of biological soil crusts in tailings ponds.

Tailings ponds resulting from mining operations have led to serious environmental hazards, and their bioremediation is an area of ongoing exploration. Primary succession represents the starting point of biotic community establishment and development, with soil carbon and nitrogen cycling being critical to this process. To investigate the soil microbial-mediated carbon and nitrogen cycling patterns accompanying primary succession, we selected three types of tailings ponds as study areas and set up sampling sites for different stages of primary succession. The results showed that primary succession promoted microbe-mediated carbon and nitrogen cycling. It also led to improvements in soil nutrient availability and enzyme activity. In primary succession, the main pathways of carbon cycling are 3HP and rTCA, and nitrogen cycling is nitrate assimilation. In the early stages, microbes mediated more anaerobic and microaerobic processes. As succession proceeded, the pattern of microbial contributions to the carbon and nitrogen cycles changed. As succession proceeds, the functional metabolic potential of the carbon cycle gradually rises, while the nitrogen cycle shows a dramatic increase after the accumulation of autotrophic biomass. In addition, we found a positive coupling pattern between the carbon and nitrogen cycles. These findings support the optimization of bioremediation strategies for tailings ponds.

Network Analysis Indicates Microbial Assemblage Differences in Life Stages of Cladophora.

Cladophora represents a microscopic forest that provides many ecological niches and fosters a diverse microbiota. However, the microbial community on Cladophora in brackish lakes is still poorly understood. In this study, the epiphytic bacterial communities of Cladophora in Qinghai Lake were investigated at three life stages (attached, floating, and decomposing). We found that in the attached stage, Cladophora was enriched with chemoheterotrophic and aerobic microorganisms, including Yoonia-Loktanella and Granulosicoccus. The proportion of phototrophic bacteria was higher in the floating stage, especially Cyanobacteria. The decomposing stage fostered an abundance of bacteria that showed vertical heterogeneity from the surface to the bottom. The surface layer of Cladophora contained mainly stress-tolerant chemoheterotrophic and photoheterotrophic bacteria, including Porphyrobacter and Nonlabens. The microbial community in the middle layer was similar to that of floating-stage Cladophora. Purple oxidizing bacteria were enriched in the bottom layer, with Candidatus Chloroploca, Allochromatium, and Thiocapsa as the dominant genera. The Shannon and Chao1 indices of epibiotic bacterial communities increased monotonically from the attached stage to the decomposing stage. Microbial community composition and functional predictions indicate that a large number of sulfur cycle-associated bacteria play an important role in the development of Cladophora. These results suggest that the microbial assemblage on Cladophora in a brackish lake is complex and contributes to the cycling of materials. IMPORTANCE Cladophora represents a microscopic forest that provides many ecological niches fostering a diverse microbiota, with a complex and intimate relationship between Cladophora and bacteria. Many studies have focused on the microbiology of freshwater Cladophora, but the composition and succession of microorganisms in different life stages of Cladophora, especially in brackish water, have not been explored. In this study, we investigated the microbial assemblages in the life stages of Cladophora in the brackish Qinghai Lake. We show that heterotrophic and photosynthetic autotrophic bacteria are enriched in attached and floating Cladophora, respectively, whereas the epiphytic bacterial community shows vertical heterogeneity in decomposing mats.

Geosmin disrupts energy metabolism and locomotor behavior of zebrafish in early life stages.

