Natural Aquatic Systems Research Articles

Are passive collectors effective samplers of microbes in natural aquatic systems?

Are passive collectors effective samplers of microbes in natural aquatic systems?

Biodegradation of poly(butylene adipate terephthalate) and poly(vinyl alcohol) within aquatic pathway

Understanding the environmental fate of biodegradable plastics in aquatic systems is crucial, given the alarming amount of plastic waste and microplastic particles transported through aquatic pathways. In particular, there is a need to analyze the biodegradation of commercialized biodegradable plastics upon release from wastewater treatment plants into natural aquatic systems. This study investigates the biodegradation behaviors of poly(butylene adipate terephthalate) (PBAT) and poly(vinyl alcohol) (PVA) in wastewater, freshwater, and seawater. Biodegradation of PBAT and PVA assessed through biochemical oxygen demand (BOD) experiments and microcosm tests revealed that the type of aquatic system governs the biodegradation behaviors of each plastic, with the highest biodegradation rate achieved in wastewater for both PBAT and PVA (25.6 and 32.2 % in 30 d, respectively). Plastic release pathway from wastewater into other aquatic systems simulated by sequential incubation in different microcosms suggested that PBAT exposed to wastewater and freshwater before reaching seawater was more prone to degradation than when directly exposed to seawater. On the other hand, PVA displayed comparable biodegradation rate regardless of whether it was directly exposed to seawater or had passed through other environments beforehand. Metagenome amplicon sequencing of 16S rRNA genes revealed distinct community shifts dependent on the type of plastics in changing environments along the simulated aquatic pathway. Several bacterial species putatively implicated in the biodegradation of PBAT and PVA are discussed. Our findings underscore the significant influence of pollution routes on the biodegradation of PBAT and PVA, highlighting the potential for wastewater treatment to facilitate rapid degradation compared to direct exposure to pristine aquatic environments.

A multi-pronged approach to assessing antimicrobial resistance risks in coastal waters and aquaculture systems

Antimicrobial resistance (AMR) is a global challenge that has impacted aquaculture and surrounding marine environments. In this study, a year-long monitoring program was implemented to evaluate AMR in two different aquaculture settings (i.e., open cage farming, recirculating aquaculture system (RAS)) and surrounding marine environment within a tropical coastal region. The objectives of this study are to (i) investigate the prevalence and co-occurrence of antibiotic-resistant bacteria (ARB), antibiotic resistance genes (ARGs), antibiotics (AB) and various associated chemical compounds at these study sites; (ii) explore the contributing factors to development and propagation of AMR in the coastal environment; and (iii) assess the AMR risks from different perspectives based on the three AMR determinants (i.e., ARB, ARGs and AB). Key findings revealed a distinct pattern of AMR across the different aquaculture settings, notably a higher prevalence of antibiotic-resistant Vibrio at RAS outfalls, suggesting a potential accumulation of microorganisms within the treatment system. Despite the relative uniform distribution of ARGs across marine sites, specific genes such as qepA, blaCTX−M and bacA, were found to be abundant in fish samples, especially from the RAS. Variations in chemical contaminant prevalence across sites highlighted possible anthropogenic impacts. Moreover, environmental and seasonal variations were found to significantly influence the distribution of ARGs and chemical compounds in the coastal waters. Hierarchical cluster analysis that was based on ARGs, chemical compounds and environmental data, categorized the sites into three distinct clusters which reflected strong association with location, seasonality and aquaculture activities. The observed weak correlations between ARGs and chemical compounds imply that low environmental concentrations may be insufficient for resistance selection. A comprehensive risk assessment using methodologies such as the multiple antibiotic resistance (MAR) index, comparative AMR risk index (CAMRI) and Risk quotient (RQ) underscored the complexity of AMR risks. This research significantly contributes to the understanding of AMR dynamics in natural aquatic systems and provides valuable insights for managing and mitigating AMR risks in coastal environments.

