Analysis of diversity and function of epiphytic bacterial communities associated with macrophytes using a metagenomic approach.

Epiphytic bacteria constitute a vital component of aquatic ecosystems, pivotal in regulating elemental cycling. Despite their significance, the diversity and functions of epiphytic bacterial communities adhering to various submerged macrophytes remain largely unexplored. In this study, we employed a metagenomic approach to investigate the diversity and function of epiphytic bacterial communities associated with six submerged macrophytes: Ceratophyllum demersum, Hydrilla verticillata, Myriophyllum verticillatum, Potamogeton lucens, Stuckenia pectinata, and Najas marina. The results revealed that the predominant epiphytic bacterial species for each plant type included Pseudomonas spp., Microbacterium spp., and Stenotrophomonas rhizophila. Multiple comparisons and linear discriminant analysis effect size indicated a significant divergence in the community composition of epiphytic bacteria among the six submerged macrophytes, with 0.3-1% of species uniquely identified. Epiphytic bacterial richness associated with S. pectinata significantly differed from that of both C. demersum and H. verticillata, although no significant differences were observed in diversity and evenness. Functionally, notable variations were observed in the relative abundances of genes associated with carbon, nitrogen, and phosphorus cycling within epiphytic bacterial communities on the submerged macrophyte hosts. Among these communities, H. verticillata exhibited enrichment in genes related to the 3-hydroxypropionate bicycle and nitrogen assimilation, translocation, and denitrification. Conversely, M. verticillatum showcased enrichment in genes linked to the reductive citric acid cycle (Arnon-Buchanan cycle), reductive pentose phosphate cycle (Calvin cycle), polyphosphate degradation, and organic nitrogen metabolism. In summary, our findings offer valuable insights into the diversity and function of epiphytic bacteria on submerged macrophyte leaves, shedding light on their roles in lake ecosystems.

Temporal Succession of Bacterial Community Structure, Co-occurrence Patterns, and Community Assembly Process in Epiphytic Biofilms of Submerged Plants in a Plateau Lake.

In shallow macrophytic lakes, epiphytic biofilms are formed on the surface of submerged plant stems and leaves because of algae and bacterial accumulation. Epiphytic biofilms significantly impact the health of the host vegetation and the biogeochemical cycling of lake elements. However, community diversity, species interactions, and community assembly mechanisms in epiphytic bacterial communities (EBCs) of plants during different growth periods are not well understood. We investigated the successional dynamics, co-occurrence patterns, and community assembly processes of epiphytic biofilm bacterial communities of submerged plants, Najas marina and Potamogeton lucens, from July to November 2020. The results showed a significant seasonal variation in EBC diversity and richness. Community diversity and richness increased from July to November, and the temperature was the most important driving factor for predicting seasonal changes in EBC community structure. Co-occurrence network analysis revealed that the average degree and graph density of the network increased from July to November, indicating that the complexity of the EBC network increased. The bacterial community co-occurrence network was limited by temperature, pH, and transparency. The phylogeny-based null model analysis showed that deterministic processes dominated the microbial community assembly in different periods, increasing their contribution. In addition, we found that as the dominance of deterministic processes increased, the microbial co-occurrence links increased, and the potential interrelationships between species became stronger. Thus, the findings provide insights into the seasonal variability of EBC assemblage and co-occurrence patterns in lacustrine ecosystems.

Occurrence of total mercury and methylmercury in rice: Exposure and health implications in Nepal.

