Influence of algal community structure on denitrification rates in periphyton cultivated on artificial substrata

Microalgae-based constructed wetland system enhances nitrogen removal and reduce carbon emissions: Performance and mechanisms

Combination of constructed wetlands (CWs) and microalgae-based technologies has been proved as effective wastewater treatment option; however, little attention was paid to investigate the optimal combination ways. This study showed that the integrated system (IS) connecting microalgal pond with CWs exhibited improved pollutant-removal efficiencies and preferred carbon reduction effects compared to other alternatives such as coupled system or independent CWs. Microbial analysis demonstrated that core microorganisms (e.g., Acinetobacter and Thermomonas) of the IS were mostly associated with carbon, nitrogen, and energy metabolism. Based on co-occurrence networks, microbial quantity with denitrification function in the IS accounted for 71.01 % of the microorganism related to nitrogen metabolism, which was higher than that of 48.84 % in the independent CWs, indicating that the presence of microalgae in IS played important role in promoting biological denitrification. These findings provide insights into the microbial mechanism and highlights the complementary effects between microalgae and CWs.

Community composition and correlations between bacteria and algae within epiphytic biofilms on submerged macrophytes in a plateau lake, southwest China

Epiphytic biofilms are complex matrix-enclosed communities comprising large numbers of bacteria and algae, which play an important role in the biogeochemical cycles in aquatic systems. However, little is known about the correlations that occur between these communities or the relative impact of environmental factors on their composition. In this study, epiphytic biofilms on three different aquatic plants were sampled in a typical plateau lake (Caohai, southwest China) in July and November of 2018. Bacterial diversity was assessed using Miseq sequencing approaches and algal communities were assessed using light microscopy. Gammaproteobacteria (54.64%), Bacteroidetes (17.50%) and Firmicutes (13.99%) were the dominant bacterial taxa and Chlorophyta (47.62%), Bacillariophyta (28.57%) and Euglenophyta (19.05%) were the dominant algae. The alpha diversity values of the epiphytic bacterial and algal communities were greater during the macrophyte decline period (November) than during the growth period (July). Microbial community composition was significantly affected by abiotic factors (water temperature, NH₄⁺, pH or TP) and biotic factors (algae or bacteria). Interestingly, in July and November, the epiphytic algal community dissimilarity was stronger than that observed for bacterial community dissimilarity, suggesting that bacterial community dissimilarity may increase more slowly with environmental change than algal community dissimilarity. Furthermore, association network analysis revealed complex correlations between algal biomass and bacteria phylotype, and that 67.83% of correlations were positive and 32.17% were negative. This may indicate that facilitative correlations between algae and bacteria are predominant in epiphytic biofilms. These results provide new information on algal-bacterial correlations as well as the possible mechanisms that drive variations in the microbial community in epiphytic biofilms in freshwater lakes.

Excess N2 and denitrification in hyporheic porewaters and groundwaters of the San Joaquin River, California

The San Joaquin River (SJR) in California is purported to receive high nitrate loadings from surrounding agricultural lands through both surface and groundwater inputs. To investigate the potential removal of nitrate (NO₃^-) from surface and ground water sources, the spatial variations in dinitrogen (N₂) gas concentrations and direct measurements of sediment denitrification potential (DNP), with amended NO₃^- and carbon (C) treatments, were investigated in the summer along a 95-km reach of the San Joaquin River. Excess N₂ in hyporheic porewaters ranged from <0.1 to 8.65 mg L^-1 and was significantly higher in porewaters from the 1.3 m (ground water source) versus 0.3 m (mixed surface and ground water) depths. In deep groundwater wells (3-7 m), median excess N₂ concentration was 5.39 mg L^-1 (range = <0.1-14.6 mg L^-1). Excess N₂ concentrations were inversely correlated with dissolved oxygen and NO₃^- concentrations suggesting denitrification as an important process in the dominantly anaerobic sediments. Hyporheic porewater NO₃^- concentrations exceeded the detection limit of 0.01 mg L^-1 in only 20% of the hyporheic porewaters, in spite of high NO₃^- concentrations measured in both surface waters (mean = 2.25 mg N L^-1) and surrounding groundwaters. Sediment DNP rates averaged 253 and 297 μg N kg^-1 hr^-1 for NO₃^- amended, and NO₃^- + C amended sediments, respectively, supporting the prevalence of denitrification in hyporheic sediments. Our results indicate that the hyporheic/riparian zones act as an anoxic barrier to nitrate transport from regional groundwater and as a location to remove NO₃^- from surface waters exchanging with the hyporheic zone.