Influence of Organic Carbon from Weathered Sediments on Triclocarban Distribution in Environmental Aqueous Systems

In this study, the chemical distribution of triclocarban (TCC), in natural aqueous systems, between water and sediment, with different chemical compositions of the aqueous phase and different percentages of organic carbon (OC%) in the sediments is presented. The influences of the temperature, of the composition of the aqueous matrices of natural waters and (OC%) in the sediment over the solubility of triclocarban, and its distribution coefficient Kd values were studied. log KD at 25 °C varied between 1.94 and 3.27 for a sediment with 5.50% OC and between 3.95 and 5.93% for a sediment with 6.75% OC, in the studied aqueous systems, with different concentrations of OC in the sediment.

A robust self-regenerating graphene-based adsorbent for pharmaceutical removal in various water environments

The presence of active pharmaceutical ingredients (APIs) in wastewater effluents and natural aquatic systems threatens ecological and human health. While activated carbon-based adsorbents, such as GAC and PAC, are widely used for API removal, they exhibit certain deficiencies, including reduced performance due to the presence of natural organic macromolecules (NOMs) and high regeneration costs. There is growing demand for a robust, stable, and self-regenerative adsorbent designed for API removal in various environments. In this study, we synthesized a self-generating metal oxide nano-composite (S-MGC) containing titanium dioxide (TiO2) and silicon dioxide (SiO2) combined with 3D graphene oxide (GO) to adsorb APIs and undergo regeneration via light illumination. We determined optimal TiO2:SiO2:GO compositions for the S-MGCs through experiments using a model contaminant, methylene blue. The physical and chemical properties of S-MGCs were characterized, and their adsorption and photodegradation capabilities were studied using five model APIs, including sulfamethoxazole, carbamazepine, ketoprofen, valsartan, and diclofenac, both in single-component and multi-component mixtures. In the absence of TiO2/SiO2, 3D graphene oxide (CGB) displayed better adsorption performance compared to GAC, and S-MGCs further improve CGB's adsorption capacity. This performance remained consistent in two complex water environments: aqueous solutions at varying NOM levels and artificial urine. TiO2 supported on the GO surface exhibits similar photocatalytic activity to suspended TiO2. In a continuous fixed-bed column test, S-MGCs demonstrated robust API adsorption performance that is maintained in the presence of NOM or urine, and can be regenerated through multiple cycles of adsorption and light illumination.

EFFICIENCY OF APPLICATION OF SURFACTANT-MODIFIED NATURAL ZEOLITE FOR REMOVAL OF METALS FROM AKHTALA AND SHNOGH RIVERS WATERS

The sorption (removal) efficiency of anionic surfactant modified natural zeolite from Nor Koghb deposit in relation to metal contaminants of water samples from Akhtala and Shnogh Rivers has been studied. It is shown that suggested anionic surfactant modified zeolites exhibit high sorption capability for metal adsorption from multi-component natural aquatic systems.

Modifying luteolin’s algicidal effect on Microcystis by virgin and diversely-aged polystyrene microplastics: Unveiling novel mechanisms through microalgal adaptive strategies

Luteolin has shown great potential in inhibiting Microcystis-dominated cyanobacterial blooms. However, widespread microplastics (MPs) in natural aquatic systems often serve as substrates for cyanobacterial growth, which could impact cyanobacterial resistance to external stresses and interfere with luteolin’s algicidal effect. This study explored the influence of virgin and diversely-aged polystyrene microplastics (PS-MPs) on inhibitory effect of luteolin on Microcystis growth and its microcystins (MCs) production/release. Moreover, the underlying mechanisms were also revealed by jointly analyzing SEM image, antioxidant response, exopolymeric substances (EPSs) production, and functional gene expression. Results suggested that 0.5, 5, and 50 mg/L virgin and diversely-aged PS-MPs almost weakened growth inhibition and oxidative damage of two doses of luteolin against Microcystisby stimulating its EPSs production and inducing self-aggregation of Microcystis cells and/or hetero-aggregation between Microcystis cells and PS-MPs. Compared to virgin PS-MPs, photo-aged PS-MPs possessed rougher flaky surfaces, and hydrothermal-aged PS-MPs showed internal cracking. These characteristics led to greater stimulation of EPS production and exhibited more significant protective effects on Microcystis. Notably, PS-MPs also decreased MCs content in aqueous phase, likely because they adsorbed some MCs. Such toxigenic hetero-aggregates formed by MCs, MPs, and Microcystis cells would directly poison grazing organisms that consume them and create more pathways for MCs into food web, posing greater eco-risks. This is the first study to clarify the influence and mechanisms of virgin and diversely-aged MPs on allelopathic algicidal effects from the perspective of microalgal inherent adaptive strategies.

