Strong associations between dissolved organic matter and microbial communities in the sediments of Qinghai-Tibetan Plateau lakes depend on salinity.

In aquatic ecosystems, dissolved organic matter (DOM) plays a vital role in microbial communities and the biogeochemical cycling of elements. However, little is known about the associations between DOM and microbial communities in lake sediments. This study investigated the composition of water-extractable organic matter and microbial communities in surface sediments of lakes with different salinities on the Qinghai-Tibet Plateau. Ultrahigh-resolution mass spectrometry and high-throughput microbial sequencing techniques were employed to assess the associations between molecular diversity and microbial diversity and the effects of salinity in 19 lakes spanning a salinity range from 0.22 ‰ to 341.87 ‰. Our results show that increasing salinity of lake water led to higher molecular diversity of DOM in surface sediments. High-salinity lakes exhibited distinct DOM characteristics, such as lower aromaticity, smaller molecular weight, and higher oxidation degree, compared to freshwater lakes. The complexity of the microbial network composition of sediments first increased and then decreased with the increase of salinity. Moreover, as salinity increases, the dominant species transitioned from Gammaproteobacteria to Bacteroidia, and this transition was accompanied by a decrease in microbial diversity and an increase in molecular diversity. Microbial factors accounted for 34.68 % of the variation in the molecular composition of DOM. Overall, this study emphasizes the significant effects of salinity on both molecular and microbial diversity in lake sediments. Furthermore, our findings underscore the importance of microbes in controlling the range of organic compounds present in lakes and deepen our knowledge of the biogeochemical cycling of DOM.

Inactivation of harmful cyanobacteria Microcystis aeruginosa by Cu2+ doped corn stalk biochar treated with different pyrolysis temperatures.

In this study, biochars (BCs) derived from corn stalk treated at various pyrolysis temperatures (350-950 °C) were prepared and then loaded with Cu2+ to form highly efficient algaecide, i.e. Cu2+-doped BC composites (Cu-BCs). The results showed BCs pyrolyzed at higher temperatures suppressed the growth of Microcystis aeruginosa in the order of BC550 ≫ BC750 > BC950, while BC350 accelerated cell growth due to the release of inorganic nutrients. The difference could be attributed to the physicochemical characteristics, including specific surface area, adsorption capacity of nutrients and the presence of particularly persistent free radicals. Furthermore, Cu-BCs exhibited the improved inactivation performance, but the 72 h growth inhibition rates and reaction activities of Cu-BCs were still influenced by the Cu2+ loading ratio and pyrolysis temperature. These results, reported for the first time, demonstrated the algae inactivation efficiency of pristine BCs, and Cu-BCs were principally manipulated by the biochar pyrolysis temperature.

Cable bacteria regulate sedimentary phosphorus release in freshwater sediments.

Previous studies have demonstrated that e-SOx can regulate the sedimentary release of phosphorus (P) in brackish and marine sediments. When e-SOx is active, an iron (Fe) and manganese (Mn) oxide rich layer is formed near the sediment surface, which prevents P release. When e-SOx becomes inactive, the metal oxide layer is reduced via sulfide-mediated dissolution, and P is subsequently released to the water column. Cable bacteria have been shown to also occur in freshwater sediments. In these sediments, sulfide production is limited, and the metal oxide layer would thus dissolve less efficiently, leaving the P trapped at the sediment surface. This lack of an efficient dissolution mechanism implies that e-SOx could play an important role in the regulation of P availability in eutrophied freshwater streams. To test this hypothesis, we incubated sediments from a eutrophic freshwater river to investigate the impact of cable bacteria on sedimentary cycling of Fe, Mn and P. High-resolution depth profiling of pH, O2 and ΣH2S complemented with FISH analysis and high-throughput gene sequencing showed that the development of e-SOx activity was closely linked to the enrichment of cable bacteria in incubated sediments. Cable bacteria activity caused a strong acidification in the suboxic zone, leading to the dissolution of Fe and Mn minerals and consequently a strong release of dissolved Fe2+ and Mn2+ to the porewater. Oxidation of these mobilized ions at the sediment surface led to the formation of a metal oxide layer that trapped dissolved P, as shown by the enrichment of P-bearing metal oxides in the top layer of the sediment and low phosphate in the pore and overlying water. After e-SOx activity declined, the metal oxide layer did not dissolve and P remained trapped at the surface. Overall, our results suggested cable bacteria can play an important role to counteract eutrophication in freshwater systems.

