A rapid and high-throughput microplate spectrophotometric method for field measurement of nitrate in seawater and freshwater

Efficient nitrogen removal via simultaneous ammonium assimilation and heterotrophic denitrification of Paracoccus denitrificans R-1

Although diverse microorganisms can remove ammonium and nitrate simultaneously, their metabolic mechanisms are not well understood. Paracoccus denitrificans R-1 showed the maximal NH4 + removal rate 9.94 mg L-1·h-1 and 2.91 mg L-1·h-1 under aerobic and anaerobic conditions, respectively. Analysis of the nitrogen balance calculation and isotope tracing experiment indicated that NH4 + was consumed through assimilation. The maximal NO3 - removal rate of strain R-1 was 18.05 and 19.76 mg L-1·h-1 under aerobic and anaerobic conditions, respectively. The stoichiometric consumption ratio of acetate to nitrate was 0.902 and NO3 - was reduced to N2 for strain R-1 through 15NO3 - isotopic tracing experiment, which indicated a respiratory process coupled with the oxidation of electron donors. Genomic analysis showed that strain R-1 contained genes for ammonium assimilation and denitrification, which effectively promoted each other. These findings provide insights into microbial nitrogen transformation and facilitate the simultaneous removal of NH4 + and NO3 - in a single reactor.

Microfluidic paper-based analytical device for the speciation of inorganic nitrogen species

A microfluidic paper-based analytical device (μPAD) utilizing gas-diffusion separation and solid-phase reduction was developed for the first time for the determination of both ammonium and nitrate, which are the dominant inorganic nitrogen species in environmental waters. The device consists of 3 filter paper layers accommodating the sample, reagent and detection zones. The reagent zone is separated from the detection zone by a semipermeable hydrophobic membrane and acts as a solid-phase reactor where nitrate is reduced to ammonia by Devarda's alloy microparticles, integrated into a μPAD for the first time. The detection zone incorporates the acid-base indicators bromothymol blue (BTB) or nitrazine yellow (NY) and changes colour in two steps. Initially the colour change is caused by ammonia generated by the reaction of ammonium and sodium hydroxide in the sample zone. This colour change is followed by a subsequent colour change as a result of the ammonia produced by the reduction of nitrate by the Devarda's alloy microparticles. The corresponding reflectance value changes are used for the quantification of the two inorganic nitrogen species in the ranges 6.5-100.0 or 2.1-15.0 mg N L-1 for ammonium and 18.2-100.0 or 4.2-15.0 mg N L-1 for nitrate when BTB or NY are used, respectively. Under optimal conditions the limits of quantification of ammonium and nitrate in the case of BTB were determined as 6.5 and 18.2 mg N L-1, respectively, while the corresponding values in the case of NY were found to be 2.1 and 4.2 mg N L-1. The newly developed μPAD was stable for 62 days when stored in a freezer and 1 day at ambient temperature. It was validated with a certified reference material and successfully applied to the determination of ammonium and nitrate in spiked environmental water samples and soil extracts.

Salinity and pH related microbial nitrogen removal in the largest coastal lagoon of Chinese mainland (Pinqing Lagoon)

Coastal lagoon is critical habitat for human and provides a wide range of ecosystem services. These vital habitats are now threatened by waste discharge and eutrophication. Previous studies suggest that the pollution mitigation of coastal lagoon relies on the water exchange with open sea, and the role of microbial processes inside the lagoon is overlooked. This study takes the Pinqing Lagoon which is the largest coastal lagoon in Chinese mainland as example. The distribution of nutrients, microbial activity of nitrogen removal and community structure of denitrifying bacteria in sediment are analyzed. The results showed that the nutrient in sediment represented by DIN (1.65-12.78 mg kg-1), TOM (0.59-8.72 %) and TN (0.14-1.93 mg g-1) are at high levels and are enriched at the terrestrial impacted zone (TZ). The microbial nitrogen removal is active at 0.27-19.76 μmol N kg-1 h-1 in sediment and denitrification is the dominate pathway taking 51.44-98.71 % of total N removal. The composition of the denitrifying microbial community in marine impacted zone (MZ) is close to that of ocean and estuary, but differs considerably with those of TZ and transition zone (TM). The denitrification activity is mainly controlled by salinity and pH, and the denitrifying bacterial community composition related to the nutrient parameters of TN, TOM, etc. Our study suggested that the distribution of nutrients, microbial activity of nitrogen removal and community structure in Lagoon are the combined effects of terrestrial input and exchange with open sea. The microbial processes play important role in the nitrogen removal of coastal lagoon.

