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.

NosZ-II-type N2O-reducing bacteria play dominant roles in determining the release potential of N2O from sediments in the Pearl River Estuary, China.

The microbial reduction of N2O serves as a "gatekeeper" for N2O emissions, determining the flux of N2O release into the atmosphere. Estuaries are active regions for N2O emissions, but the microbial functions of N2O-reducing bacteria in estuarine ecosystems are not well understood. In this study, the 15N isotope tracer method, qPCR, and high-throughput sequencing were used to analyze N2O production, reduction, and emission processes in surface sediments of the Pearl River Estuary. The 15N isotope tracer experiment showed that the N2O production rates declined and the N2O reduction potential (Rr, the ratio of N2O reduction rates to N2O production rates) increased from upstream to downstream of the Pearl River Estuary, leading to a corresponding decrease of the N2O emission rates from upstream to downstream. The gene abundance ratio of nosZ/nir gradually increased from upstream to downstream and was negatively correlated with the water N2O saturation. The gene abundance of nosZ II was significantly higher than that of nosZ I in the estuary, and the nosZ II/nosZ I abundance ratio was positively correlated with N2O reduction potential. Furthermore, the community composition of NosZ-I- and NosZ-II-type N2O-reducing bacteria shifted from upstream to downstream. NosZ-II-type N2O-reducing bacteria, especially Myxococcales, Thiotrichales, and Gemmatimonadetes species, contributed to the high N2O reduction potential in the downstream. Our results suggest that NosZ-II-type N2O-reducing bacteria play a dominant role in determining the release potential of N2O from sediments in the Pearl River Estuary. This study provides a new insight into the function of microbial N2O reduction in estuarine ecosystems.

Effect of Ocean Acidification on Bacterial Metabolic Activity and Community Composition in Oligotrophic Oceans, Inferred From Short-Term Bioassays.

Increasing anthropogenic CO2 emissions in recent decades cause ocean acidification (OA), affecting carbon cycling in oceans by regulating eco-physiological processes of plankton. Heterotrophic bacteria play an important role in carbon cycling in oceans. However, the effect of OA on bacteria in oceans, especially in oligotrophic regions, was not well understood. In our study, the response of bacterial metabolic activity and community composition to OA was assessed by determining bacterial production, respiration, and community composition at the low-pCO2 (400 ppm) and high-pCO2 (800 ppm) treatments over the short term at two oligotrophic stations in the northern South China Sea. Bacterial production decreased significantly by 17.1-37.1 % in response to OA, since bacteria with high nucleic acid content preferentially were repressed by OA, which was less abundant under high-pCO2 treatment. Correspondingly, shifts in bacterial community composition occurred in response to OA, with a high fraction of the small-sized bacteria and high bacterial species diversity in a high-pCO2 scenario at K11. Bacterial respiration responded to OA differently at both stations, most likely attributed to different physiological responses of the bacterial community to OA. OA mitigated bacterial growth efficiency, and consequently, a larger fraction of DOC entering microbial loops was transferred to CO2.

Shift of DNRA bacterial community composition in sediment cores of the Pearl River Estuary and the impact of environmental factors.

Dissimilatory nitrate reduction to ammonia (DNRA) process, competing with denitrification and anaerobic ammonia oxidation (anammox) for nitrate, is an important nitrogen retention pathway in the environment. Previous studies on DNRA bacterial diversity and composition focused on the surface sediments in estuaries, but studies on the deep sediments are limited, and the linkage between DNRA community structure and complex estuarine environment remains unclear. In this study, through high-throughput sequencing of nrfA gene followed by high-resolution sample inference, we examined spatially and temporally the composition and diversity of DNRA bacteria along a salinity gradient in five sediment cores of the Pearl River Estuary (PRE). We found a higher diversity and richness of DNRA bacteria in sediments with lower organic carbon, where sea water intersects fresh water. Moreover, the DNRA bacterial communities had the specific spatially distribution coupling with their metabolic difference along the salinity gradient of the Pearl River Estuary, but no obvious difference along the sediment depth. The distribution of DNRA bacteria in the PRE was largely driven by various environmental factors, including salinity, Oxidation-Reduction Potential (ORP), ammonium, nitrate and Corg/NO3-. Furthermore, dominant DNRA bacteria were found to be the key populations of DNRA communities in the PRE sediments by network analysis. Collectively, our results showed that niche difference of DNRA bacteria indeed occurs in the Pearl River Estuary.