What drives the change of nitrogen and phosphorus loads in the Yellow River Basin during 2006-2017?

The Yellow River Basin (YRB) plays a very important role in China's economic and social development and ecological security. In particular, the ecosystem of the YRB is sensitive to climate change. However, the change of nutrient fluxes in this region during the past years and its main driving forces remain unclear. In this study, a hydrologic model R System for Spatially Referenced Regressions on Watershed Attributes (RSPARROW) was employed to simulate the spatio-temporal variations in the fluxes of total nitrogen (TN) and total phosphorus (TP) during the period of 2006-2017. The results suggested that the TN and TP loads increased by 138% and 38% during 2006-2014, respectively, and decreased by 66% and 71% from 2015 to 2017, respectively. During the period of 2006-2017, the annual mean fluxes of TN and TP in the YRB were in the range of 3.9 to 591.6 kg/km2/year and 1.7 to 12.0 kg/km2/year, respectively. TN flux was low in the upstream area of the Yellow River, and presented a high level in the middle and lower reaches. However, the flux of TP in Gansu and Ningxia section was slightly higher than that in the lower reaches of the Yellow River. Precipitation and point source are the key drivers for the inter-annual changes of TN loads in most regions of the YRB. While the inter-annual variations of TP loads in the whole basin are mainly driven by the point source. This study demonstrates the important impacts of climate change on nutrient loads in the YRB. Moreover, management measures should be taken to reduce pollution sources and thus provide solid basis for control of nitrogen and phosphorus in the YRB.

Hemocytes of bivalve mollusks as cellular models in toxicological studies of metals and metal-based nanomaterials.

Understanding the impacts of environmental pollutants on immune systems is indispensable in ecological and health risk assessments due to the significance of normal immunological functions in all living organisms. Bivalves as sentinel organisms with vital ecological importance are widely distributed in aquatic environments and their innate immune systems are the sensitive targets of environmental pollutants. As the central component of innate immunity, bivalve hemocytes are endowed with specialized endolysosomal systems for particle internalization and metal detoxification. These intrinsic biological features make them a unique cellular model for metal- and nano-immunotoxicology research. In this review, we firstly provided a general overview of bivalve's innate immunity and the classification and immune functions of hemocytes. We then summarized the recent progress on the interactions of metals and nanoparticles with bivalve hemocytes, with emphasis on the involvement of hemocytes in metal regulation and detoxification, the interactions of hemocytes and nanoparticles at eco/bio-nano interface and hemocyte-mediated immune responses to the exposure of metals and nanoparticles. Finally, we proposed the key knowledge gaps and future research priorities in deciphering the fundamental biological processes of the interactions of environmental pollutants with the innate immune system of bivalves as well as in developing bivalve hemocytes into a promising cellular model for nano-immuno-safety assessment.

Century-Long Homogenization of Algal Communities Is Accelerated by Nutrient Enrichment and Climate Warming in Lakes and Reservoirs of the North Temperate Zone.

Anthropogenic pressures can threaten lake and reservoir ecosystems, leading to harmful algal blooms that have become globally widespread. However, patterns of phytoplankton diversity change and community assembly over long-term scales remain unknown. Here, we explore biodiversity patterns in eukaryotic algal (EA) and cyanobacterial (CYA) communities over a century by sequencing DNA preserved in the sediment cores of seven lakes and reservoirs in the North Temperate Zone. Comparisons within lakes revealed temporal algal community homogenization in mesotrophic lakes, eutrophic lakes, and reservoirs over the last century but no systematic losses of α-diversity. Temporal homogenization of EA and CYA communities continued into the modern day probably due to time-lags related to historical legacies, even if lakes go through a eutrophication phase followed by a reoligotrophication phase. Further, algal community assembly in lakes and reservoirs was mediated by both deterministic and stochastic processes, while homogeneous selection played a relatively important role in recent decades due to intensified anthropogenic activities and climate warming. Overall, these results expand our understanding of global change effects on algal community diversity and succession in lakes and reservoirs that exhibit different successional trajectories while also providing a baseline framework to assess their potential responses to future environmental change.

