Denitrification potential of the Northern Arabian Gulf--an experimental study

A preliminary study on benthic nutrient exchange across sediment-water interfaces in a shallow marine protected area of the Northwestern Arabian Gulf

Sedimentary processes are expected to play a crucial role in macronutrient cycling of the shallow Arabian Gulf. To investigate this aspect, sediment cores were collected from the shallow intertidal and subtidal expanses of the first Marine Protected Area (MPA) of Kuwait in the Northwestern Arabian Gulf (NAG). Porewater nutrient profiling and whole core incubation experiments were conducted to measure the nutrient fluxes, both with and without the addition of the nitrification inhibitor allylthiourea (ATU). The porewater data confirmed the potential of sediments to host multiple aerobic and anaerobic pathways of nutrient regeneration. The average (±SD) of net nutrient fluxes from several incubation experiments indicated that ammonium (NH4+) predominantly fluxed out of the sediment (3.81 ± 2.53 mmol m-2 d-1), followed by SiO44- (3.07 ± 1.21 mmol m-2 d-1). In contrast, the average PO43- flux was minimal, at only 0.06 ± 0.05 mmol m-2 d-1. Fluxes of NO3- (ranged from 0.07 ± 0.005 to 1.16 ± 0.35 mmol m-2 d-1) and NO2- (0.03 ± 0.003 to 0.71 ± 0.21 mmol m-2 d-1) were moderate, which either reduced or reversed in the presence of ATU (-0.001 ± 0.0001 to 0.01 ± 0.0001 mmol m-2 d-1 and -0.001 ± 0.0003 to 0.006 ± 0.001 mmol m-2 d-1 for NO3- and NO2- respectively). Thus, this study provides preliminary experimental evidence that nitrification can act as a source of NO3- and NO2- as well as contribute towards the relatively high concentrations of NO2- (>1 in the gulf waters.

The Reproductive Capacities of the Calanoid Copepods Parvocalanus crassirostis and Acartia pacifica under Different pH and Temperature Conditions

The increasing atmospheric CO₂ concentrations and warming of marine waters have encouraged experiments on multi-stressor interactions in marine organisms. We conducted a multigenerational experiment to assess reproductive capacities regarding egg production in calanoid copepods Parvocalanus crassirostis and Acartia pacifica under different pH and temperature conditions. The experimental set-up allowed assessing the tandem effect of warming and acidification on the number of eggs produced by healthy copepod pairs under two pH conditions of 8.20 and 7.50 (hard selection) as well as with a gradual reduction of 0.05 pH units at each generation (soft selection). The results are quite interesting, with very diverse performance across temperatures. The number of eggs produced under hard selection was higher at pH 8.20 compared to pH 7.50 for both species, with the maximum number of eggs produced at 24-28 °C, whereas under soft selection, there was no significant difference in the egg production rate at 24-28 °C across generations and there was an improvement in the number of eggs produced at 8-16 °C. The results provide evidence that in a future ocean scenario of lower pH and higher temperature, the two species, and possibly the copepod population at large, might not decrease. Copepod populations might be resilient, and the transcriptomic evidence of adaptation to increased temperature and lower pH is a ray of hope. We believe further studies are needed to provide more robust datasets to underpin the hypothesis of adaptation to climate change.

Metagenomes from Coastal Sediments of Kuwait: Insights into the Microbiome, Metabolic Functions and Resistome

Coastal sediments in the proximity of wastewater and emergency outfalls are often sinks of pharmaceutical compounds and other organic and inorganic contaminants that are likely to affect the microbial community. The metabolites of these contaminants affect microbial diversity and their metabolic processes, resulting in undesirable effects on ecosystem functioning, thus necessitating the need to understand their composition and functions. In the present investigation, we studied the metagenomes of 12 coastal surface sediments through whole genome shot-gun sequencing. Taxonomic binning of the genes predicted about 86% as bacteria, 1% as archaea, >0.001% as viruses and Eukaryota, and 12% as other communities. The dominant bacterial, archaeal, and fungal genera were Woeseia, Nitrosopumilus, and Rhizophagus, respectively. The most prevalent viral families were Myoviridae and Siphoviridae, and the T4 virus was the most dominant bacteriophage. The unigenes further aligned to 26 clusters of orthologous genes (COGs) and five carbohydrate-active enzymes (CAZy) classes. Glycoside hydrolases (GH) and glycoside transferase (GT) were the highest-recorded CAzymes. The Kyoto Encyclopedia of Genes and Genomes (KEGG) level 3 functions were subjugated by purine metabolism > ABC transporters > oxidative phosphorylation > two-component system > pyrimidine metabolism > pyruvate metabolism > quorum sensing > carbon fixation pathways > ribosomes > and glyoxalate and dicarboxylate metabolism. Sequences allying with plasmids, integrons, insertion sequences and antibiotic-resistance genes were also observed. Both the taxonomies and functional abundances exhibited variation in relative abundances, with limited spatial variability (ANOVA p > 0.05; ANOSIM-0.05, p > 0.05). This study underlines the dominant microbial communities and functional genes in the marine sediments of Kuwait as a baseline for future biomonitoring programs.