Potential bioremediation effects of seaweed Gracilaria lemaneiformis on heavy metals in coastal sediment from a typical mariculture zone.

Seaweeds are good bio-monitors of heavy metals pollution in coastal seawater. In the present study, the potential bioremediation effects of cultivated Gracilaria lemaneiformis on heavy metals in Nan'ao coastal sediment from a typical mariculture zone, South China were evaluated. Sediment samples were collected from five different zones (Gracilaria cultivation zone, G; Fish culture zone, F; Shellfish culture zone, S; Transition zone, T; Control zone, C) from December 2014 to July 2015. The concentrations of Cd, Pb, Cu, and Zn in the sediments were significantly different among the various types of mariculture areas. The concentrations varied widely: Cd (0.04-1.02) µg g-1; Cu (1.19-37.70) µg g-1; Pb (8.45-74.45) µg g-1; Zn (36.80-201.24) µg g-1. The lowest heavy metal concentrations in the sediment were occurred at Gracilaria cultivation zone, while higher concentrations occurred at control zones and fish culture zones. The pollution load index, principal components and cluster analysis showed that heavy metal concentrations were the highest at fish culture zone, while the concentrations were the lowest at Gracilaria cultivation zone, and Gracilaria cultivation affects the heavy metals in the sediments. Gracilaria had strong adsorption capacities for heavy metals from seawater, showing the highest heavy metal Bioconcentration Factors in May (higher seaweed biomass period). Consequently, the results suggested that Gracilaria cultivation influences the heavy metal concentrations in sediments from the typical coastal mariculture zone. Gracilaria cultivation has the potential to bioremediate heavy metals in the coastal sediments. Therefore, Gracilaria cultivation can add environmental advantages and ecological values to coastal mariculture zones.

Nitrogen removal in the sediments of the Pearl River Estuary, China: Evidence from the distribution and forms of nitrogen in the sediment cores.

In this study, the spatial distribution and forms of nitrogen in sediment cores collected from the Pearl River Estuary were analyzed. Exchangeable nitrogen (Nex) comprised only a small proportion of total nitrogen (Ntot), with a mean of 3.54% in the sediment cores. NH4+ was the main form of Nex. No obvious change was observed in the vertical content of fixed ammonia (Nfix) in the sediments, and the mean Nfix in all five sediment cores was 141.23 mg·kg-1. The organic nitrogen (Norg), strongly related to organic carbon (Corg), was the main form in Ntot. The dissolved inorganic nitrogen in sediment pore water was much lower than that in estuarine water and no significant variation was observed from upstream to downstream. Our results indicated that most nitrogen deposited on surface sediments from overlying water was rapidly removed by a series of microbial processes, reducing the extent of nitrogen returning to overlying waters.

Protection of dietary selenium-enriched seaweed Gracilaria lemaneiformis against cadmium toxicity to abalone Haliotis discus hannai.