Effects of sulfide availability on the metabolic activity and population dynamics of cable bacteria in freshwater sediment.

Cable bacteria occur in many natural environments, and their electrogenic sulfide oxidation (e-SOx) may influence sediment biogeochemistry. The environmental factors determining the growth and diversity of cable bacteria are poorly known, especially in freshwater sediments. We conducted a laboratory incubation experiment, using freshwater sediments with different sulfide supply levels, to study how sulfide availability in sediment affects the metabolic activity and population dynamics of cable bacteria. A moderate increase in the sulfide availability in sediment significantly promoted metabolic activity and the proliferation of the cable bacteria population, as revealed by enhanced e-SOx intensity and increased bacteria abundance. In high-sulfide treatments there was a more significant increase in the population of cable bacteria in the deeper sediment layers, indicating that increased sulfide availability may expand the vertical scale impact of cable bacteria activities on sediment biogeochemistry. The relative proportions of co-existing species in the cable bacteria population also changed with sulfide supply levels, indicating that sulfide availability can be involved in determining the interspecies relationships of cable bacteria. Our findings provide new insight into the relationship between sediment sulfide availability and the growth, depth distribution, and species composition of cable bacteria, implying the consideration of regulating environmental sulfide availability as a potential management practice for the development of cable bacteria-based environmental biotechnologies.

NanoSIMS Imaging of Bioaccumulation and Subcellular Distribution of Manganese During Oyster Gametogenesis.

Many bivalve mollusks display remarkable sex differentiation of gonadal accumulation of manganese (Mn), but the underlying processes responsible for such differences have seldom been explored. In this study, the accumulation of Mn in male and female gonads during the reproductive cycle of oysters was first examined, and the distributions of Mn in oocytes and sperm cells at different developmental stages were imaged by the nanoscale secondary ion mass spectrometry (NanoSIMS) at the subcellular level. We found that the distribution and accumulation of Mn during oogenesis were closely associated with the formation and translocation of cortical granules. This is the first time that the enrichment of Mn was directly visualized in cortical granules, which was identified as the major storage site of Mn in oocytes of oysters. Yolk granules were revealed as another storage pool of Mn in oyster oocytes with lower accumulation. In contrast, Mn was mainly distributed in the nucleus of sperm cells with accumulation levels much lower than those in cortical and yolk granules of oocytes. These results demonstrated great differences of the subcellular localization and accumulation capacity of Mn between oocytes and sperm cells in oysters, implying the sex differentiation in susceptibility of reproductive response to Mn stress. Our study also highlights the importance of gender difference in future biomonitoring and ecotoxicological studies of Mn in marine bivalves.

Novel Insights into the Role of Copper in Critical Life Stages of Oysters Revealed by High-Resolution NanoSIMS Imaging.

Copper (Cu) is a key trace element for many biological processes, but there is little available information regarding its requirements and functions in critical life stages of marine bivalves, during which dramatic morphological and physiological changes occur. In this study, the ontogeny pattern of Cu accumulation across the life history of oysters was explored for the first time and the distributions of Cu in oysters at critical life stages (pediveliger and early settled spat) were in situ mapped by nanoscale secondary ion mass spectrometry (NanoSIMS) with high lateral resolution. We first demonstrated that the late pelagic stage to early settled stage was the critical stage requiring Cu during oyster development. NanoSIMS imaging revealed a significant elevation of intercellular Cu levels along with mitochondrial calcium overload and obvious structural degradation of mitochondria in velar cells at the pediveliger stage, implying the possible role of Cu in cell apoptosis of the velum during larval metamorphosis. Furthermore, an obvious enrichment of Cu together with calcium was observed in the nucleus and mitochondria of ciliated cells of gill tissue at the early settled stage. Their accumulation in the gill cells was significantly higher than that in the juveniles, indicating the potential role of Cu in sustaining the fast growth of the gill filament and the concomitant acceleration of energy metabolism during early benthic development. Our findings offer new insights into the understanding of the interactions between trace metals and marine bivalves.

Seasonal fluctuations of metal bioaccumulation and reproductive health of local oyster populations in a large contaminated estuary.