Contrasting impact of elevated atmospheric CO2 on nitrogen cycle in eutrophic water with or without Eichhornia crassipes (Mart.) Solms

The elevation of atmospheric CO₂ is an inevitable trend that would lead to significant impact on the interrelated carbon and nitrogen cycles through microbial activities in the aquatic ecosystem. Eutrophication has become a common trophic state of inland waters throughout the world, but how the elevated CO₂ affects N cycles in such eutrophic water with algal bloom, and how vegetative restoration helps to mitigate N₂O emission remains unknown. We conducted the experiments to investigate the effects of ambient and elevated atmospheric CO₂ (a[CO₂], e[CO₂]; 400, 800 μmol﹒mol^-1) with and without the floating aquatic plant, Eichhornia crassipes (Mart.) Solms, on N-transformation in eutrophic water using the ¹⁵N tracer method. The nitrification could be slightly inhibited by e[CO₂], due mainly to the competition for dissolved inorganic carbon between algae and nitrifiers. The e[CO₂] promoted denitrification and N₂O emissions from eutrophic water without growth of plants, leading to aggravation of greenhouse effect and forming a vicious cycle. However, growth of the aquatic plant, Eichhornia crassipes, slightly promoted nitrification, but reduced N₂O emissions from eutrophic water under e[CO₂] conditions, thereby attenuating the negative effect of e[CO₂] on N₂O emissions. In the experiment, the N transformation was influenced by many factors such as pH, DO and algae density, except e[CO₂] and plant presence. The pH could be regulated through diurnal photosynthesis and respiration of algae and mitigated the acidification of water caused by e[CO₂], leading to an appropriate pH range for both nitrifying and denitrifying microbes. Algal respiration at night could consume DO and enhance abundance of denitrifying functional genes (nirK, nosZ) in water, which was also supposed to be a critical factor affecting denitrification and N₂O emissions. This study clarifies how the greenhouse effect caused by e[CO₂] mediates N biogeochemical cycle in the aquatic ecosystem, and how vegetative restoration mitigates greenhouse gas emission.

The secretion of organics by living Microcystis under the dark/anoxic condition and its enhancing effect on nitrate removal

Recent studies indicated that the algal decomposition produces particulate and dissolved organic carbon (DOC), and can enhance denitrification in eutrophic lakes. However, the effects of the living cyanobacteria on nitrogen cycling in eutrophic lakes were still an unknown question. This study explores a new underlying mechanism of nitrate removal which is driven by living Microcystis. The results suggested that living Microcystis significantly enhanced the nitrate removal at sediment-water interface, with a nitrate removal rate of 0.54 d^-1, which was 2.57 times higher than the nitrate removal rate in the treatment without the addition of Microcystis. Measurements of Chl a and Fv/Fm confirmed that Microcystis was tolerant to the dark/anoxic condition, and the recovery experiments suggested that Microcystis could survive under such stress conditions for at least seven days. Meanwhile, DOC secreted by living Microcystis reached to 4.55 mg C mg^-1 Chl a. These secretions were biodegradable hydrophilic and contained carbohydrates and proteins. Our study indicated that during blooms, sinking Microcystis cells could directly provide DOC as carbon source, then consequently enhanced the denitrification at sediment-water interface, and the interactive relationship between living cyanobacteria and permanent nitrate removal should be taken into account while studying nitrogen cycling in aquatic ecosystem.