Seaweed Gracilaria lemaneiformis is the main dietary source of the abalone mariculture industry in China. In this study, we examined the protection of selenium (Se)-enriched G. lemaneiformis against cadmium (Cd) toxicity in the abalone, Haliotis discus hannai, using various indices including metal concentration (Se and Cd), growth rate, GPx enzymatic antioxidants, and metallothionein (MT) concentration over a period of 28 days of exposure. The growth rates and Se contents increased significantly in abalones fed with Se-enriched G. lemaneiformis, while the toxicity of Cd was reduced. Seven to 12 days Cd exposure to the Se-enriched G. lemaneiformis not only affected GPx activity but the MT levels fluctuated irregularly. MT concentrations increased after 3 days exposure and then gradually decreased to the control level after Day 7. There were statistically significant positive correlations between MT levels, GPx activity and Se concentrations, and negative relationships between MT levels, GPx activity and Cd levels in abalones. These findings suggest that Se-enriched Gracilaria protects abalone against Cd toxicity. The possible mechanism is the induction of MT with a concomitant increased capacity of GPx enzymatic antioxidants.

Characteristics of Phytoplankton Biomass, Primary Production and Community Structure in the Modaomen Channel, Pearl River Estuary, with Special Reference to the Influence of Saltwater Intrusion during Neap and Spring Tides.

In recent decades, increasing frequency and intensity of saltwater intrusion in the Modaomen Channel has threatened the freshwater supply in the surrounding cities of the Pearl River Estuary, and ulteriorly changed the environmental conditions of the estuarine waters. Phytoplankton biomass, primary production (PP) and species composition, as well as hydrological and chemical parameters were examined along a downstream transect in the Modaomen Channel during neap tide (NT) and spring tide (ST), when a strong saltwater intrusion event occurred in late September, 2011. A total of 46 species phytoplankton were identified, including Bacillariophyta (25 species), Dinoflagellate (14 species), Chlorophyta (4 species), Cyanophyta (2 species) and Euglenozoa (1 species). The dominant species were shifted from freshwater diatoms (e.g., Melosira granulata and Melosira granulata var. angustissima) in the upper reaches to saline water diatoms (e.g., Skeletonema costatum and Coscinodiscus sp.) in the river mouth. Generally, phytoplankton density, biomass (chl-a) and PP decreased from the upper to lower reaches along the channel, and were significantly higher in NT than those of ST. There was a shift from large-sized phytoplankton (>20 µm) in the upper reaches to relative small-sized cells (5-20 µm) in the lower reaches. Compared to NT, low discharge and flow velocity, coupled with strong easterly winds during ST specially aggravated saltwater intrusion further to the upstream (~50 km from the estuary). The intruded saltwater diluted nutrients, N/P ratios, chl-a, and phytoplankton abundances, and thereby led to a decline in PP during ST.

[Chemical characteristics of precipitation in South China Sea].

Rainwater samples were collected in the summer on "Shiyan 3" during the 2012 South China Sea Sectional Scientific Survey. The concentrations of anion and cation, and pH in precipitation were determined and backward trajectories of air mass were simulated to analyze the chemical characteristics of ions and examine the source of ions. The results indicated that the mean pH value of precipitation was 6.3, with 5.6 of minimal value in summer in South China Sea. The order of anion and cation abundance was Cl(-) > S04(2-) > NO3(-) and Na(+) > Mg(2+) > Ca(2+) > K(+). Cl(-) was the major anion and Na(+) was the major cation, with concentrations of 2 637.5 microeq x L(-1) and 2095.5 microeq x L(-1), respectively, showing that they were the characteristics of marine atmospheric precipitation. There was a good linear relationship between each pair of 7 ions, with correlation coefficient above 0.9, suggesting that they may have a common source. However, the correlation coefficients were lower between NO3(-) and other ions than the others, suggesting that NO3(-) had more complex sources. The concentrations of Ca(2+) and K(+) in precipitation may be related to coral environment in South China Sea. The backward trajectories in 6 stations showed that the air mass was from south and southwest of South China Sea, without passing through above the continent. These results suggested that precipitation affected by human ion source can be ignored in summer in South China Sea.

Different hydrodynamic processes regulated on water quality (nutrients, dissolved oxygen, and phytoplankton biomass) in three contrasting waters of Hong Kong.