Despite of much evidence of trace metal pollution in the Pearl River Estuary (PRE), the seasonal dynamics of metal bioavailability as well as the potential impacts of metal pollution on the local marine organisms in this estuary is poorly understood. In the present study, the accumulation of trace metals and reproductive states of three populations of oyster Crassostrea hongkongensis, a keystone bivalve species in the PRE, were for the first time investigated throughout a one-year field study. Significant temporal fluctuations of metal accumulation were observed in the somatic tissues of oysters, suggesting seasonal variations of metal bioavailability in the PRE. A major feature of the seasonal variations was the increased levels of metal bioaccumulation in the summer season for the contaminated sites nearby the major river inlets. High riverine inputs accompanied by relatively lower salinity in summer may greatly contribute to such variations. Furthermore, oyster populations from two contaminated sites had a poor reproductive condition in comparison with the reference oyster population, as reflected by a significant decrease of gonad-somatic index (GSI) and gonad cover area (GCA), as well as an obvious change of sex ratios. Gonadal metal accumulation of Cu, Zn, Ni, Co and Pb in the contaminated oysters was much higher than that in the relatively uncontaminated oysters. Especially, the concentrations of these metals in the gonad during the breeding season had significantly negative correlations with the gonad condition indexes (GSI and GCA). Our results suggested strong seasonal fluctuations of bioavailability of trace metals in this highly contaminated estuary as well as an adverse effect of metal pollution on the reproduction of local oyster populations.

Zn Isotope Fractionation in the Oyster Crassostrea hongkongensis and Implications for Contaminant Source Tracking.

Variations in stable isotope ratios have been used to trace sources of contaminants as well as their biogeochemical pathways in the environment. In this study, we investigated the influences of internal redistribution among tissues and ambient water conditions on Zn isotope fractionation in oysters. There was no significant difference in Zn isotope ratios during in vivo Zn transportation among various oyster tissues. Estuarine oysters were exposed to additional Zn either at different salinities or at different Zn concentrations, following which the Zn isotope ratios in the oysters were measured. Results showed no significant difference in δ66/64Zn values in the oysters exposed at different salinities. Tissue Zn accumulation increased with increasing Zn levels in water over the 30 day exposure. Within this period, there was a nearly 0.3‰ difference in averaged δ66/64Zn values in the exposed oysters compared to the initial δ66/64Zn values in the oysters prior to exposure. However, there was no evidence of significant difference in δ66/64Zn values in oysters exposed at different Zn levels, with postexposure signatures similar to the δ66/64Zn values of the Zn solution added. Our results suggested that the δ66/64Zn values measured in the oysters were approaching the δ66/64Zn values of the "source" faster with increasing Zn concentrations added in the water. This study highlighted the absence of Zn isotope fractionation during Zn internal distribution and in vivo transport in oysters. The calculation of the contributions of different Zn sources demonstrated that oysters can be a sentinel animal for Zn source tracking in marine environments.

Trace metals in oysters: molecular and cellular mechanisms and ecotoxicological impacts.

Oysters are important benthic bivalves in coastal and estuarine environments. They are widely farmed due to their rapid growth and taste; they are also widely applied in environmental monitoring of coastal pollution due to their accumulation of contaminants. Most importantly, oysters are among the few marine organisms that are considered to be hyper-accumulators of many toxic metals, such as cadmium, copper and zinc. As such, there is a tremendous call to study the interactions between metals and oysters, especially due to the increasing metal pollution in many coastal and estuarine waters. Over the past decades, many studies have focused on metal accumulation in oysters as well as the ecotoxicological effects of metals on oysters. In this review, we summarize the recent progress in our understanding of the molecular and cellular mechanisms of metal accumulation, sequestration and toxicity in oysters. Applications of modern technologies such as omics and nanoscale imaging have added significantly to our knowledge of metal biology in oysters. Variations between different metals also demonstrate the diversity of the interactions between oysters and metals. Despite this recent progress, however, there is a need for further study of the molecular mechanisms of metal uptake and toxicity as well as the joint effects of metal mixtures on oyster populations. Oysters have higher numbers of stress responsive genes than most animals, which may have been induced by gene duplication during the evolution of their intertidal environmental adaptations. The divergent expression of stress responsive genes may explain the different tolerances for metals among different species. These fundamental studies may eventually provide promising solutions for reducing toxic metal concentrations in oysters for safe consumption by humans. To conclude, the complexity of life history and metal chemistry of oysters coupled with emerging pollution and application of modern techniques represents an important and exciting research area in modern ecotoxicology.