Vivianite oxidation is not photocatalyzed

Vivianite is a hydrous Fe(II) phosphate mineral found in anoxic environments that plays an integral role in P cycling and environmental biogeochemistry. Imbued in the vivianite literature dating as far back as 1918 is the notion that vivianite is light-sensitive and susceptible to rapid photooxidation induced entirely by solar light. However, despite an exhaustive search, to our knowledge this conventionally accepted assumption has never been directly tested, nor has the band gap ever been measured. Resolving this question of photochemical control of vivianite redox chemistry is important for performing and interpreting experiments involving vivianite reactivity with contaminants, and potentially iron and phosphate cycling in the photic zone of aquatic systems. The present study was specifically designed to address this question. Pure synthetic vivianite samples were irradiated in aqueous suspensions with a Xe lamp at conditions mimicking the solar spectrum and compared to dark controls. Particles stirred in anoxic suspensions for up to 66 days showed no detectable oxidation irrespective of light exposure, according to colorimetric Fe(II)/Fe(III) assays, micro X-ray diffraction, and Raman spectroscopy. In contrast, the same experiment performed using air-equilibrated suspensions showed systematic oxidation of up to ∼80 mol% Fe(II) at 66 days, but again with no detectable light exposure effect. To test for UV reactivity, further experiments on vivianite powders under air versus N2 atmosphere were performed using a Hg lamp with a longpass filter. In contrast to oxidation in air where no effect of light was detected, after 24 h irradiation anoxically, an additional 0.8 % Fe was oxidized compared to dark controls, and after 13 days only an additional 0.3 % Fe was oxidized. The band gap of pure vivianite was measured by the Tauc method to be 3.5 eV. Contrary to previous reports, solar light has minimal impact on the oxidation of pure vivianite; any such effect would be insignificant when compared to its relatively fast oxidation by oxygen exposure. As such, light does not need to be excluded when running typical vivianite laboratory experiments. Long-term storage of natural vivianite in museums or private collections should continue protective measures given that a possible photocatalytic role of impurities cannot yet be ruled out.Nonetheless, at all depths in natural aquatic systems, including the photic zone, it appears that the primary abiotic control on the reactivity and half-life of vivianite particles would be the dissolved oxygen concentration, not light exposure.

Acute and Chonic Effects of Silver Nanoparticles (AgNPs) on Unio delicatus

The widespread use of silver nanoparticles (AgNPs) including water filters, paints, cosmetics, deodorants, clothing, textiles, food packaging, electrical appliances and medical devices inevitably leads to their release into the natural environment, bioaccumulation in organisms and persistent accumulation in natural aquatic systems. The aim of this study is to investigate the acute and chronic effects of silver nanoparticles, which can contaminate aquatic ecosystems, in freshwater mussels, one of the aquatic invertebrate organisms. The model organism of the study, Unio delicatus, was obtained from Gölbaşı Lake (Hatay). After that acclimation was performed in the laboratory for two weeks. The mussels were then exposed to 1 and 10 mg/L AgNPs for 7 and 21 days. At the end of the exposure period, hemolymph and tissue samples of the mussels were taken. Total hemocyte count from hemolymph samples, lipid peroxidation and glutathione levels from tissue samples (digestive gland and gill) were investigated. Acute exposure resulted in an increase in the total hemocyte counts, while chronic exposure resulted in a significant decrease (P

Thermodynamic property estimations for aqueous primary, secondary, and tertiary alkylamines, benzylamines, and their corresponding aminiums across temperature and pressure are validated by measurements from experiments