Geosmin has been commonly detected both in various aquatic environments and biota, but its exact toxicological mechanisms to organisms need further experimentation. In the present study, zebrafish embryos were exposed to geosmin at nominal concentrations of 50, 500 and 5000 ng/L for 120 h post-fertilization (hpf), followed by locomotor activity and biochemical parameter examination, and multi-omics investigation of the transcriptome and metabolome. The results showed that geosmin exposure significantly reduced the mitochondrial electron transport chain (ETC) complexes I-V, ATP content and mitochondrial respiration and suppressed the locomotor behavior of zebrafish larvae. Transcriptomics analysis revealed that the transcripts of genes involved in oxidative phosphorylation, glycolysis, and lipid metabolism were significantly affected, indicating that geosmin disrupts energy metabolism. Furthermore, metabolomics results showed that 3 classes of lipids, namely glycerophospholipids (GPs), sphingolipids (SLs) and fatty acyls (FAs) were significantly decreased after geosmin exposure. This study provides novel insight into the underlying mechanisms of geosmin-induced energy metabolism and highlights the need for concern about geosmin exposure.

Gradient rise in seepage pollution levels in tailings ponds shapes closer linkages between phytoplankton and bacteria.

A large number of tailings ponds formed by slag accumulation have become serious environmental hazards. Spatially high potential energy and long-term accumulation may result in gradient-changing seepage pollution. The assemblages of phytoplankton and bacteria are widely used as assessment indicators. In this study, we investigate the changes in phytoplankton and bacterial assemblages in tailing pollution. The results showed that there are temporal and spatial variabilities in seepage pollution. The abundance and diversity of phytoplankton and bacteria decreased with increasing pollution. However, Synedra acus (diatom) and Polynucleobacter (bacteria) were positively correlated with pollution levels (r = 0.37, P < 0.05; r = 0.24, P < 0.05). Heavy metals are the main contributors to bacterial changes (16.46%), while nutrients are for algae (13.24%). Tailings pond pollution reduced the number of phytoplankton and bacterial linkages. However, more pollution broke the originally independent modules of phytoplankton and bacteria, and they produced more positive correlations (79.39%; 87.68%). Microcystis sp. and Limnobacter were the key nodes of the co-occurrence network in the polluted areas. Exploring the interactions between bacteria and phytoplankton within different pollution levels could provide insights into biological interaction patterns and the bioremediation of tailings ponds.

Insights into the spatiotemporal differences in tailings seepage pollution by assessing the diversity and metabolic functions of the soil microbial community.

The formation of tailings ponds depends on the long-term accumulation of tailing and high terrain. Its seepage pollution characteristics may have gradient variations on spatiotemporal scales. Used three nearby metal tailings ponds with different service times, we aimed to reveal seepage pollution trends on spatiotemporal scales and the response of soil microbial community. The results showed that the degree of seepage pollution was negatively correlated with the distance from the tailings pond on the spatial scale, while the seepage pollution showed higher levels in tailings ponds with longer service times on the temporal scale (RI = 248.04-2109.85). The pollution effect of seepage persisted after the tailings pond was discontinued (RI = 226.72). Soil microbial diversity increased with spatial scale expansion. The proportion of Actinomyces gradually increased and Proteobacteria decreased. Cr (r = 0.21) and Fe (r = 0.22) contributed more to the microbial community changes. Functional predictions showed that pathways related to signal transduction and energy metabolism were more abundant in the tailings pond. In contaminated areas, the proportion of nitrate respiration and cellulolysis functional communities had decreased, and some potentially pathogenic human taxa had accumulated. These results emphasized that there was pollution accumulation on temporal scale and pollution dispersion on spatial scale around tailings ponds, and the response of the microbial community further illustrated these trends.

ß-Ionone causes endocrine disruption, hyperpigmentation and hypoactivity in zebrafish early life stages.

In nature, the odorous substance ß-ionone has been widely detected in aquatic ecosystems. However, little is known about its ecotoxicological effects on freshwater vertebrates. In this study, we aimed to assess the acute toxicity of ß-ionone in zebrafish (Danio rerio) embryos from 2 to 120 h post fertilization (hpf) and investigate embryo development, locomotor behavior and pigmentation under different concentrations. The results showed that exposure to ß-ionone had an acute toxicity to early life stages of zebrafish and induced a decrease in hatching rate and an increase in the mortality and malformation rate. The median lethal concentration (LC50) of ß-ionone at 96 h was observed as 1321 µg/L. In addition, ß-ionone not only affected the body length of zebrafish larvae but also regulated the transcription of genes and the levels of hormones involved in the growth hormone/insulin-like growth factor (GH/IGF) and the hypothalamic-pituitary-thyroid (HPT) axes. Moreover, exposure to ß-ionone induced significant decreases in locomotor activity and catecholamine neurotransmitters levels. Furthermore, ß-ionone stimulated pigmentation via regulation of tyrosinase activity and melanin-related gene expression. Overall, this research could provide new insights into the potential risk of odorants to aquatic organisms.