Smartphone-based detection of nitrate in seawater samples with the resorcinol method: Comparison with the vanadium reduction method

The presence of nitrate (NO3-) in the aquatic environment has raised a major concern for scientists and environmental managers. In this study, a smartphone-based resorcinol method was developed for the determination of NO3- in seawater. Simple custom-made devices were used in the method, and the reaction temperature, reaction time, and smartphone camera settings were optimized. Salinity variation did not show any major impact on the determination of NO3- using the proposed method, and hence the incorporation of a correction factor was also not required. The detection limit for this method was observed to be 1.3 µM, and the working range was observed to be 5-60 µM, with a relative standard deviation of 0.7% (5 µM, n = 7), which was adequate for the determination of NO3- in most estuarine and coastal seawater samples. The proposed method was compared with the frequently used vanadium chloride (VCl3) reduction method under the same experimental conditions, and both methods were found to be beneficial. The proposed method procedure was simple and easy to use. It was successfully applied for the determination of NO3- in seawater samples, and the results showed that it was practical and can be used potentially for on-site analysis.

Aerobic denitrifying bacterial-fungal consortium mediating nitrate removal: Dynamics, network patterns and interactions

In recent years, nitrogen removal by mixed microbial cultures has received increasing attention owing to cooperative metabolism. A natural bacterial-fungal consortium was isolated from mariculture, which exhibited an excellent aerobic denitrification capacity. Under aerobic conditions, nitrate removal and denitrification efficiencies were up to 100% and 44.27%, respectively. High-throughput sequencing and network analysis suggested that aerobic denitrification was potentially driven by the co-occurrence of the following bacterial and fungal genera: Vibrio, Fusarium, Gibberella, Meyerozyma, Exophiala and Pseudoalteromonas, with the dominance of Vibrio and Fusarium in bacterial and fungal communities, respectively. In addition, the isolated consortium had a high steady aerobic denitrification performance in our sub-culturing experiments. Our results provide new insights on the dynamics, network patterns and interactions of aerobic denitrifying microbial consortia with a high potential for new biotechnology applications.

Denitrification contributes to N2O emission in paddy soils

Denitrification is vital to nitrogen removal and N₂O release in ecosystems; in this regard, paddy soils exhibit strong denitrifying ability. However, the underlying mechanism of N₂O emission from denitrification in paddy soils is yet to be elucidated. In this study, the potential N₂O emission rate, enzymatic activity for N₂O production and reduction, gene abundance, and community composition during denitrification were investigated using the ¹⁵N isotope tracer technique combined with slurry incubation, enzymatic activity detection, quantitative polymerase chain reaction (qPCR), and metagenomic sequencing. Results of incubation experiments showed that the average potential N₂O emission rates were 0.51 ± 0.20 μmol⋅N⋅kg^-1⋅h^-1, which constituted 2.16 ± 0.85% of the denitrification end-products. The enzymatic activity for N₂O production was 2.77-8.94 times than that for N₂O reduction, indicating an imbalance between N₂O production and reduction. The gene abundance ratio of nir to nosZ from qPCR results further supported the imbalance. Results of metagenomic analysis showed that, although Proteobacteria was the common phylum for denitrification genes, other dominant community compositions varied for different denitrification genes. Gammaproteobacteria and other phyla containing the norB gene without nosZ genes, including Actinobacteria, Planctomycetes, Desulfobacterota, Cyanobacteria, Acidobacteria, Bacteroidetes, and Myxococcus, may contribute to N₂O emission from paddy soils. Our results suggest that denitrification is highly modular, with different microbial communities collaborating to complete the denitrification process, thus resulting in an emission estimation of 13.67 ± 5.44 g N₂O⋅m^-2⋅yr^-1 in surface paddy soils.