Influence of light and temperature on the development and denitrification potential of periphytic biofilms

Periphytic biofilms are microbial aggregates commonly present in submerged aquatic environments and play a significant role in nutrient cycling. In recent years, utilization of natural periphytic biofilms in wastewater treatment and water restoration attracts growing research interests. Light and temperature are two important environmental factors known to affect the development of periphytic biofilms and can be manipulated for the regulation of the biofilm properties. In this work, effects of light and temperature on the development and function (denitrification potential) of periphytic biofilms were investigated using a microcosm experiment. Results showed that thicker periphytic biofilms with higher Chlorophyll a, extracellular polymeric substances (EPS), and total phosphorus contents were developed under higher temperature. Whereas, biomass accumulation was more rapid for periphytic biofilms under higher irradiance. The denitrification potential rate was negatively associated with irradiance, which can be linked to the influence of irradiance on biofilm structure and microbial composition. A relatively lower irradiance is recommended when using periphytic biofilms in nitrogen removal from wastewater.

The sensitivity and stability of bacterioplankton community structure to wind-wave turbulence in a large, shallow, eutrophic lake

Lakes are strongly influenced by wind-driven wave turbulence. The direct physical effects of turbulence on bacterioplankton community structure however, have not yet been addressed and remains poorly understood. To examine the stability of bacterioplankton communities under turbulent conditions, we simulated conditions in the field to evaluate the responses of the bacterioplankton community to physical forcing in Lake Taihu, using high-throughput sequencing and flow cytometry. A total of 4,520,231 high quality sequence reads and 74,842 OTUs were obtained in all samples with α-proteobacteria, γ-proteobacteria and Actinobacteria being the most dominant taxa. The diversity and structure of bacterioplankton communities varied during the experiment, but were highly similar based on the same time of sampling, suggesting that bacterioplankton communities are insensitive to wind wave turbulence in the lake. This stability could be associated with the traits associated with bacteria. In particular, turbulence favored the growth of bacterioplankton, which enhanced biogeochemical cycling of nutrients in the lake. This study provides a better understanding of bacterioplankton communities in lake ecosystems exposed to natural mixing/disturbances.

Influences of anthropogenic land use on microbial community structure and functional potentials of stream benthic biofilms

Stream ecosystems are the primary receivers of nutrient and organic carbon exported from terrestrial ecosystems and are profoundly influenced by the land use of the surrounding landscape. The aquatic impacts of anthropogenic land use are often first observed in stream benthic biofilms. We studied the benthic biofilms in streams flowing through forest (upstream) and anthropogenic land use (downstream) areas in southwestern China. The results showed that anthropogenic land use increased nutrient and organic carbon in both stream water and benthic biofilms, which are closely related to the differences in the microbial communities. The taxonomic dissimilarity of the communities was significantly correlated with the functional gene dissimilarity, and the upstream sites had more distinct functional genes. Network analysis showed that upstream sites had more highly connected microbial networks. Furthermore, downstream sites had higher relative abundances of anammox and denitrification suggesting stronger nitrogen removal than upstream sites. Increased nutrients in both the stream water and biofilms caused by anthropogenic land use had severe impacts on the nitrogen cycle in stream ecosystems. Downstream sites also had stronger carbon metabolism than upstream sites. This study provides insights into the influences of anthropogenic land use on microbial community structure and functions of stream benthic biofilms.

Algicidal and denitrification characterization of Acinetobacter sp. J25 against Microcystis aeruginosa and microbial community in eutrophic landscape water

Acinetobacter sp. J25 exhibited good denitrification and high algicidal activity against toxic Microcystis aeruginosa. Response surface methodology (RSM) experiments showed that the maximum algicidal ratio occurred under the following conditions: temperature, 30.46°C; M. aeruginosa density, 960,000cellsmL(-1); and inoculum, 23.75% (v/v). Of these, inoculum produced the maximum effect. In the eutrophic landscape water experiment, 10% bacterial culture was infected with M. aeruginosa cells in the landscape water. After 24days, the removal ratios of nitrate and chlorophyll-a were high, 100% and 87.86%, respectively. The denitrification rate was approximately 0.118mgNO3(-)-N·L(-1)·h(-1). Moreover, the high-throughput sequencing result showed that Acinetobacter sp. J25 was obviously beneficial for chlorophyll-a and nitrate removal performance in the eutrophic landscape water treatment. Therefore, strain J25 is promising for the simultaneous removal of chlorophyll-a and nitrate in the eutrophic landscape water treatment.