Environmental factors associated with the filamentous green algae Cladophora blooms: A mesocosm experiment in a shallow eutrophic lake.

The process of ecological restoration in eutrophic lakes, often results in the blooming of the filamentous green algae Cladophora. This consequently affects the growth of submerged plants and the restoration of vegetation. However, the blooming process of Cladophora and the environmental factors affecting their growth are poorly understood. This has become a difficult problem in the management of lakes. The study therefore focused on succession process of Cladophora blooms and their driving factors through mesocosm experiments in Caohai Lake. The results of our experiment indicated that Cladophora growth was mainly affected by water temperature, turbidity and soluble reactive phosphorus concentration of the habitat where Elodea nuttallii and Cladophora coexist. Nuisance Cladophora was mainly affected by turbidity (>19.24 NTU) when the water temperature was above 15.7 °C. With increasing Cladophora biomass and decreasing turbidity (<4.88 NTU), Cladophora biomass accumulation was mainly limited by the soluble reactive phosphorus concentration (<3.2 µg/L). Recorded turbidity range of 9.54-13.19 NTU was found to cause dramatic changes in the biomass of Cladophora. The results also showed that the outbreak of Cladophora blooms was mainly attributed to turbidity when the water temperature was appropriate in eutrophic lakes. These findings suggest that successful management efforts should strengthen the monitoring of transparency change in addition to controlling the phosphorus concentration to limit the Cladophora overgrowth on lake ecological restoration.

Precipitation Mediates the Distribution but Not the Taxonomic Composition of Phytoplankton Communities in a Tributary of Three Gorges Reservoir.

Precipitation is a driver of changes in the spatiotemporal distribution of phytoplankton communities. The ecological consequence of precipitation is important, but the underlying processes are not clear. Here we conducted an immediate prior- and after-event short-interval investigation in the Three Gorges Reservoir region, to test whether the short-term changes in the phytoplankton communities and functional groups could be predicted based on the precipitation level. We found that precipitation of moderate and high levels immediately changed the phytoplankton distribution and altered functional groups. According to structural equation model, the vertical velocity (λ = -0.81), light availability (Zeu/Zmix, λ = 0.47) and relative water column stability (RWCS, λ = 0.38) were important parameters for phytoplankton distribution during the precipitation event. Water quality did not directly affect phytoplankton distribution (λ = -0.11) and effects of precipitation on the water quality only lasted 1-2 days. The phytoplankton community was redistributed with some tolerance functional groups appearance, such as groups F, Lo, M and groups M, MP, TB, W1 appeared during- and after- precipitation event, respectively. We also found that mixing rather than flushing was the driving force for the decrease of phytoplankton biomass. Our study provided valuable data for reservoir regulation and evidence for predictions of phytoplankton during the precipitation events under different climate change scenarios.

Physiological and transcriptomic changes of zebrafish (Danio rerio) embryos-larvae in response to 2-MIB exposure.