Temporal variability in Cu speciation, phytotoxicity, and soil microbial activity of Cu-polluted soils as affected by elevated temperature.

Global warming has obtained increasing attentions due to its multiple impacts on agro-ecosystem. However, limited efforts had been devoted to reveal the temporal variability of metal speciation and phytotoxicity of heavy metal-polluted soils affected by elevated temperature under the global warming scenario. In this study, effects of elevated temperature (15 °C, 25 °C, and 35 °C) on the physicochemical properties, microbial metabolic activities, and phytotoxicity of three Cu-polluted soils were investigated by a laboratory incubation study. Soil physicochemical properties were observed to be significantly altered by elevated temperature with the degree of temperature effect varying in soil types and incubation time. The Biolog and enzymatic tests demonstrated that soil microbial activities were mainly controlled and decreased with increasing incubation temperature. Moreover, plant assays confirmed that the phytotoxicity and Cu uptake by wheat roots were highly dependent on soil types but less affected by incubation temperature. Overall, the findings in this study have highlighted the importance of soil types to better understand the temperature-dependent alternation of soil properties, Cu speciation and bioavailability, as well as phytotoxicity of Cu-polluted soils under global warming scenario. The present study also suggests the necessary of investigating effects of soil types on the transport and accumulation of toxic elements in soil-crop systems under global warming scenario.

In Situ Subcellular Imaging of Copper and Zinc in Contaminated Oysters Revealed by Nanoscale Secondary Ion Mass Spectrometry.

Determining the in situ localization of trace elements at high lateral resolution levels in the biological system is very challenging, but critical for our understanding of metal sequestration and detoxification. Here, the cellular and subcellular distributions of Cu and Zn in contaminated oysters of Crassostrea hongkongensis were for the first time mapped using nanoscale secondary ion mass spectrometry (nanoSIMS). Three types of metal-containing cells were revealed in the gill and mantle of oysters, including Cu-specific hemocytes, Cu and Zn-containing granular hemocytes, and Cu and Zn-containing calcium cells. Obvious intercellular distribution of Cu was found in the gill tissue, indicating the potential role of hemolymph in the transportation of Cu in oysters. The distribution of Cu showed a strong colocalization with sulfur and nitrogen in Cu-specific hemocyte and intercellular hemolymph. In the Cu and Zn-containing granular hemocytes and calcium cells, the co-occurrence of Cu and Zn with phosphorus and calcium was also found. Different relationships of distributions between Cu/Zn and macronutrient elements (nitrogen, sulfur and phosphorus) implied the differential metal complexation in oysters. Interestingly, quantitative analysis of the ratios of 32S-/12C14N- and 31P-/12C14N- of metal-deposited sites suggested the dynamic process of transfer of Cu and Zn from the metabolized protein pool to a more thermodynamically stable and detoxified form.

Dynamics of maternally transferred trace elements in oyster larvae and latent growth effects.

Understanding the maternal transfer of contaminants and their potential effects has great implications for a valid ecological assessment of environmental pollution. However, relevant studies on marine bivalves are very limited. Here, we examined the maternal transfer of trace metals in populations of oyster Crassostrea hongkongensis with contrasting metal exposure histories. Elevated accumulation of trace metals was observed in eggs and larvae from contaminated sites, suggesting maternal transfer of multi-metals in natural oyster populations. The dynamics of maternally transferred metals was for the first time documented in this study. We demonstrated that excessively transferred metals in contaminated larvae were rapidly eliminated during the early developmental stage, and the efflux rate of metals in larvae was greatly dependent on environmental contamination level. These results provided the first field evidence of modified metal biokinetics in offsprings due to exposure history of adults in marine bivalves. Moreover, egg production was negatively correlated with the contamination level of metals in eggs. There was a further lagged growth in the contaminated larvae, indicating the potential adverse and latent effects of maternally transferred metals on the viability of oyster offspring. Our findings highlighted the importance of transgenerational studies on long-term metal exposure in marine bivalves.

Reproductive responses and detoxification of estuarine oyster Crassostrea hongkongensis under metal stress: a seasonal study.