Metagenomic sequencing of marine periphyton: taxonomic and functional insights into biofilm communities

Periphyton communities are complex phototrophic, multispecies biofilms that develop on surfaces in aquatic environments. These communities harbor a large diversity of organisms comprising viruses, bacteria, algae, fungi, protozoans, and metazoans. However, thus far the total biodiversity of periphyton has not been described. In this study, we use metagenomics to characterize periphyton communities from the marine environment of the Swedish west coast. Although we found approximately ten times more eukaryotic rRNA marker gene sequences compared to prokaryotic, the whole metagenome-based similarity searches showed that bacteria constitute the most abundant phyla in these biofilms. We show that marine periphyton encompass a range of heterotrophic and phototrophic organisms. Heterotrophic bacteria, including the majority of proteobacterial clades and Bacteroidetes, and eukaryotic macro-invertebrates were found to dominate periphyton. The phototrophic groups comprise Cyanobacteria and the alpha-proteobacterial genus Roseobacter, followed by different micro- and macro-algae. We also assess the metabolic pathways that predispose these communities to an attached lifestyle. Functional indicators of the biofilm form of life in periphyton involve genes coding for enzymes that catalyze the production and degradation of extracellular polymeric substances, mainly in the form of complex sugars such as starch and glycogen-like meshes together with chitin. Genes for 278 different transporter proteins were detected in the metagenome, constituting the most abundant protein complexes. Finally, genes encoding enzymes that participate in anaerobic pathways, such as denitrification and methanogenesis, were detected suggesting the presence of anaerobic or low-oxygen micro-zones within the biofilms.

Nitrate removal in shallow, open-water treatment wetlands

The diffuse biomat formed on the bottom of shallow, open-water unit process wetland cells contains suboxic zones that provide conditions conducive to NO3(-) removal via microbial denitrification, as well as anaerobic ammonium oxidation (anammox). To assess these processes, nitrogen cycling was evaluated over a 3-year period in a pilot-scale wetland cell receiving nitrified municipal wastewater effluent. NO3(-) removal varied seasonally, with approximately two-thirds of the NO3(-) entering the cell removed on an annual basis. Microcosm studies indicated that NO3(-) removal was mainly attributable to denitrification within the diffuse biomat (i.e., 80 ± 20%), with accretion of assimilated nitrogen accounting for less than 3% of the NO3(-) removed. The importance of denitrification to NO3(-) removal was supported by the presence of denitrifying genes (nirS and nirK) within the biomat. While modest when compared to the presence of denitrifying genes, a higher abundance of the anammox-specific gene hydrazine synthase (hzs) at the biomat bottom than at the biomat surface, the simultaneous presence of NH4(+) and NO3(-) within the biomat, and NH4(+) removal coupled to NO2(-) and NO3(-) removal in microcosm studies, suggested that anammox may have been responsible for some NO3(-) removal, following reduction of NO3(-) to NO2(-) within the biomat. The annual temperature-corrected areal first-order NO3(-) removal rate (k20 = 59.4 ± 6.2 m yr(-1)) was higher than values reported for more than 75% of vegetated wetlands that treated water in which NO3(-) was the primary nitrogen species (e.g., nitrified secondary wastewater effluent and agricultural runoff). The inclusion of open-water cells, originally designed for the removal of trace organic contaminants and pathogens, in unit-process wetlands may enhance NO3(-) removal as compared to existing vegetated wetland systems.

Impact of streambed morphology on the abundance and activity of ammonia-oxidizing bacteria

Ammonia oxidizers catalyze the first step of nitrification. Combined microbial nitrification-denitrification activities are essential for the removal of excess nitrogen from water bodies. In sandy streambeds, bed form structures are created by water flow and lead to the creation of heterogeneous microenvironments. The objective of this study, therefore, was to investigate the effect of bed form morphology on the abundance and activity of ammonia-oxidizing bacteria (AOB) within a benthic biofilm. An 8-month-old benthic biofilm was established in a recirculating laboratory flume under controlled flow conditions and frequent amendment with ammonium. The sand bed was arranged into bed form structures. The highest concentrations of chlorophyll a (indicative of algae) were measured on the upstream side of the bed forms. The biofilm was dominated by Nitrosospira species, and amoA gene abundance was higher on the downstream sides of the bed forms with no significant difference in oxygen consumption between the upstream and downstream sections of the bed form. In contrast, potential ammonium oxidation rates were higher on the upstream sides of the bed forms. The results suggest that bed form morphology can affect the spatial distribution and activity of AOB, possibly through the creation of distinct microhabitats. These results contribute to our understanding of nitrogen transformations and removal from streams.