2-Methylisoborneol (2-MIB), a natural odorous substance, is widely distributed in water environment, but there is a paucity of information concerning its systemic toxicity. Herein, we investigated the effects of 2-MIB exposure on developmental parameters, locomotive behavior, oxidative stress, apoptosis and transcriptome of zebrafish. Zebrafish embryos exposed to different concentrations (0, 0.5, 5 and 42.8 µg/L) of 2-MIB showed no changes in mortality, hatchability, and malformation rate, but the body length of zebrafish larvae was significantly increased in a dose-dependent manner, and accompanied by the changes of growth hormone/insulin-like growth factor (GH/IGF) axis and the hypothalamic-pituitary-thyroid (HPT) axis genes. Moreover, the swimming activity of zebrafish larvae increased, which may be due to the increase of acetylcholinesterase (AChE) activity. Meanwhile, 2-MIB caused oxidative stress and apoptosis in zebrafish larvae by altering the NF-E2-related factor 2 (Nrf2) and mitochondrial signaling pathways, respectively. Transcriptome sequencing assay showed that the phototransduction signaling pathway was significantly enriched, and most of the genes in this pathway exhibited enhanced expression after exposure to 2-MIB. These findings provide an important reference for risk assessment and early warning to 2-MIB exposure.

Spatiotemporal distribution of microplastics in surface water, biofilms, and sediments in the world's largest drinking water diversion project.

Investigations of microplastics have increased exponentially over the past decade, yet no information is currently available on the status of microplastics in strictly regulated, artificial bodies of water. The Middle Route of the South-to-North Water Diversion Project (SNWDP) in China, a highly regulated canal, supplies water to 19 cities and more than 53.10 million residents since 2014, as part of the world's largest inter-basin drinking water diversion project. In this study, the spatiotemporal distribution, characteristics, and polymer types of microplastics were surveyed for the first time in the Middle Route of the SNWDP. On average, microplastics were distributed at abundances of 516 items m-3 in water, 20 items kg-1 in biofilms (wet weight), and 24 items kg-1 in sediments (wet weight), lower than that in other typical inland waters. Both sites and seasons significantly affected microplastic abundance in water, biofilms, and sediments; microplastic abundance in water was also significantly positively correlated with distance to the headwork. The main microplastics in the canal were small (0.05-1 mm) polyethylene terephthalate (PET) fibers. Interestingly, microplastics were concentrated in biofilms, indicating that biofilms could serve as a sink for microplastics in the canal. Vehicular harvester was used to demonstrate the practicality of biofilm harvest to mitigate contamination with microplastics. Our results showed that microplastics are consistently transported long distances through the canal, biofilms play an important role in the fate of microplastics in the canal, and that biofilm harvest could be potentially used to mitigate microplastic pollution.

Environmentally relevant concentrations of geosmin affect the development, oxidative stress, apoptosis and endocrine disruption of embryo-larval zebrafish.

Geosmin (trans-1, 10-dimethyl-trans-9-decalol), a volatile organic compound, has been widely detected in aquatic ecosystems. However, the ecological effects of geosmin are not clear. Here, using zebrafish (Danio rerio) embryo as a model, we investigated biological activity effects of environmentally relevant concentrations (50, 500, 5000 ng/L) of geosmin on the developing zebrafish starting from 2 h post-fertilization (hpf) to 96 hpf. Results showed geosmin had no effect on hatchability, malformations and mortality. However, we observed that geosmin exposure significantly increased zebrafish body length in a concentration dependent manner. This effect was possibly due to up-regulation of expression of genes along the growth hormone/insulin-like growth factor (GH/IGF) axis and hypothalamic-pituitary-thyroid (HPT) axis. In addition, superoxide dismutase (SOD) activities and catalase (CAT) activities significantly increased at 96 hpf when the embryos were exposed to 500 and 5000 ng/L of geosmin. The malondialdehyde (MDA) contents and glutathione S-transferase (GST) activities decreased significantly after the exposure to 5000 ng/L geosmin. Simultaneously, exposure to geosmin resulted in significant increase in cell apoptosis, mainly in the heart area. The mRNA levels of the genes related to oxidative stress and apoptosis were also altered significantly after geosmin exposure. These findings indicated that geosmin can simultaneously induce multiple responses during zebrafish embryonic development, including oxidative stress, apoptosis, and endocrine disruption.