Depositional time trends of phosphorous accumulation in a dated sediment core from the northwestern Arabian Gulf: Can phosphorous be used to support 210Pb chronologies in coastal aquatic sediments?

The use of ²¹⁰Pb dating for the reconstruction of contaminant profiles in undisturbed sediments is the most widely acceptable technique historically. Due to the uncertainties associated with the technique, dating of sediments have often been supported by ancillary evidence such as an alternative historical marker in the catchment that is preserved in the sedimentary records. The most widely used verification marker is ¹³⁷Cs, which is attributed to global fallout from past nuclear weapons testing, and the Chernobyl accident. In the southern hemisphere, and the mid-latitudes, the ¹³⁷Cs signal from Chernobyl fallout is often absent from the sedimentary records making it very difficult to verify the ²¹⁰Pb chronologies in these natural archives. This study reports the spatial and temporal variability of total phosphorus (TP) and its forms in sediments from Kuwait Bay and provides evidence of the possibility of using Phosphorous as a potential marker to support dates derived from ²¹⁰Pb dating. In the current study we report the spatial and temporal variations in the concentrations of Phosphorus from near-shore sediments from Kuwait. The mean (and range) of TP concentration in surficial sediments is 19.4 (12.2 to 24) μmol-P g^-1 dry weight. The highest concentrations were measured in the deepest portions of the Bay which is characterized by fine grained sediments. The vertical profile of TP in the sediments was characterized by a gradual increase in concentration from lower layers to a subsurface maximum (at approximately 4-5 cm) followed by an exponential decrease to the sediment-water interface. The sedimentary TP profile correlated remarkably well with changes in the population of Kuwait over the last three decades, suggesting that wastewater inputs into the Bay from treatment plants are an important source of phosphorus in the coastal waters. The decrease in concentration in sediment deposited over the last five years of the core is explained by the commissioning of an improved wastewater treatment plant with higher phosphorus removal efficiency. These fluctuations preserved in the sedimentary records linked to identifiable historical events provides powerful evidence that this nutrient associated with wastewater inputs in coastal aquatic environments can be used as ancillary information to support ²¹⁰Pb dating.

Microplastics in wastewater outlets of Bandar Abbas city (Iran): A potential point source of microplastics into the Persian Gulf

Wastewater discharge is considered to be a significant point source of microplastic (MPs) release into the marine environment. This study is the first attempt to quantify MPs released from the wastewater outfall from Bandar Abbas City into the Persian Gulf. Two wastewater discharge stations at Gursuzan and Suru were sampled. MPs were isolated by an oxidative procedure and subsequent density separation using ZnCl₂ solution. The average MP concentration in wastewater and sludge were 70.66 (±14.12, SD) MP.35 L^-1 and 6070 (±807.25) MPs.kg^-1, respectively at Confidence Level (CL) (95.0%). The most commonly recovered polymers were polyethylene (PE) and polypropylene (PP) in all size classes. Our findings provides a baseline of MP concentration in wastewater streams and slurry that is discharged from the Bandar Abbas wastewater treatment facility into the Persian Gulf. This highlights the need to undertake more studies at water treatment plants in the region for a realistic assessment of MP discharge into the Persian Gulf.

Heterotrophic consumption may mask increasing primary production fuelled by anthropogenic nutrient loading in the northern Arabian/Persian Gulf

Monthly measurements of nitrate, nitrite, ammonium and phosphate at three stations off Kuwait during 2002-2015 revealed considerable inter-annual variability, broadly corresponding to fluctuations in the Shatt-al-Arab River discharge, but a lack of secular increasing trend. Nutrient enrichment experiments during two seasons revealed nitrate uptake, chlorophyll build-up and growth of micro-phytoplankton, even in the presence of ammonium, provided the availability of phosphate. Primary production was mostly nitrogen limited, but anthropogenic nitrogen supply may eventually make it phosphorus limited, especially in summer and in the open Gulf. Anthropogenic nutrient inputs appear to have enhanced biological productivity of the northern Gulf, but heterotrophic consumption, indicated by high respiration rates, probably prevented accumulation of phytoplankton biomass, accounting for the observed lack of chlorophyll increase over the past three decades. Consequently high total organic carbon and emerging hypoxia in the Gulf may lead to expansion/intensification of the oxygen minimum zone of the Arabian Sea.