The subtropical Hong Kong (HK) waters are located at the eastern side of the Pearl River Estuary. Monthly changes of water quality, including nutrients, dissolved oxygen (DO), and phytoplankton biomass (Chl-a) were routinely investigated in 2003 by the Hong Kong Environmental Protection Department in three contrasting waters of HK with different prevailing hydrodynamic processes. The western, eastern, and southern waters were mainly dominated by nutrient-replete Pearl River discharge, the nutrient-poor coastal/shelf oceanic waters, and mixtures of estuarine and coastal seawater and sewage effluent of Hong Kong, respectively. Acting in response, the water quality in these three contrasting areas showed apparently spatial­temporal variation pattern. Nutrients usually decreased along western waters to eastern waters. In the dry season, the water column was strongly mixed by monsoon winds and tidal currents, which resulted in relatively low Chl-a (<5 µg l(−1)) and high bottom DO (>4 mg l(−1)), suggesting that mixing enhanced the buffering capacity of eutrophication in HK waters. However, in the wet season, surface Chl-a was generally >10 µg l(−1) in southern waters in summer due to halocline and thermohaline stratification, adequate nutrients, and light availability. Although summer hypoxia (DO <2 mg l(−1)) was episodically observed near sewage effluent site and in southern waters induced by vertical stratification, the eutrophication impacts in HK waters were not as severe as expected owing to P limitation and short water residence time in the wet season.

Bacterioplankton dynamics along the gradient from highly eutrophic Pearl River Estuary to oligotrophic northern South China Sea in wet season: implication for anthropogenic inputs.

Bacterioplankton abundance (BA) and biomass (BB) from the eutrophic Pearl River Estuary (PRE) to the oligotrophic northern South China Sea (NSCS) were studied in the wet season. BA was significantly higher (p < 0.05) in PRE (12.51 ± 3.52 x 108 cells L⁻¹), than in the continental shelf neritic province (CSNP, 4.95 ± 2.21 x 108 cells L⁻¹) and in the deep oceanic province (OP, 3.16 ± 1.56 x 108 cells L⁻¹). Nutrient-replete PRE waters (DIN > 100 µM and PO4 > 1 µM) resulted in high chl a and BB, whereas nutrient-depleted offshore waters (DIN < 5 µM and PO4 < 0.5 µM) had low biomass. Temperature (> 26 °C) was not the controlling factor of BA. BB was significantly correlated with chl a biomass both in PRE and NSCS. The bacteria to phytoplankton biomass (BB/PB) ratio increased clearly along the gradient from near-shore PRE (0.15) to offshore CSNP (0.93) and deep OP (2.75), indicating the important role of small cells in the open ocean compared to estuarine and coastal zones.

Short-term metal accumulation and MTLP induction in the digestive glands of Perna virdis exposed to Zn and Cd.

Time-dependent Zn and Cd accumulation and metallothionein like protein (MTLP) induction in the digestive glands of mussels, Perna virdis, were measured under different exposure conditions. The initial uptake rate at start of chase (rho0) and mean residence time (tau) were calculated to determine the physiological response of organisms and their potential detoxification mechanisms. It was found that in digestive glands, Zn had obviously higher rho0 and shorter mean residence time than Cd, indicating that these two metals had different accumulation dynamics even though they were very close in the periodic element table. MTLP levels in digestive glands varied from 0.51 to 1.05 microg/g ww (wet weight). The MTLP level increased continuously when mussels were exposed to low and middle levels of Zn and Cd media, and reached maximal levels at day 4, then decreased when they were exposed to high level Zn and Cd solutions. With regard to the fraction of Zn and Cd accumulated in the digestive glands, the ratios of soluble metal to total metal decreased continuously after exposure in low and middle levels of Zn and Cd media, and decreased continuously in the first 4 days and then to level off when mussels were exposed to media with high concentration of Zn and Cd. Results suggested that both MTLP induction and metal insolubilization were detoxification processes in digestive glands of mussels.

[Study on optical characteristics of chromophoric dissolved organic matter (CDOM) in rainwater by fluorescence excitation-emission matrix and absorbance spectroscopy].