Temperature-Related N2O Emission and Emission Potential of Freshwater Sediment

Nitrous oxide (N2O) is a major radiative forcing and stratospheric ozone-depleting gas. Among natural sources, freshwater ecosystems are significant contributors to N2O. Although temperature is a key factor determining the N2O emissions, the respective effects of temperature on emitted and dissolved N2O in the water column of freshwater ecosystems remain unclear. In this study, 48 h incubation experiments were performed at three different temperatures; 15 °C, 25 °C, and 35 °C. For each sample, N2O emission, dissolved N2O in the overlying water and denitrification rates were measured, and N2O-related functional genes were quantified at regular intervals. The highest N2O emission was observed at an incubation of 35 °C, which was 1.5 to 2.1 factors higher than samples incubated at 25 °C and 15 °C. However, the highest level of dissolved N2O and estimated exchange flux of N2O were both observed at 25 °C and were both approximately 2 factors higher than those at 35 °C and 15 °C. The denitrification rates increased significantly during the incubation period, and samples at 25 °C and 35 °C exhibited much greater rates than those at 15 °C, which is in agreement with the N2O emission of the three incubation temperatures. The NO3− decreased in relation to the increase of N2O emissions, which confirms the dominant role of denitrification in N2O generation. Indeed, the nirK type denitrifier, which constitutes part of the denitrification process, dominated the nirS type involved in N2O generation, and the nosZ II type N2O reducer was more abundant than the nosZ I type. The results of the current study indicate that higher temperatures (35 °C) result in higher N2O emissions, but incubation at moderate temperatures (25 °C) causes higher levels of dissolved N2O, which represent a potential source of N2O emissions from freshwater ecosystems.

Portable Colorimetric Hydrogel Test Kits and On-Mobile Digital Image Colorimetry for On-Site Determination of Nutrients in Water

Portable colorimetric hydrogel test kits are newly developed for the on-site detection of nitrite, nitrate, and phosphate in water. Griess-doped hydrogel was prepared at the bottom of a 1.5 mL plastic tube for nitrite detection, a nitrate reduction film based on zinc powder was placed on the inner lid of a second 1.5 mL plastic tube for use in conjunction with the Griess-doped hydrogel for nitrate detection, and a molybdenum blue-based reagent was entrapped within a poly(vinyl alcohol) hydrogel matrix placed at the bottom of a third 1.5 mL plastic tube to detect phosphate. These test kits are usable with on-mobile digital image colorimetry (DIC) for the on-site determination of nutrients with good analytical performance. The detection limits were 0.02, 0.04, and 0.14 mg L−1 for nitrite, nitrate, and phosphate, respectively, with good accuracy (<4.8% relative error) and precision (<1.85% relative standard deviation). These test kits and on-mobile DIC were used for the on-site determination of nutrients in the Pak Bang and Bang Yai canals, the main canals in Phuket, Thailand. The concentrations of nitrite, nitrate, and phosphate were undetectable to 0.60 mg L−1, undetectable to 2.98 mg L−1, and undetectable to 0.52 mg L−1, respectively.

Existence and distribution of novel phylotypes of Nitrospira in water columnsof the South China Sea

In the biological nitrogen cycle, nitrite oxidation is performed by nitrite oxidation bacteria, of which Nitrospira is widespread and diverse. Communities of Nitrospira were collected at 25-1500 m depths in the South China Sea. Phylogenetic diversity, community composition, and environmental factors were investigated using high-throughput sequencing targeting the nxrB gene and statistical analyses. The community composition of Nitrospira varied spatially and by depth. Among the 24 OTUs with relatively high abundance, 70% were unclassified and not affiliated with the known Nitrospira genus, suggesting a previously unrecognized high diversity of marine Nitrospira. Five known Nitrospira genera were detected, of which the common marine Nitrospira marina was not the dominant species, whereas Candidatus Nitrospira lenta and Candidatus Nitrospira defluvii dominated in shallow habitats. Comammox Candidatus Nitrospira nitrosa was discovered in the marine ecosystem. The niche differentiation of versatile Nitrospira species was mainly shaped by nitrate, temperature, and DO.