Warming-induced changes in denitrifier community structure modulate the ability of phototrophic river biofilms to denitrify

Microbial denitrification is the main nitrogen removing process in freshwater ecosystems. The aim of this study was to show whether and how water warming (+2.5 °C) drives bacterial diversity and structuring and how bacterial diversity affects denitrification enzymatic activity in phototrophic river biofilms (PRB). We used water warming associated to the immediate thermal release of a nuclear power plant cooling circuit to produce natural PRB assemblages on glass slides while testing 2 temperatures (mean temperature of 17 °C versus 19.5 °C). PRB were sampled at 2 sampling times during PRB accretion (6 and 21days) in both temperatures. Bacterial community composition was assessed using ARISA. Denitrifier community abundance and denitrification gene mRNA levels were estimated by q-PCR and qRT-PCR, respectively, of 5 genes encoding catalytic subunits of the denitrification key enzymes. Denitrification enzyme activity (DEA) was measured by the acetylene-block assay at 20 °C. A mean water warming of 2.5 °C was sufficient to produce contrasted total bacterial and denitrifier communities and, therefore, to affect DEA. Indirect temperature effect on DEA may have varied between sampling time, increasing by up to 10 the denitrification rate of 6-day-old PRB and decreasing by up to 5 the denitrification rate of 21-day-old PRB. The present results suggest that indirect effects of warming through changes in bacterial community composition, coupled to the strong direct effect of temperature on DEA already demonstrated in PRB, could modulate dissolved nitrogen removal by denitrification in rivers and streams.

Biological characteristics of biofilms formed on different substrata in a shallow lake in Haihe basin (China)

Natural biofilms were simultaneously grown on glass, plexi-glass, glass fiber and activated carbon fiber for 60 days in a shallow Lake in Haihe basin. Biofilm production and abundance of algae were influenced by the substratum. Magnitude of the substratum effect was glass < plexi-glass < glass fiber < activated carbon fiber. The bacteria communities in biofilms of nature substrata were similar with the biofilms on plexi-glass and glass fiber after 60-day growing, as well as the ones on activated carbon fiber after 14-day growing. On basis of algological and bacteriological investigations, as well as literature data, the activated carbon fiber substratum is recommended for biomonitoring the lake in Haihe basin.

Algal exudates and stream organic matter influence the structure and function of denitrifying bacterial communities

Within aquatic ecosystems, periphytic biofilms can be hot spots of denitrification, and previous work has suggested that algal taxa within periphyton can influence the species composition and activity of resident denitrifying bacteria. This study tested the hypothesis that algal species composition within biofilms influences the structure and function of associated denitrifying bacterial communities through the composition of organic exudates. A mixed population of bacteria was incubated with organic carbon isolated from one of seven algal species or from one of two streams that differed in anthropogenic inputs. Pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) revealed differences in the organic composition of algal exudates and stream waters, which, in turn, selected for distinct bacterial communities. Organic carbon source had a significant effect on potential denitrification rates (DNP) of the communities, with organics isolated from a stream with high anthropogenic inputs resulting in a bacterial community with the highest DNP. There was no correlation between DNP and numbers of denitrifiers (based on nirS copy numbers), but there was a strong relationship between the species composition of denitrifier communities (as indicated by tag pyrosequencing of nosZ genes) and DNP. Specifically, the relative abundance of Pseudomonas stutzeri-like nosZ sequences across treatments correlated significantly with DNP, and bacterial communities incubated with organic carbon from the stream with high anthropogenic inputs had the highest relative abundance of P. stutzeri-like nosZ sequences. These results demonstrate a significant relationship between bacterial community composition and function and provide evidence of the potential impacts of anthropogenic inputs on the structure and function of stream microbial communities.