The optical characteristics of chromophoric dissolved organic matter (CDOM) were determined in rain samples collected in Xiamen Island, during a rainy season in 2007, using fluorescence excitation-emission matrix spectroscopy associated with UV-Vis absorbance spectra. Results showed that the absorbance spectra of CDOM in rain samples decreased exponentially with wavelength. The absorbance coefficient at 300 nm [a(300)] ranged from 0.27 to 3.45 m(-1), which would be used as an index of CDOM abundance, and the mean value was 1.08 m(-1). The content of earlier stage of precipitation events was higher than that of later stage of precipitation events, which implied that anthropogenic sources or atmospheric pollution or air mass types were important contributors to CDOM levels in precipitation. EEMs spectra showed 4 types of fluorescence signals (2 humic-like fluorescence peaks and 2 protein-like fluorescence peaks) in rainwater samples, and there were significant positive correlations of peak A with C and peak B with S, showing their same sources or some relationship of the two humic-like substance and the two protein-like substance. The strong positive correlations of the two humic-like fluorescence peaks with a(300), suggested that the chromophores responsible for absorbance might be the same as fluorophores responsible for fluorescence. Results showed that the presence of highly absorbing and fluorescing CDOM in rainwater is of significant importance in atmospheric chemistry and might play a previously unrecognized role in the wavelength dependent spectral attenuation of solar radiation by atmospheric waters.

Composition and sources of organic matter and its solvent extractable components in surface sediments of a bay under serious anthropogenic influences: Daya Bay, China.

Organic matter of nine surface sediments from the Daya Bay was Soxhlet-extracted with a mixture of 2:1 (v/v) dichloromethane-methanol and separated into five fractions: non-aromatic hydrocarbons, aromatic hydrocarbons, ketones, alcohols, and fatty acids and asphaltenes, and analyzed to determine their bulk and biomarker composition. Marine autogenic input appears to be a major source of organic matter. Generally, non-aromatic hydrocarbons are the most dominant fraction of solvent-extractable organic matter (EOM) followed by the other four fractions in decreasing amounts: fatty acids and asphaltenes, ketones, alcohols and aromatic hydrocarbons. On average, both non-aromatic hydrocarbon fraction and fatty acid and asphaltene fraction account for approximately 40% of EOM. The sources of acyclic methyl ketones, alkanols and fatty acids were examined. For n-alkan-2-ones, allochthonous input is a more important source than marine autogenetic input; the reverse is true for n-fatty acids; for n-alkanols, allochthonous and autogenetic inputs seem comparable. Both n-alkan-2-ones and n-fatty acids in the surface sediments of different areas appear to be derived from common sources.

[Metal accumulation and MTLP induction in the digestive glands of Perna viridis exposed to Cu].

Mussels have been proposed as appropriate biomonitors of marine pollution, especially for monitoring metallic pollution based on variations of metallothionein as biomarkers. Under 2 exposure levels (12.7 microg/L, 63.5 microg/L), Cu accumulation and metallothionein-like protein (MTLP) induction by mussel (Perna viridis) digestive glands were investigated and simulated into dynamic models in the present work, and the soluble and total Cu burden of digestive glands were also determined. Calculated mean Cu uptake rates by mussel target organ were 2.045 and 7.028 microg x (g x d)(-1) respectively, and the theoretical equilibrium kinetic BCFs of Cu were 2074 and 1619 correspondingly. And within the exposure duration, different changing trends of ratio of soluble Cu to total Cu in digestive glands were observed in the two groups. The MTLP level of control samples was (0.551 +/- 0.037) mg/g, and the counterparts are 0.407 - 0.699 mg/g, 0.826 - 0.942 mg/g respectively when mussels were exposed to 12.7 microg/L and 63.5 microg/L Cu solutions. Statistically significant MTLP induction (p < 0.001) was observed under higher exposure level. MTLP contents in digestive glands increased with the exposure Cu concentration and body accumulation of metal. There is a significantly negative exponential rise relationship (p < 0.000 1) between MTLP and Cu concentrations accumulated in the digestive glands of mussels.