Nitrate Addition Increases the Activity of Microbial Nitrogen Removal in Freshwater Sediment

Denitrification and anammox occur widely in aquatic ecosystems serving vital roles in nitrogen pollution removal. However, small waterbodies are sensitive to external influences; stormwater runoff carrying nutrients and oxygen, flows into waterbodies resulting in a disruption of geochemical and microbial processes. Nonetheless, little is known about how these short-term external inputs affect the microbial processes of nitrogen removal in small waterbodies. To investigate the effects of NO₃⁻, NH₄⁺, dissolved oxygen (DO) and organic C on microbial nitrogen removal in pond sediments, regulation experiments have been conducted using slurry incubation experiments and ¹⁵N tracer techniques in this study. It was demonstrated the addition of NO₃⁻ (50 to 800 μmol L⁻¹) significantly promoted denitrification rates, as expected by Michaelis-Menten kinetics. Ponds with higher NO₃⁻ concentrations in the overlying water responded more greatly to NO₃⁻ additions. Moreover, N₂O production was also promoted by such an addition of NO₃⁻. Denitrification was significantly inhibited by the elevation of DO concentration from 0 to 2 mg L⁻¹, after which no significant increase in inhibition was observed. Denitrification rates increased when organic C was introduced. Due to the abundant NH₄⁺ in pond sediments, the addition demonstrated little influence on nitrogen removal. Moreover, anammox rates showed no significant changes to any amendment.

Niche differentiation of ammonia-oxidizing archaea and related autotrophic carbon fixation potential in the water column of the South China Sea

The significant primary production by ammonia-oxidizing archaea (AOA) in the ocean was reported, but the carbon fixation process of AOA and its community composition along the water depth remain unclear. Here, we investigated the abundance, community composition, and potential carbon fixation of AOA in water columns of the South China Sea. Higher abundances of the amoA and accA genes of AOA were found below the euphotic zone. Similarly, higher carbon fixation potential of AOA, evaluated by the ratios of amoA to accA gene, was also observed below euphotic zone and the ratios increased with increasing water depth. The vertical niche differentiation of AOA was further evidenced, with the dominant genus shifting from Nitrosopelagicus in the epipelagic zone to uncultured genus in the meso- and bathypelagic zones. Our findings highlight the higher carbon fixation potential of AOA in deep water and the significance of AOA to the ocean carbon budget.

Waterborne Polyurethane Acrylates Preparation towards 3D Printing for Sewage Treatment

Conventional immobilized nitrifying bacteria technologies are limited to fixed beds with regular shapes such as spheres and cubes. To achieve a higher mass transfer capacity, a complex-structured cultivate bed with larger specific surface areas is usually expected. Direct ink writing (DIW) 3D printing technology is capable of preparing fixed beds where nitrifying bacteria are embedded in without geometry limitations. Nevertheless, conventional bacterial carrier materials for sewage treatment tend to easily collapse during printing procedures. Here, we developed a novel biocompatible waterborne polyurethane acrylate (WPUA) with favorable mechanical properties synthesized by introducing amino acids. End-capped by hydroxyethyl acrylate and mixed with sodium alginate (SA), a dual stimuli-responsive ink for DIW 3D printers was prepared. A robust and insoluble crosslinking network was formed by UV-curing and ion-exchange curing. This dual-cured network with a higher crosslinking density provides better recyclability and protection for cryogenic preservation. The corresponding results show that the nitrification efficiency for printed bioreactors reached 99.9% in 72 h, which is faster than unprinted samples and unmodified WPUA samples. This work provides an innovative immobilization method for 3D printing bacterial active structures and has high potential for future sewage treatment.

Data for assessment of sediment, soil, and water quality at Ashfield flats reserve, Western Australia

Sediment and water samples were collected using transects and grids within sampling strata, in 2019, 2020, and 2021 from a riparian reserve adjoining the Swan River estuary in Western Australia. Different sampling designs were used each year, with transects and/or grids designed to assess changes in sediment and water quality across assumed environmental gradients such as salinity or distance from possible contaminant sources. Sediments were from 0-10cm; pH and electrical conductivity were measured on suspensions, 32 elements measured by ICP-OES on HNO₃/HCl digests, and microplastics counted microscopically after Fenton digestion and density separation. Surface water was from wetland ponds and stormwater drains, with pH, EC measured in-situ. Filtered acidified water subsamples used to measure nitrate + nitrite and dissolved phosphate spectrophotometrically and 26 elements using ICP-OES. Reported data include metadata and are for 231 sediment/soil samples and 172 water samples, including sampling strata categories and UTM and Longitude-Latitude coordinates. Elemental concentrations have been censored based on blank subtraction and calculated lower detection limits, with censored data presented with missing value codes.