Nitrogen limitation significantly reduces the competitive advantage of toxic Microcystis at high light conditions.

Microcystis is a notorious cyanobacterial genus due to its rapid growth rate, huge biomass, and producing toxins in some eutrophic freshwater environments. To reveal the regulatory factors of interspecific competition between toxic and non-toxic Microcystis, three dominant Microcystis strains were selected, and their photosynthesis, population dynamics and microcystins (MCYST) production were measured. The results suggested that nitrogen-limitation (N-limitation) had a greater restriction for the growth of toxic Microcystis than that of non-toxic Microcystis, especially when cultured at high light or high temperature based on the weight analysis of key factors. Comparison of photosynthesis showed that low light or N-rich would favor the competitive advantage of toxic Microcystis while high light combined with N-limitation would promote the competitive advantage of non-toxic Microcystis, and these two competitive advantages could be further amplified by temperature increase. Mixed competitive experiments of toxic and non-toxic Microcystis were conducted, and the results of absorption spectrum (A485/A665) and qPCR (real-time quantitative PCR) suggested that the proportion of toxic Microcystis and the half-time of succession process were significantly reduced by 69.4% and 28.4% (p < 0.01) respectively by combining N-limitation with high light intensity than that measured under N-limitation condition. N-limitation led to a significant decrease of MCYST cellular quota in Microcystis biomass, which would be further decreased to a lower level by the high light. Based on above mentioned analysis, to decrease the MCYST production of Microcystis blooms, we should control nutrient, especial nitrogen through pollutant intercepting and increase the light intensity through improving water transparency.

Sediment phosphorus release in response to flood event across different land covers in a restored wetland.

The phosphorus (P) fraction and its release characteristics from sediment in response to flood events across different land covers (i.e., reclaimed land with dominant vegetation of Phragmites australis and/or Typha orientalis, grassland with dominant vegetation of annual and perennial forbs, and bare land) in the lakeshore of Chaohu Lake were investigated. The results indicated that the re-flooding of a restored wetland led to P release. IP (inorganic P) was the major P fraction in the soils pre-flood and post-flood. For all the soil samples, the rank order of P fractions was Ca-P (P associated with calcium) > OP (organic P) > Fe/Al-P (P bound to Al, Fe, and Mn oxides and hydroxides). During flooding, Fe/Al-P contributed the most as the P release source in the soils and to the P sources for the overlying water. In reclaimed land, Fe/Al-P release correlated significantly with soil pH. In grassland, Fe/Al-P release correlated significantly with soil pH and Al content. In bare land, Fe/Al-P release correlated significantly with Al and clay content. The max TP release rates were also significantly influenced by land cover, and the values in bare land, grassland, and reclaimed land were 9.91 mg P m-2 day-1, 8.10 mg P m-2 day-1, and 5.05 mg P m-2 day-1, respectively. The results showed that the P release processes might be regulated by different factors across different land covers, and that the re-introduction of vegetation during wetland restoration must be taken into account prior to flood events to avoid an undesirable degradation of water quality.

The relative role of spatial and environmental processes on seasonal variations of phytoplankton beta diversity along different anthropogenic disturbances of subtropical rivers in China.