On Earth and beyond, organic chemistry often occurs in the presence of water within environments that deviate drastically from ambient conditions (25 °C and 1 bar). Accurately predicting aqueous organic reaction pathways is crucial toward understanding planetary scale processes such as the cycling of elements crucial for life (e.g., carbon and nitrogen). Advanced thermodynamic modeling can be utilized to determine the favorability of various organic reactions based on geologically relevant ranges of temperature and pressure, as well as compositional variables (e.g., pH). However, data that would otherwise allow for a diversity of organic compounds and environmental conditions to be modeled are sparse and rarely tested with experiments, particularly with regard to organic-nitrogen compounds. In this work, we develop a framework to estimate thermodynamic properties at ambient conditions that can then be extrapolated across ranges of temperature and pressure for aqueous primary, secondary, and tertiary amines and aminiums (protonated amines), specifically those structures containing linear alkyl chains and benzyl functional groups. We also performed hydrothermal experiments (250 °C, ∼40 bar) involving reactions of methylamines to test our resulting thermodynamic models, and we compare our models for other alkylamines and benzylamines to previous empirical measurements from the literature. Specifically, we use existing thermodynamic data along with our estimates at ambient conditions in combination with a variety of existing extrapolation methods related to the revised Helgeson-Kirkham-Flowers (HKF) equations of state to generate temperature- and pressure-dependent predictions of acid dissociation constants (i.e., pKa values) that strongly agree with previous empirical measurements. We use similar methods to predict product distributions for reactions involving primary, secondary, and tertiary amines/aminiums, as well as ammonia/ammonium and corresponding alcohols whose collective distributions depend on reversible substitution reactions. Our predictions are in good agreement with our experimental results involving the methylamine reaction system as well as previous experiments involving the benzylamine reaction system, for which we also produced thermodynamic estimates involving benzyl alcohol. The agreement between independent theoretical predictions and experimental measurements suggests that our estimated properties can be applied to modeling amine chemistry in other experimental and natural aqueous systems that range in temperature and pressure, providing new tools for planetary exploration.

Effect of UV-LED Wavelength on Reactive Species Photogeneration from Dissolved Organic Matter

The photogeneration of reactive species from dissolved organic matter (DOM) plays a crucial role in the photochemical and photobiochemical processes in natural aquatic systems. However, the impact of the ultraviolet (UV) wavelength on the photogeneration of reactive species by different sources of DOM remains unclear. In this study, UV light at four wavelengths (365 nm, 310 nm, 280 nm, and 260 nm) provided by UV-LEDs were irradiated onto three types of DOM: humic acid (HA), fulvic acid (FA), and effluent organic matter (EfOM). Three reactive species produced by DOM, including excited triplet-state DOM (3DOM*), singlet oxygen (1O2), and hydroxyl radicals (•OH), were determined. UV365 proved to be the most efficient wavelength for generating 1O2 and •OH, with formation rates of 3.47 × 10−6 M s−1 and 1.67 × 10−8 M s−1, respectively, with the addition of FA and EfOM. The highest steady-state concentrations of all three reactive species were also generated under UV365, reaching 3.00 × 10−13 M (3DOM*) and 1.64 × 10−11 M (1O2) with the FA addition, and 1.44 × 10−10 M (•OH) with the EfOM. Across the different DOM sources, UV365 obtained the maximum quantum yields of reactive species, indicating the stronger effect of UV365 on inducing the photosensitization of DOM compared to the other shorter wavelengths. This study expands our understanding of the photochemistry of DOM in aquatic environments.