How habitat heterogeneity shapes bacterial and protistan communities in temperate coastal areas near estuaries

In this study, we investigated microbial communities (bacteria and protist) in two coastal areas near the estuaries of the Liaohe (LH) River and Yalujiang (YLJ) River in the Northwestern Pacific Ocean. Due to the existence of Liaodong Peninsula and different levels of urbanization, geographical segregation and significant environmental heterogeneity were observed between these two areas. There were significantly different regional species pools and biogeographic patterns for both bacterial and protistan communities between LH and YLJ coastal areas. Species turnover was the main mechanism driving β-diversity patterns of both bacterial and protistan communities in each area. In addition, the contributed ratio of nestedness to the β-diversity patterns was significantly higher for protists compared to bacteria. Variation in regional species pools was found to be the dominant driver of differences of bacterial and protistan communities between the LH and YLJ coastal areas. For a single-studied area, local community assembly mechanisms, including heterogeneous selection and dispersal limitation, were found to shape the bacterial and protistan communities through calculation of the β-deviation index. Among them, the relative importance of heterogeneous selection and dispersal limitation on the community assembly varied according to microorganism type and habitat.

Variations in nitrogen removal rates and microbial communities over sediment depth in Daya Bay, China

Depth-related variations in the activities, abundances, and community composition of denitrification and anaerobic ammonia oxidation (anammox) bacteria in coastal sediment cores remain poorly understood. In this study, we used ¹⁵N-labelled incubation, quantitative polymerase chain reaction (qPCR), and high-throughput sequencing techniques to reveal the structure and function of denitrifiers and anammox bacteria in sediment cores (almost 100 cm depth) collected in winter and summer from four locations in Daya Bay. The results indicated that the activities and abundances of both denitrifiers and anammox bacteria were detected even in deeper sediments with low concentrations of dissolved inorganic nitrogen (DIN). The potential rates, abundances, and community compositions of denitrifiers and anammox bacteria only varied spatially. In the surface sediment (top 2 cm), denitrifiers had significantly higher activities and abundances than anammox bacteria, but the relative contribution of anammox bacteria to nitrogen loss increased to >60% in the subsurface sediments. Phylogenetic analysis revealed that nirS-type denitrifiers were affiliated to 10 different clusters and Candidatus Scalindua dominated the anammox community in the whole sediments. Furthermore, both denitrification and anammox bacterial communities in the subsurface sediments were distinct from those in the surface sediments. Coupled nitrification and denitrification or anammox may play significant roles in removing fixed N, and the availability of electronic acceptors (e.g. nitrite and nitrate) strongly influenced the N loss activities in the subsurface sediment, emphasising its role as a sink for buried N.

Spatial variability pattern of the anaerobic ammonia-oxidizing bacterial community across a salinity gradient from river to ocean

In natural habitats, the diversity of anaerobic ammonia-oxidizing (anammox) bacteria could be affected by multiple environmental variables. In this study, we investigated the distribution of the anammox bacterial community in surface sediment from the Dongjiang River (riverine sediment, DJ) to the Pearl River Estuary (estuarine sediment, PRE) and then to the South China Sea (coastal sediment, SCS). The results revealed evident differences in the structural diversity of anammox bacteria in three different habitats. Candidatus Brocadia accounted for approximately 90% of the total anammox bacteria in DJ, conversely, Ca. Scalindua dominated in the SCS. Nevertheless, Ca. Scalindua, Ca. Brocadia and Ca. Kuenenia coexisted in the PRE. The qPCR results indicated that anammox bacterial 16S rRNA gene abundance ranged from 2.23 × 10⁵ to 1.19 × 10⁷ copies g^-1 of wet weight, but no significant correlation was found between the abundances and environmental variables (p > 0.05). The relative abundances of Ca. Brocadia gradually decreased with increasing salinity, and Ca. Scalindua showed the opposite trend, suggesting that salinity was a crucial factor in sculpturing the community composition of anammox bacteria in natural environments. Ca. Brocadia should be able to live in freshwater ecosystems, but it can also tolerate a certain level of salinity. Ca. Scalindua was halophilic anammox bacterium and exists only in saline environments. Ca. Kuenenia could adapt to a wide range of salinity and preferred to live in high DIN level conditions according to our search. The distribution pattern of anammox bacteria may be the result of microbial migration and long-term adaptation to salinity.