Bacteria and diatom co-occurrence patterns in microbial mats from polar desert streams

The ephemeral stream habitats of the McMurdo Dry Valleys of Antarctica support desiccation and freeze-tolerant microbial mats that are hot spots of primary productivity in an otherwise inhospitable environment. The ecological processes that structure bacterial communities in this harsh environment are not known; however, insights from diatom community ecology may prove to be informative. We examined the relationships between diatoms and bacteria at the community and taxon levels. The diversity and community structure of stream microbial mats were characterized using high-throughput pyrosequencing for bacteria and morphological identification for diatoms. We found significant relationships between diatom communities and the communities of cyanobacteria and heterotrophic bacteria, and co-occurrence analysis identified numerous correlations between the relative abundances of individual diatom and bacterial taxa, which may result from species interactions. Additionally, the strength of correlations between heterotrophic bacteria and diatoms varied along a hydrologic gradient, indicating that flow regime may influence the overall community structure. Phylogenetic consistency in the co-occurrence patterns suggests that the associations are ecologically relevant. Despite these community- and taxon-level relationships, diatom and bacterial alpha diversity were inversely correlated, which may highlight a fundamental difference between the processes that influence bacterial and diatom community assembly in these streams. Our results therefore demonstrate that the relationships between diatoms and bacteria are complex and may result from species interactions as well as niche-specific processes.

Development of associations between microalgae and denitrifying bacteria in streams of contrasting anthropogenic influence

We compared the development of microalgal and bacterial-denitrifier communities within biofilms over 28 days in a restored-prairie stream (RP) and a stream receiving treated wastewater effluent (DER). Inorganic nutrient concentrations were an order of magnitude greater in DER, and stream waters differed in the quality of dissolved organics (characterized via pyrolysis-GC/MS). Biofilm biomass and the densities of algae and bacteria increased over time in both systems; however, algal and denitrifier community composition and the patterns of development differed between systems. Specifically, algal and denitrifier taxonomic composition stabilized more quickly in DER than RP, whereas the rates of algal and denitrifier succession were more closely coupled in RP than DER. We hypothesize that, under unenriched conditions, successional changes in algal assemblages influence bacterial denitrifiers due to their dependence on algal exudates, while under enriched conditions, this relationship is decoupled. Between-system differences in organic signatures supported this, as RP biofilms contained more labile, aliphatic compounds than DER. In addition, potential denitrification rates (DNP) were negatively correlated with the percentage of aromatic compounds within the biofilm organic signatures, suggesting a significant relationship between algal exudate composition and denitrification. These results are significant because anthropogenic factors that affect biofilm community composition may alter their capacity to perform critical ecosystem services.

Temporal patterns in glycolate-utilizing bacterial community composition correlate with phytoplankton population dynamics in humic lakes

Previous observations of correlated community dynamics between phytoplankton and bacteria in lakes indicate that phytoplankton populations may influence bacterial community structure. To investigate the possibility that bacterial use of phytoplankton exudates contributes to observed patterns of community change, we characterized the diversity and dynamics of heterotrophic bacterioplankton with genetic potential to use glycolate, a photorespiration-specific exudate, in five lakes over a 15-week period. Culture-independent approaches were used to track different bacterial phylotypes represented by DNA sequence variation in the functional gene glycolate oxidase subunit D (glcD). glcD gene sequences from freshwater bacteria exhibited broad phylogenetic diversity, including sequences representing the Alpha-, Beta-, and Gammaproteobacteria, Actinobacteria, Bacteroidetes, Firmicutes, and Verrucomicrobia. The majority of glcD gene sequences were betaproteobacterial, with 48% of the sequences clustering with the glcD gene from the cosmopolitan freshwater species Polynucleobacter necessarius. Terminal restriction fragment length polymorphism fingerprinting of the glcD gene revealed changes in glycolate-utilizing assemblages over time. An average of 39% of within-lake temporal variation in glycolate-utilizing assemblages across five lakes was explained by phytoplankton community composition and dynamics. The interaction between phytoplankton populations and the environment explained an additional 17% of variation on average. These observations offer new insight into the diversity and temporal dynamics of freshwater bacteria with genetic potential to use glycolate and support the hypothesis that algal exudates influence the structure of bacterial communities.