The phytoplankton community structure is potentially influenced by both environmental and spatial processes. In addition, the relative importance of these two processes to phytoplankton assemblage will be affected by hydrological connectivity. However, the impacts of anthropogenic activities on phytoplankton beta diversity and the relative importance of these two processes to phytoplankton are still poorly understood, especially in water conservation areas. Here, we examined the relative importance of local and regional environmental control and spatial structuring of phytoplankton communities in five rivers with different degrees of disturbance during wet and dry seasons. We found that community structure and local environmental conditions varied greatly in seasons and rivers. The reference river (with minimum disturbance) had the highest homogeneity of environmental conditions and phytoplankton assemblage, while the excessive disturbance rivers (sand mining activities) had the greatest environmental heterogeneity and species dissimilarity between sites. Variation partitioning analysis showed that the phytoplankton community variation was mainly explained by the spatial variables in the wet season (summer and autumn) and winter, while the local environmental variables explained the largest variation of phytoplankton community in the dry season (spring). However, broad-scale variables were selected by redundancy analysis in both dry and wet seasons, which indicates that long-distance scales always have low river connectivity, regardless of whether the river is overflowing or drying up. Local environmental processes explained the most variation in phytoplankton community within all of the rivers, suggesting that deterministic processes usually work on relatively small spatial scales. However, this effect would be weakened by anthropogenic activities, especially sand mining activities. We inferred that sand mining activities increased the environmental heterogeneity and species dissimilarity between sites by causing watercourse habitat patches and obstructing river connectivity. On the other hand, as the excessive disturbance, sand mining activities significantly reduced the species richness and abundance of phytoplankton.

Nitrogen-cycling microbial community functional potential and enzyme activities in cultured biofilms with response to inorganic nitrogen availability.

Biofilms mediate crucial biochemical processes in aquatic ecosystems. It was hypothesized that eutrophication may promote the growth of biofilms, resulting in larger numbers of functional genes. However, the metabolic activity and the roles of biofilms in N cycling will be affected by ambient inorganic nitrogen availability, not by the abundance of functional genes. Biofilms were cultured either with replete inorganic nitrogen (N-rep) or without exogenous inorganic nitrogen supply (N-def) in a flow incubator, and the N-cycling gene abundances (nifH, N2 fixation; amoA, ammonia oxidation, archaea and bacteria; nirS and nirK, denitrification) and enzyme activities (nitrogenase and nitrate reductase) were analyzed. The results showed that, comparing the N-def and N-rep biofilms, the former contained lower nifH gene abundance, but higher nitrogenase activity (NA), while the latter contained higher nifH gene abundance, but lower NA. Different patterns of NA diel variations corresponded to the dynamic microbial community composition and different stages of biofilm colonization. Ammonia oxidizing bacteria (AOB), detected only in N-def biofilms, were responsible for nitrification in biofilms. N-rep biofilms contained high nirS and nirK gene abundance and high denitrification enzyme activity, but N-def biofilms contained significantly lower denitrification gene abundance and activity. In general, the strong N2 fixation in N-def biofilms and strong denitrification in N-rep biofilms assured the balance of aquatic ecosystems. The results suggested that evaluation of the functional processes of N cycling should not only focus on genetic potential, but also on the physiological activity of biofilms.

The Species-Specific Responses of Freshwater Diatoms to Elevated Temperatures Are Affected by Interspecific Interactions.

Numerous experimental simulations with different warming scenarios have been conducted to predict how algae will respond to warming, but their conclusions are sometimes contradictory to each other. This might be due to a failure to consider interspecific interactions. In this study, the dominant diatom species in a seasonal succession were isolated and verified to adapt to different temperature ranges by constant temperature experiment. Both unialgal and mixed cultures were exposed to two fluctuant temperature treatments that simulated the temperature variations from early spring to summer, with one treatment 4 °C higher (warming scenario) than the other. We found that the specific response of diatoms to warming was affected by interspecific interactions. Spring warming had no significant effect on eurythermal species and had a positive effect on the abundance of warm-adapted diatom species, but interspecific interactions reduced this promotional effect. Cold-adapted species had a negative response to spring warming in the presence of other diatom species but had a positive response to early spring warming in the absence of interspecific interactions. In addition, warming resulted in the growth of all diatom species peaking earlier in unialgal cultures, but this effect could be weakened or amplified by interspecies interactions in mixed cultures. Our results suggest that the specific diatom species with different optimal growth temperature ranges responding to warming were expected if there were no interspecific interactions. However, in natural environments, the inevitable and complex interspecific interactions will influence the responses of diatoms to warming. This important factor should not be ignored in the prediction of organism responses to climate warming.