Harnessing Natural Aquifer Filtration for Large-Scale Water Purification: Opportunities and Challenges

The research investigates the potential benefits of utilizing natural aquifer systems as a means of water treatment on an extensive basis. The present study analyzes the mechanisms via which naturally aquifers properly filter water, examining the possibilities to be practical financially effective ways for dealing with the growing demand for safe water. The paper highlights the potential advantages associated with natural aquifers filtration, such as its small environmental impact and its ability to sustain water quality. At the same time, it recognizes the challenges that have to be overcome, including the risk of pollution, the complex nature of laws and regulations, and the necessity of successfully controlling aquifer recharge. This study incorporates many geological, hydrological, and ecological engineering perspectives in order to offer an in-depth study of natural aquifer filter systems. This study aims to examine case studies and present practices in order to provide an in-depth strategy for effective use of these systems in various global environments. It also takes into consideration the significant potential of these mechanisms as well as the obstacles that need to be solved.

Reactivity of hydrogen sulfide toward organic compounds with sulfur-sulfur bonds

The presence of organic molecules containing sulfur-sulfur bonds was documented in the water columns and sediments of the natural aquatic systems as well as in fossil fuels. These compounds range from small molecules of volatile organic compounds to polysulfide cross-linked macromolecules in the sediments. While processes leading to formation of these compounds were intensively studied during the last decades, kinetics and mechanisms of reactions which lead to their decomposition are poorly understood. In this work, the kinetics and products of the reactions of dimethyl disulfide, dimethyl trisulfide, and cyclic polysulfide lenthionine (1,2,3,5,6-pentathiepane) with hydrogen sulfide at the environmental pH and temperature ranges were studied. It was shown that at pH ≥ 5, the reaction of bisulfide anion (HS–) rather than that of hydrogen sulfide controls the overall reaction rates. The activation energy and the order of the reaction with respect to bisulfide anion is dimethyl disulfide– < dimethyl trisulfide < lenthionine, while the order of the reaction with respect to organosulfur compounds is lenthionine < dimethyl trisulfide < dimethyl disulfide. The obtained results suggest that bisulfide anion is responsible for fast decomposition of organosulfur compounds in aphotic natural aquatic systems. This observation leads to the conclusion that sulfurization of sedimentary organic matter during the early diagenesis is not an unidirectional but a dynamic process, and concentrations of organic compounds with sulfur-sulfur bonds are controlled by the polysulfide to sulfide sulfur concentrations ratio in the sedimentary pore waters. In addition, the cyclic polysulfides were shown to be more stable than their linear analogs that results in preferential preservation of these compounds during the maturation process.

Progress in Research on the Bioavailability and Toxicity of Nanoplastics to Freshwater Plankton

The present review critically examines the advancements in the past 5 years regarding research on the bioavailability and toxicity of the nanoplastics (NPLs) to freshwater plankton. We discuss the recent progress in the understanding of adsorption, absorption, trophic transfer, and biological effects in phyto- and zooplankton induced by NPLs exposure. The influence of plankton on NPLs’ bioavailability via the excretion of biomolecules and formation of eco-corona is also examined. Despite important research developments, there are still considerable knowledge gaps with respect to NPLs’ bioavailability and trophic transfer by plankton as well as a potential adverse effect in natural aquatic systems. As plankton play a critical role in primary production, nutrient cycling, and food web structure, understanding the interactions between NPLs and plankton is essential in assessing the potential implications of NPLs pollution for aquatic ecosystem biodiversity and services.

Comparative analysis of five convolutional neural networks and transfer learning classification approach for microplastics in wastewater treatment plants

Wastewater treatment plants offer an important pathway for microplastics (MPs) to enter natural aquatic systems. Traditional MP analysis often involves manual sorting of MPs, which can be time-consuming and labor-intensive and is susceptible to human errors, particularly when handling large sample sizes. Herein, five convolutional neural networks—EfficientNet_b7, Inception_v3, Resnet_v2_50, Resnet_v2_101, and Mobilenet_v3—were employed using transfer learning (TL) methods to categorize MPs extracted from municipal wastewater treatment plants into four morphologies: fibers, films, fragments, and pellets. A Samsung Galaxy S22 Android smartphone was used to capture pictures of the MPs using a stereomicroscope, which were then used to train the neural networks. First, TL methods were employed using an 80:20 split of training and validation datasets and testing sets, and the effectiveness of the methods was evaluated via five-fold cross validation. Second, the effect of the sizes of the training and validation sets on the performances of different neural networks was estimated through augmentation of these sets to 300% and their combination with the original dataset, yielding 5104 images. The overall performances of the considered neural networks were evaluated in terms of accuracy, precision, recall, F1 score, and training time. The proposed approach achieved accuracies of 92%–96% and 94%–98% in classifying the original and augmented datasets, respectively, based on MP morphologies. Notably, EfficientNet_b7, Inception_v3, and Mobilenet_v3 achieved the highest classification accuracy of 98% when trained on the augmented dataset. Mobilenet_v3 achieved a competitive accuracy on our datasets while being considerably smaller and more computationally efficient, leading to its selection for deployment in Android mobile and web applications. Overall, Mobilenet_v3 can promptly classify MPs in real time, offering an efficient, effective solution for MP image classification on a smartphone.