Emerging studies have indicated that the consumption of rice could be the major methylmercury (MeHg) contributor to human mercury (Hg) exposure. Nonetheless, few studies are available on Hg in rice around the world, especially in countries with heavy rice diet. In this study, total Hg (THg) and MeHg levels in rice samples (n = 172) across Nepal were first investigated. The geometric mean THg was 7.05 ± 7.71 µg/kg with a range of 0.622 µg/kg to 158 µg/kg, and the maximum THg level was up to 791% of the Chinese National Standard Limit for THg in rice (20 µg/kg). The geometric mean MeHg was 0.820 ± 0.660 µg/kg with a range of 0.189 µg/kg to 8.59 µg/kg. Overall, the mean MeHg exposure (0.00445 ± 0.00477 µg/kg bw/day) and inorganic Hg (IHg) exposure (0.0360 ± 0.0739 µg/kg bw/day) were lower than the reference dose (RfD) of 0.1 µg/kg bw/day for MeHg and the provisional tolerable weekly intake (PTWI) of 0.571 µg/kg bw/day for IHg, respectively. Concerning different groups of vulnerable populations, the highest MeHg exposure (0.126 µg/kg bw/day) and IHg exposure (1.57 µg/kg bw/day) of preschoolers (37-50 months old) were approximately 126% of the RfD for MeHg and 275% of the PTWI for IHg. When the pregnant mothers eat the rice without awareness of the Hg content in rice, the mean and highest intelligence quotients (IQs) losses were 9554 and 118659 points, respectively, and the corresponding economic costs due to IQ loss could be 15.1 million USD and 188 million USD in Nepal. The results of rice THg and MeHg levels and corresponding exposure in populations highlighted the occurrence of rice THg and MeHg pollution issues in Nepal. More efforts should be made to protect younger groups in Nepal from high rice Hg exposure. CAPSULE: Owing to the high rice consumption rates relative to body mass, preschoolers (37-50 months) may meet the 126% reference dose (0.1 µg/kg bw/day) for MeHg and 275% provisional tolerable weekly intake (0.571 µg/kg bw/day) for IHg exposure in Nepal.

Diversity and distribution of autotrophic microbial community along environmental gradients in grassland soils on the Tibetan Plateau.

Soil microbial autotrophs play a significant role in CO2 fixation in terrestrial ecosystem, particularly in vegetation-constrained ecosystems with environmental stresses, such as the Tibetan Plateau characterized by low temperature and high UV. However, soil microbial autotrophic communities and their driving factors remain less appreciated. We investigated the structure and shift of microbial autotrophic communities and their driving factors along an elevation gradient (4400-5100 m above sea level) in alpine grassland soils on the Tibetan Plateau. The autotrophic microbial communities were characterized by quantitative PCR, terminal restriction fragment length polymorphism (T-RFLP), and cloning/sequencing of cbbL genes, encoding the large subunit for the CO2 fixation protein ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO). High cbbL gene abundance and high RubisCO enzyme activity were observed and both significantly increased with increasing elevations. Path analysis identified that soil RubisCO enzyme causally originated from microbial autotrophs, and its activity was indirectly driven by soil water content, temperature, and NH4 (+) content. Soil autotrophic microbial community structure dramatically shifted along the elevation and was jointly driven by soil temperature, water content, nutrients, and plant types. The autotrophic microbial communities were dominated by bacterial autotrophs, which were affiliated with Rhizobiales, Burkholderiales, and Actinomycetales. These autotrophs have been well documented to degrade organic matters; thus, metabolic versatility could be a key strategy for microbial autotrophs to survive in the harsh environments. Our results demonstrated high abundance of microbial autotrophs and high CO2 fixation potential in alpine grassland soils and provided a novel model to identify dominant drivers of soil microbial communities and their ecological functions.

Loss of submerged macrophytes in shallow lakes alters bacterial and archaeal community structures, and reduces their co-occurrence networks connectivity and complexity.

Introduction: Bacteria and archaea are important components in shallow lake ecosystems and are crucial for biogeochemical cycling. While the submerged macrophyte loss is widespread in shallow lakes, the effect on the bacteria and archaea in the sediment and water is not yet widely understood. Methods: In this study, 16S rRNA gene sequencing was used to explore the bacteria and archaea in samples taken from the sediment and water in the submerged macrophyte abundant (MA) and submerged macrophyte loss (ML) areas of Caohai Lake, Guizhou, China. Results: The results showed that the dominant bacterial phyla were Proteobacteria and Chloroflexi in the sediment; the dominant phyla were Proteobacteria, Actinobacteriota, and Bacteroidota in the water. The dominant archaea in sediment and water were the same, in the order of Crenarchaeota, Thermoplasmatota, and Halobacterota. Non-metric multidimensional scaling (NMDS) analyses showed that bacterial and archaeal community structures in the water were significantly affected by the loss of submerged macrophytes, but not by significant changes in the sediment. This suggests that the loss of submerged macrophytes has a stronger effect on the bacterial and archaeal community structures in water than in sediment. Furthermore, plant biomass (PB) was the key factor significantly influencing the bacterial community structure in water, while total nitrogen (TN) was the main factor significantly influencing the archaeal community structure in water. The loss of submerged macrophytes did not significantly affect the alpha diversity of the bacterial and archaeal communities in either the sediment or water. Based on network analyses, we found that the loss of submerged macrophytes reduced the connectivity and complexity of bacterial patterns in sediment and water. For archaea, network associations were stronger for MA network than for ML network in sediment, but network complexity for archaea in water was not significantly different between the two areas. Discussion: This study assesses the impacts of submerged macrophyte loss on bacteria and archaea in lakes from microbial perspective, which can help to provide further theoretical basis for microbiological research and submerged macrophytes restoration in shallow lakes.