Understanding the impacts of metal stress on the reproduction of dominant species, such as oysters, in seriously contaminated estuarine environments has great ecological implications. In the present study, the reproductive conditions were examined monthly for 1 year in oysters Crassostrea hongkongensis from a heavily metal-contaminated site (Baijiao, mainly by Cu and Zn) in the Jiulong River estuary and a relatively clean nearby estuary (Jiuzhen). Oysters sampled in the contaminated site showed a delayed gametogenesis, a relatively shorter spawning period, and a lower gonad condition index in comparison to the oysters sampled in the reference site. In particular, we found that the proportion of females increased significantly in the contaminated oysters, which provided the first evidence that the feminization in wild oyster populations could be related to trace metal pollution. Additionally, the potential detoxification mechanism of trace metals in oysters was also investigated. Compartmentalization of trace metals in membrane-limited vesicles in hemocytes could be an important detoxification mechanism for the contaminated oysters. Our findings indicated that the long-term metal exposure may greatly influence the reproduction of the oysters and finally affect the recruitment and population of this species.

Endosulfan isomers and sulfate metabolite induced reproductive toxicity in Caenorhabditis elegans involves genotoxic response genes.

Endosulfan is enlisted as one of the persistent organic pollutants (POPs) and exists in the form of its α and ß isomers in the environment as well as in the form of endosulfan sulfate, a toxic metabolite. General endosulfan toxicity has been investigated in various organisms, but the effect of the isomers and sulfate metabolites on reproductive function is unclear. This study was aimed at studying the reproductive dysfunction induced by endosulfan isomers and its sulfate metabolite in Caenorhabditis elegans (C. elegans). We also determined a role for the DNA-damage-checkpoint gene hus-1. Compared to ß-endosulfan and its sulfate metabolite, α-endosulfan caused a dramatically higher level of germ cell apoptosis, which was regulated by DNA damage signal pathway. Both endosulfan isomers and the sulfate metabolite induced germ cell cycle arrest. Loss-of-function studies using hus-1, egl-1, and cep-1 mutants revealed that hus-1 specifically influenced the fecundity, hatchability, and sexual ratio after endosulfan exposure. Our data provide clear evidence that the DNA-checkpoint gene hus-1 has an essential role in endosulfan-induced reproductive dysfunction and that α-endosulfan exhibited the highest reproductive toxicity among the different forms of endosulfan.

Variations of trace metals in two estuarine environments with contrasting pollution histories.

In the present study, seasonal variations of metal concentrations were examined in two oyster species, Crassostrea hongkongensis and Crassostrea sikamea, and in seawater from a heavily contaminated estuary and a reference estuary. Strong temporal fluctuations of metal concentration were observed in oyster tissues as well as in seawater from both estuaries, especially for the contaminated estuary and for C. hongkongensis with higher accumulation ability. A closer inter-element correlation was observed for Cu, Zn, Cr and Ni in both the dissolved and particulate phases from the contaminated site, indicating that these metals originated from the same industrial sources. Seasonal variations of metals in oysters were not significantly related to their concentrations in seawater, but were related to the variations of condition index. Our results showed that the seasonal patterns of metal concentrations in oysters were largely controlled by the biological process, while the variation as well as the concentration were dependent on the contamination levels and bioaccumulation ability.

Improved tolerance of metals in contaminated oyster larvae.

Environmental stress experienced by parents may make a significant difference in the response of their offspring. However, relevant studies on marine bivalves are very limited especially for the field populations. In the present study, we examined the relative metal tolerance of offspring produced by four natural populations of oyster Crassostrea sikamea that were contaminated by metals to different degrees. We demonstrated that the resistance of oyster offspring to copper and zinc was correlated with the level of metal pollution experienced by the parent oysters. Specifically, the oyster embryo and larvae produced by adult oysters from contaminated sites had a much higher tolerance to metal stress than those from the reference sites. Furthermore, tissue concentration-dependent maternal transfer of Cu and Zn was found in this study, and the metallothionein concentrations in eggs were positively related to the total concentrations of maternally transferred Cu and Zn. Thus, the maternally transferred metals inducing high level of MT synthesis in eggs was one of the possible mechanisms responsible for the enhanced metal tolerance of oyster embryos and larvae from heavily contaminated sites. We concluded that environmental exposure history of adult oysters significantly influenced the ability of their offspring to cope with metal stress. Our findings offered the field evidence of the possible transfer of metal tolerance from adults to offspring in marine bivalves.