Differential utilisation of dissolved organic matter compound fractions by different biofilter microbial communities

Abstract Dissolved organic matter (DOM) is a complex mixture of carbon-based compounds present in natural aquatic systems, which significantly affects drinking water treatment processes. Biofiltration, utilising biologically active beds of porous medium, offers a low-energy and low-chemical solution for controlling bioavailable DOM. However, the impact of microbial community composition on DOM degradation in biofilters remains poorly understood. This study aimed to explore the abilities of microbial communities from the top, middle, and bottom (TOP, MID, and BOT) of a biofilter to process DOM. We showed varying growth rates on the DOM, with bottom community exhibiting the highest cell abundance at the end of the experiment (1.83 × 106 ± 9 × 103; 2.06 × 106 ± 1 × 104; 2.15 × 106 ± 7 × 103 cells/mL for the TOP, MID, and BOT, respectively). The three communities showed different preferences for utilising specific DOM fractions, with the bottom community targeting more complex ones. The microbial communities from the bottom of the biofilter had a higher relative abundance of the Curvibacter genus, suggesting it could play a crucial role in degrading complex DOM fractions. These findings highlight the influence of microbial community composition on DOM degradation in biofilters, providing valuable insights for optimising their performance.

Low-cost field particle image velocimetry for quantifying environmental turbulence

To date, there is a lack of in-situ turbulence data in natural aquatic systems. Adapting particle image velocimetry (PIV) to field use is of interest to provide such data. Past field PIV systems have been successful, but suffer from high costs, lack of portability, and flow disruption. We present a new field PIV system designed to address these limitations and capture turbulence metrics significant to aquatic biota: velocity, turbulent kinetic energy (TKE), vorticity, eddy size, and eddy circulation. Velocity measurements by the novel PIV system were compared to measurements by an acoustic doppler velocimeter (ADV) and an electromagnetic flowmeter for validation, and its results agreed with these other two methods. The PIV’s TKE results were compared to those of an ADV, and they agreed as well. Vorticity, eddy size, and eddy circulation were validated in the field through repeating past measurements. Each of these data sets generated by the novel PIV fell within the range of the validation measurements. The novel system thus provides accurate turbulence and velocity measurements; further, its design is low-cost, uses widely available materials, is minimally disruptive to the flow, and is highly portable and adjustable, offering an accessible method for acquiring meaningful field turbulence data.

Preparation of an electrochemical sensor utilizing graphene-like biochar for the detection of tetracycline

Tetracycline (TC), which is ubiquitous in the aquatic environment, can cause ecological imbalance and adversely affect human health. Therefore, a quick, inexpensive, and easy method for the detection of TC in water systems is highly desirable. This study reports the development of a novel electrochemical sensor from waste peanut shell for the quick detection of TC in water. Raman and TEM lattice mapping analyses confirmed the successful preparation of graphene -like biochar from waste peanut shells (PSs) via hydrothermal and pyrolysis processes. An electrochemical sensor, PS@glassy carbon electrode (PS@GCE), was then developed by coating the prepared graphene-like biochar on the surface of a glass electrode to enhance its conductivity. The feasibility of using this sensor for the detection of TC in the aqueous system was investigated. The PS@GCE sensor exhibited excellent sensitivity with a low detection limit of 3.6 × 10-−9 nM and a linear range of 10−10–102 μM. These results were attributed to the large specific surface area and high conductivity, of the PS biochar. The stability of the PS@GCE sensor was also investigated in the presence of TC (10−4 M) and interfering species (10−2 M) and recovery rates in the range of 86.4%–116.0% were achieved, thus indicating the absence of an interference range of range of 84.3%–98.2% with relative standard deviation lower than 6% were achieved upon the detection of TC in natural water samples using the designed sensor, thus confirming the superior repeatability of the PS@GCE sensor. Consequently, the designed electrode has a high potential for application in the detection of TC in natural aqueous systems.