Photocatalytic degradation of tetracycline hydrochloride by a Fe3O4/g-C3N4/rGO magnetic nanocomposite mechanism: modeling and optimization.

The photocatalytic degradation of antibiotics requires a good separation efficiency of photogenerated electron-hole pairs and a wide visible light absorption range. Current studies have discussed the successful preparation of ferroferric oxide/graphite carbon nitride/reduced graphene oxide (Fe3O4/g-C3N4/rGO). The phase structure and morphology of the Fe3O4/g-C3N4/rGO composites were characterized by XRD, HR-TEM, SEM, and EDS. The obtained composites were used to degrade tetracycline hydrochloride (TCH) to evaluate its photocatalytic activity. The effects of four variables on the degradation of TCH were analyzed by the response surface method and artificial intelligence (gradient regression tree, random forest, artificial neural network, etc.). The results showed that the graphite carbon nitride in the catalyst maintained its original structure and that the photocatalytic activity was significantly improved. The degradation rate of TCH was 86.7% under the optimal conditions (the Fe3O4/g-C3N4/rGO dosage was 0.1 g, pH = 7.0, the initial concentration of TCH was 20 mg/L, and the visible light irradiation time was 60 min). At the same time, the degradation rate of TCH changed little after the material was used five times, which indicates that the stability and recyclability of the Fe3O4/g-C3N4/rGO photocatalyst were excellent. Finally, a possible photocatalytic mechanism of the Fe3O4/g-C3N4/rGO photocatalyst is proposed in this paper.

Assessment of heavy metal pollution and exposure risk for migratory birds- A case study of Caohai wetland in Guizhou Plateau (China).

Increasing heavy metal pollution in wetland ecosystems around the world pose significant health risks to waterbirds, especially the endangered species. We analyzed heavy metal pollution in bird foraging area of Caohai wetland in Guizhou Plateau (China), and used an integrated exposure risk model for assessing heavy metal (Cd, Pb, Cr, Ni, Zn, Sb) exposure risk in birds from the soil, water, plants and benthic invertebrates. There is considerable variation in the extent of heavy metal contamination across the different sampling sites, and Cd and Sb are the main contaminants. The mussel Anodonta showed greater heavy metal accumulation (except for Zn) compared to the snail species C. cathayensis. The different plant species also varied in terms of amount for accumulated heavy metals. The phytophagous together with omnivorous birds were exposed to Cd, Pb, Zn and Ni through plants rather than the soil, whereas the maximum Zn exposure in the omnivorous and carnivorous birds was through consumption of benthic invertebrates. Furthermore, the phytophagous black-necked cranes (Grus nigricollis) were less risk to heavy metal exposure compared to the omnivorous bar-headed goose (Anser indicus) and carnivorous ruddy shelducks (Tadorna ferruginea). The exposure risk of Cr (8.1) was highest, followed by Pb (5.1), Zn (3.8), Sb (1.0), Cd (0.33) and Ni (0.28). The heavy metal assessment heavy metal exposure risk for migratory birds should take into account the exposure from food and soil. Our findings provide new insights into developing measures to minimize heavy metal contamination in migratory birds.

Prokaryotic Community Succession in Bulk and Rhizosphere Soils Along a High-Elevation Glacier Retreat Chronosequence on the Tibetan Plateau.