Microbial nitrogen loss by coupled nitrification to denitrification and anammox in a permeable subterranean estuary at Gloucester Point, Virginia

The nitrogen (N) loss processes have not been well examined in subterranean estuaries (STEs) between land and sea. We utilized a ¹⁵N isotope tracer method, q-PCR, and high-throughput sequencing to reveal the activities, abundances, and community compositions of N loss communities in a STE in Gloucester Point, Virginia, US. The highest activities, abundances and diversity of denitrifiers and anammox bacteria were detected at 50-60 cm depth in the aerobic-anaerobic transition zone (AATZ) characterized by sharp redox gradients. nirS-denitrifiers and anammox bacteria were affiliated to 10 different clusters and three genera, respectively. Denitrification and anammox played equal roles with an estimated N loss of 13.15 mmol N m^-3 day^-1. A positive correlation between ammonia oxidizing prokaryote abundances and DO as well as NO_x^- suggested that nitrification produces NO_x^- which supports the hotspot of denitrification and anammox within the AATZ. Overall, these results highlight the roles of N loss communities in STEs.

Environmental filtering dominates bacterioplankton community assembly in a highly urbanized estuarine ecosystem

Estuaries are important in terms of biodiversity, biogeochemical function, and ecological balance due to their intense land-sea interactions. The sustainability of estuarine ecosystem function relies on a good understanding of the ecological processes related to microbial communities. However, microbial community assembly in such ecosystems is still not well understood. Here, based on 16S rRNA sequencing, we investigated the bacterioplankton community structure in the Pearl River Estuarine system during the wet and dry seasons. Results showed that there were significant seasonal and spatial variations in the bacterioplankton communities of the estuary, with seasonal variations being more remarkable. Multiple bacterioplankton with different abundances in the wet and dry seasons were observed, e.g., the class Actinobacteria and Oxyphotobacteria were enriched in the wet season, whereas Alphaproteobacteria and Saccharimonadia were more abundant in the dry season. Both variation partitioning and null model analysis revealed that environmental filtering dominated the bacterioplankton community assembly in the Pearl River Estuary. Water physical properties (e.g., salinity and temperature), nutrient content (e.g., nitrate), and upstream land use (e.g., urban land cover) together determined the distribution of the bacterioplankton composition in this highly urbanized estuarine ecosystem. These findings would help improve our understanding of the bacterioplankton communities in estuarine ecosystems and provide a theoretical foundation for estuarine ecological health management.

Direct Potentiometric Sensing of Anion Concentration (Not Activity)

Potentiometric probes used in direct potentiometry are attractive sensing tools. They give information on ion activities, which is often uniquely useful. If, instead, concentrations are desired as sensor output, the ionic strength of the sample must be precisely known, which is often not possible. Here, for the first time, direct potentiometry can be made to report concentrations, rather than activities. It is demonstrated for the detection of monovalent anionic species by using a self-referencing Ag/AgI pulstrode as the reference element instead of a traditional reference electrode. This reference pulstrode releases a discrete quantity of iodide ions from the electrode and the resulting reference potential varies with the activity coefficient of iodide. The effects of activity coefficient on the indicator and reference electrode are therefore compensated and the observed cell potential may now be described in a Nernstian manner against anion concentration, rather than activity. Theoretical simulations and experimental results support the validity of this approach. For most monovalent anions of practical relevance, the potential difference between this approach and from a traditional activity coefficient calculation is less than 0.5 mV. The concept is validated with an all-solid-state nitrate sensor as well as a commercial fluoride-selective electrode, giving Nernstian responses in different ionic strength backgrounds against concentration without the need for correcting activity coefficients or liquid junction potentials.