Distinct Radiocarbon Ages Reveal Two Black Carbon Pools Preserved in Large River Estuarine Sediments.

Black carbon (BC), a group of environmentally concentrated organic pollutants, is widely distributed in marine sediments via riverine run-off and atmospheric deposition. The fate of BC transformation and cycling in marine sediments, however, has not been well studied. Here, we present radiocarbon measurements for sedimentary solid-phase BC (SBC) and porewater-dissolved BC (DBC) in surface sediments collected from the Yangtze and Yellow River estuaries and their adjacent coastal regions. Radiocarbon results revealed that two distinct BC pools in the sediments of the SBC had ancient radiocarbon ages (7110-15,850 years BP) that were 5370-14,935 years older than the 14C ages of porewater DBC. Using a radiocarbon mass balance model, we calculated that modern biomass-derived BC contributed 77-97% of the DBC pool and that fossil material-produced BC accounted for 61-87% of the SBC pools. This discrepancy between modern and dead BC contributions was associated with the BC budget after particulate BC (PBC) deposition; 38 ± 13% of the PBC was transferred to porewater as DBC and 62 ± 13% was sequestrated as SBC in sediments, serving as an important CO2 sink in marine sediments. We also provide evidence suggesting that DBC likely comprises some very fine particulate forms that are not completely dissolved as molecules. The nature and transformation mechanisms of DBC in natural aquatic systems need to be further studied.

Dynamic and systemic regulatory mechanisms in rainbow trout (Oncorhynchus mykiss) in response to acute hypoxia and reoxygenation stress

Hypoxia frequently occurs in natural aquatic systems and aquaculture environments due to extreme climate fluctuations, high-density farming, environmental pollution and global warming, resulting in serious ecological damage and enormous economic losses. Rainbow trout (Oncorhynchus mykiss), a key economic fish species worldwide, is extremely sensitive to hypoxia. However, the regulatory mechanisms in response to environmental hypoxia and reoxygenation stress remain unknown. Herein, rainbow trout liver transcriptomes and biochemical parameters were investigated in response to hypoxia for different durations (3, 12 and 24 h), reoxygenation (hypoxia for 24 h followed by reoxygenation for 3 h) and normoxia to highlight dynamic changes in molecular regulation and oxidative stress. In RNA-seq analysis, 5906 differentially expressed genes, two significantly expressed gene sets (profiles 10 and 12) and two hypoxia specific modules (MEgreen and MEturquoise) were screened by differential expression analysis, short time-series expression miner (STEM) and weighted gene co-expression network analysis (WGCNA), respectively. The intersection of the above analyses further identified several key hub genes related to hypoxia, including hif-1α, epo, igfbp1, ddit4, pck1, g6pc, cyp1a1 and dusp1. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses revealed different metabolic strategies at different stress stages; signal transmission and protein synthesis process were strongly influenced by hypoxia. The immune system was inhibited, and a switch from programmed to pathological cell death was observed from hypoxia for 12 h to 24 h. After reoxygenation for 3 h, mitochondrial oxidative phosphorylation was strongly activated to provide more cellular ATP. Enzymatic and non-enzymatic antioxidant systems clearly coordinate to alleviate oxidative stress injury caused by hypoxia. This study reveals dynamic and systemic regulatory mechanisms in rainbow trout liver, and lays a foundation for further study on the molecular mechanisms governing responses to environmental hypoxia and reoxygenation stress.