Non-aromatic hydrocarbons in surface sediments near the Pearl River estuary in the South China Sea.

Surface sediment samples at 4 sites along an offshore transect from outer continental shelf off the Pearl River estuary to the shelf slope region of the northern South China Sea, have been analyzed for total organic carbon (TOC), total nitrogen (TN), solvent extractable organic matter (EOM) and non-aromatic hydrocarbons. TOC, TN and EOM show distinct spatial variations. Their highest values are all recorded at the shelf slope region. EOM varies from 18.70-38.58 microgg(-1) dry sediment and accounts for 0.20-0.72% of the TOC contents. The non-aromatic hydrocarbons are an important fraction of EOM. Their contents range from 3.43-7.06 microgg(-1) dry sediment. n-Alkanes with carbon number ranging from 15-38 are identified. They derive from both biogenic and petrogenic sources in different proportions. Results of isoprenoid hydrocarbons, hopanes and steranes also suggest possible petroleum contamination.

Assimilation and bioconcentration of Ag and Cd by the marine black bream after waterborne and dietary metal exposure.

We determined the aqueous uptake and dietary assimilation of Cd and Ag by the marine black bream Acanthopagrus schlegeli following one to four weeks' exposure (or conditioning) to waterborne or dietary Cd or Ag at different concentrations. The concentrations of metals and metallothioneins (MT) in different tissues also were determined. The viscera contained the highest Ag, Cd, and MT concentrations after metal exposure. After exposure to waterborne metals, the metal and MT concentrations in the gills were higher than those in the remaining tissues (mainly muscles and bones), but this pattern was reversed following exposure to dietary metals. The assimilation efficiencies (AEs) of Cd and Ag ranged from 6 to 24% and 15 to 30%, respectively. The rate constant of uptake from the dissolved phase (ku) of Cd and Ag ranged from 2.2 to 7.5 and 8.0 to 31.7 L kg(-1) d(-l), respectively. In all the exposure experiments, the ku and AE increased with induced MT concentration and tissue metal concentration. Increasing metal accumulation may have been due to the increased available binding sites following the induction of MT in the fish. Furthermore, the MT induced by either Cd or Ag was not specific, but was able to bind with both metals and enhance bioaccumulation. Exposure to dissolved and dietary metals may increase metal accumulation, which potentially may lead to metal toxicity, although the fish may develop a tolerance to metals due to the apparent induction of MT.

Copper speciation in the gill microenvironment of carp (Cyprinus carpio) at various levels of pH.

The fish gill microenvironment of Cyprinus carpio under stress of copper exposure was investigated. pH and other parameters including free copper activity, alkalinity, and inorganic and organic carbons in the surrounding water (inspired water) and in the gill microenvironment (expired water) were measured or calculated at various levels of pH and varying total copper concentrations. The chemical equilibrium calculation (from MINEQA2) and complexation modeling (mucus-copper) were coupled to calculate both species distribution. The results indicate that the pH in the fish gill microenvironment was different from that in the surrounding water with a balance point around 6.9. The secretion of both CO(2) and mucus was affected in both linear and nonlinear ways when the fish were exposed to elevated concentrations of copper. The complexation capacity of the gill mucus was characterized by a conditional stability constant (logk(Cu-mucus)) of 5.37 along with a complexation equivalent concentration (L(Cu-mucus)) of 0.96 mmol Cu/mg C. For both the fish microenvironment and the surrounding water, the dominant copper species shifted from Cu(2+) to CuCO(3)(0) and to Cu(OH)(2)(0) when the pH of the surrounding water changed from 6.12 to 8.11. The change in copper speciation in the gill microenvironment is smaller than that in the surrounding water due to the pH buffering capacity of the fish gills.