Microliter Operation for Determination of Nitrate-Nitrogen via Simple Zinc Reduction and Color Formation in a Well Plate with a Smartphone

We propose a simple greener colorimetric method for the determination of nitrate-nitrogen by operating on a 96-well microplate and using a smartphone camera as a simple detector. A slurry containing 0.3 mg zinc was used for reduction of nitrate to nitrite, the reduction solution was transferred to a 96-well microplate to react with Griess reagent to form a pink azo dye product. The color product image was captured and processed by a smartphone camera and ImageJ software, respectively. The limit of detection and limit of quantitation were 0.04 and 0.10 mg/L nitrate-nitrogen, respectively, for the smartphone camera. Application to real samples was demonstrated. The proposed method results showed no significant difference (at 95% confidence) with the hydrazine reduction method. The proposed method could be used as an alternative method for on-site analysis due to the advantages of portability and rapidity; duplicate run of 20 samples could be carried out simultaneously in 12 min.

Spatial Variability and Co-acclimation of Phytoplankton and Bacterioplankton Communities in the Pearl River Estuary, China

Phytoplankton and bacterioplankton play significant roles in estuarine systems. It is important to demonstrate the spatial variability of bacterial and microalgal communities and understand the co-acclimation of these organisms to different environmental factors. In this study, MiSeq sequencing and morphological identification were applied to analyze the variations in bacterial and microalgal communities in the Pearl River Estuary, respectively. Molecular ecological network analysis was used to investigate the potential interactions between microalgae and bacteria and illustrate the responses of these interactions to environmental gradients. The results revealed that microalgal/bacterial communities in freshwater samples were distinct from those in mesohaline water samples. Microalgae affiliated to the genus Skeletonema dominated the mesohaline water phytoplankton communities, while Melosira was the more abundant genus in freshwater communities. Actinobacteria, Alphaproteobacteria, Betaproteobacteria, and Acidimicrobiia dominated bacterial communities in freshwater samples, while Gammaproteobacteria, Bacilli, and Synechococcophycideae were more abundant in mesohaline water samples. Tightly correlations were observed between phytoplankton and bacterioplankton. These interactions were regarded to be key factors in shaping the community structures. Further, the KEGG database and PICRUSt were used to predict the functions of bacterioplankton in the process of nitrogen cycling. The results indicated that denitrification could play an important role in nitrogen loss and might alleviate the eutrophication in the Pearl River Estuary. Collectively, the results in this study revealed that substantial changes in phytoplankton and bacterioplankton communities were correlated with the gradients of environmental parameters in the Pearl River Estuary. The results also demonstrated that the interactions between phytoplankton and bacterioplankton were important for these organisms to acclimate to changing environments.

Anaerobic Ammonium Oxidation in Acidic Red Soils

Anaerobic ammonium oxidation (anammox) has been proven to be an important nitrogen removal process in terrestrial ecosystems, particularly paddy soils. However, the contribution of anammox in acidic red soils to nitrogen loss has not been well-documented to date. Here, we investigated the activity, abundance, and distribution of anammox bacteria in red soils collected from nine provinces of Southern China. High-throughput sequencing analysis showed that Candidatus Brocadia dominates the anammox bacterial community (93.03% of sequence reads). Quantification of the hydrazine synthase gene (hzsB) and anammox 16S rRNA gene indicated that the abundance of anammox bacteria ranged from 6.20 × 10⁶ to 1.81 × 10⁹ and 4.81 × 10⁶ to 4.54 × 10⁸ copies per gram of dry weight, respectively. Contributions to nitrogen removal by anammox were measured by a ¹⁵N isotope-pairing assay. Anammox rates in red soil ranged from 0.01 to 0.59 nmol N g⁻¹ h⁻¹, contributing 16.67–53.27% to N₂ production in the studied area, and the total amount of removed nitrogen by anammox was estimated at 2.33 Tg N per year in the natural red soils of southern China. Pearson correlation analyses revealed that the distribution of anammox bacteria significantly correlated with the concentration of nitrate and pH, whereas the abundance and activity of anammox bacteria were significantly influenced by the nitrate and total nitrogen concentrations. Our findings demonstrate that Candidatus Brocadia dominates anammox bacterial communities in acidic red soils and plays an important role in nitrogen loss of the red soil in Southern China.