Seasonal variation of potential denitrification rate and enhanced denitrification performance via water-lifting aeration technology in a stratified reservoir-A case study of Zhoucun reservoir

Zhoucun reservoir is one of the major water resources in Zao Zhuang city, northern China. The seasonal distribution of surface sediment denitrification rate and enhanced performance of denitrification via water-lifting aeration technology were explored using the acetylene-inhibition technique. Surface sediment denitrification rates ranged from 2.57 ± 1.32 to 923.90 ± 86.81 nmol N₂/(g dw·h), with the highest rates in November (ANOVA, p < 0.05) and significantly low rates in June, July, and August (ANOVA, p < 0.05), mainly because of the seasonal differences in nitrate concentration, water temperature, and sediment total nitrogen (STN). Meanwhile, the N₂/(N₂+N₂O) ratio (83.44-91.70% for the highest sediment denitrification period) indicated that N₂ accounted for a majority of denitrification. Correlation analysis between various environmental factors and denitrification was conducted, and nitrate concentration, STN, low molecular weight organic carbon, the number of aerobic denitrifying bacteria, and the environmental parameters of oxidation-reduction potential (ORP), pH, electrical conductivity (EC), and chlorophyll a (Chl-a) presented significant relationships during the entire study period. On the basis of the multiple regression model, nitrate and low molecular weight organic carbon concentration were the most influential factors on denitrification variability. Moreover, the denitrification rates of the surface sediment clearly increased, from 5.28 to 13.22 nmol N₂/(g dw·h) to 1117.02-3129.47 nmol N₂/(g dw·h), which were higher than those in the non-operating year. This suggests that the denitrification in the sediment system could be enhanced in situ by water-lifting and aeration technology in the reservoir ecosystem.

Denitrification rates in estuarine sediments of Ashtamudi, Kerala, India

Estuarine sediments are important sites for denitrification, which is microbially mediated reduction of nitrate to dinitrogen that also influences global climate change by co-production of nitrous oxide, a potent greenhouse gas. Physicochemical properties and nutrients of sediment samples that influence denitrification rate were studied in Ashtamudi estuarine sediments. They were pH, electrical conductivity (EC), salinity, nitrate-nitrogen (NO₃^--N), exchangeable ammonia (NH₃^--N), total kjeldahl nitrogen (TKN) and organic carbon (Corg). Sediment samples were collected from six stations during summer, monsoon of 2013 and 13 stations from monsoon 2014 and summer 2015. The sedimentary denitrification potential ranged from 0.49 ± 0.05 to 4.85 ± 0.782 mmol N₂O m^-2 h^-1. Maximum denitrification was observed in S4, which is attributed to a local anthropogenic source coupled with intense rainfall episode preceding the sampling season of monsoon 2013. However, this trend was not repeated in the subsequent monsoon samples. This shows that in Ashtamudi, monsoonal effects do not influence sedimentary denitrification. Among the various environmental variables, NO₃^--N, Corg and NH₃-N were the key factors that influence denitrification in the Ashtamudi estuarine sediments. Among these key factors, NO₃^--N was the limiting factor for denitrification, and hence, it is of prime importance to understand the source of NO₃^--N that fuel denitrification in the sediments. In Ashtamudi, the concentration of NO₃^--N in overlying water was very less, which suggests reduced nitrogen yield in the estuary from the fluvial input of Kallada River and agricultural runoff. Sedimentary NO₃^--N correlated with denitrification which reveals that denitrification is coupled with nitrification in the sediments. This is further explained by the fact that NH₃-N positively correlated with denitrification. The anoxic sediments were the source of ammonia for nitrous oxide production by nitrogen mineralisation. Also, the Corg in sediment samples were sufficient to support denitrification and Corg was an important factor favouring but not limiting denitrification. The results of sediment denitrification in Ashtamudi can be a model for tropical estuaries experiencing unpredictable rainfall as well as high temperature than temperate systems.

The capability of estuarine sediments to remove nitrogen: implications for drinking water resource in Yangtze Estuary

Water in the Yangtze Estuary is fresh most of the year because of the large discharge of Yangtze River. The Qingcaosha Reservoir built on the Changxing Island in the Yangtze Estuary is an estuarine reservoir for drinking water. Denitrification rate in the top 10 cm sediment of the intertidal marshes and bare mudflat of Yangtze Estuarine islands was measured by the acetylene inhibition method. Annual denitrification rate in the top 10 cm of sediment was 23.1 μmol m(-2) h(-1) in marshes (ranged from 7.5 to 42.1 μmol m(-2) h(-1)) and 15.1 μmol m(-2) h(-1) at the mudflat (ranged from 6.6 to 26.5 μmol m(-2) h(-1)). Annual average denitrification rate is higher at mashes than at mudflat, but without a significant difference (p = 0.084, paired t test.). Taking into account the vegetation and water area of the reservoir, a total 1.42 × 10(8) g N could be converted into nitrogen gas (N2) annually by the sediment, which is 97.7 % of the dissolved inorganic nitrogen input through precipitation. Denitrification in reservoir sediment can control the bioavailable nitrogen level of the water body. At the Yangtze estuary, denitrification primarily took place in the top 4 cm of sediment, and there was no significant spatial or temporal variation of denitrification during the year at the marshes and mudflat, which led to no single factor determining the denitrification process but the combined effects of the environmental factors, hydrologic condition, and wetland vegetation.