Early colonization and succession of soil microbial communities are essential for soil development and nutrient accumulation. Herein we focused on the changes in pioneer prokaryotic communities in rhizosphere and bulk soils along the high-elevation glacier retreat chronosequence, the northern Himalayas, Tibetan Plateau. Rhizosphere soils showed substantially higher levels of total organic carbon, total nitrogen, ammonium, and nitrate than bulk soils. The dominant prokaryotes were Proteobacteria, Actinobacteria, Acidobacteria, Chloroflexi, Crenarchaeota, Bacteroidetes, and Planctomycetes, which totally accounted for more than 75% in relative abundance. The dominant genus Candidatus Nitrososphaera occurred at each stage of the microbial succession. The richness and evenness of soil prokaryotes displayed mild succession along chronosequene. Linear discriminant analysis effect size (LEfSe) analysis demonstrated that Proteobacteria (especially Alphaproteobacteria) and Actinobacteria were significantly enriched in rhizosphere soils compared with bulk soils. Actinobacteria, SHA_109, and Thermoleophilia; Betaproteobacteria and OP1.MSBL6; and Planctomycetia and Verrucomicrobia were separately enriched at each of the three sample sites. The compositions of prokaryotic communities were substantially changed with bulk and rhizosphere soils and sampling sites, indicating that the communities were dominantly driven by plants and habitat-specific effects in the deglaciated soils. Additionally, the distance to the glacier terminus also played a significant role in driving the change of prokaryotic communities in both bulk and rhizosphere soils. Soil C/N ratio exhibited a greater effect on prokaryotic communities in bulk soils than rhizosphere soils. These results indicate that plants, habitat, and glacier retreat chronosequence collectively control prokaryotic community composition and succession.

Metal(loid) accumulation levels in submerged macrophytes and epiphytic biofilms and correlations with metal(loid) levels in the surrounding water and sediments.

The pollution of wetlands with metal(loid) s is a major ecological and environmental problem all over the world. However, the accumulation characteristics of metal(loid)s in submerged macrophytes and epiphytic biofilms in wetland systems where sediments are polluted by metal(loid)s are still unclear. In July (the wet season) and November (the dry season) 2018, surface water, sediments, submerged macrophytes (Potamogeton lucens L. and Myriophyllum verticillatum L.) and their epiphytic biofilms were collected to analyze the levels of Pb, Cd, Cu, Cr, Hg and As in Caohai wetland (China). Metal(loid) concentrations in sediments were ranked as follows: Cr > Pb > Cu > As > Cd > Hg. Although Pb, Cd and Hg levels exceeded the sediment background threshold levels of Guizhou Province, the water was not polluted by metal(loid)s. Except for Hg and Cr, most of the metal(loid) concentrations in epiphytic biofilms were higher than those in submerged macrophytes. No significant correlations were found between any of the metal(loid) concentrations in submerged macrophytes or biofilms and the metal(loid) concentrations in the surrounding water and sediments. Although the accumulation of As and Hg in submerged macrophytes had a very significant negative correlation with a few elements, the correlation between other elements was not significant. No co-accumulation phenomenon was found in submerged macrophytes; however, co-accumulation and competition among different metal(loid)s did occur in the epiphytic biofilms, which may be related to the different accumulation mechanisms of metal(loid)s in submerged macrophytes and epiphytic biofilms. This study enriches our understanding of the accumulation of metal(loid)s in submerged macrophytes and epiphytic biofilms in wetlands.

Health risk assessment of heavy metals in soils and screening of accumulating plants around the Wanshan mercury mine in Northeast Guizhou Province, China.

The Wanshan mercury mine, which is an abandoned mine located in northeastern Guizhou Province in Southwest China, has introduced serious Hg pollution to the local ecosystem resulting from previous mining and smelting activities. However, it is not clear to date whether soil pollution has actually improved after treatment by related departments. Therefore, the present study investigates the vegetation community and heavy metal contents of the soil and plants in the Wanshan mercury mining area. The results showed that most of Hg, Cd, As, Cu, and Zn contents in soil samples were higher than those of Soil Environment Quality Risk Control Standard for Soil Contamination of Agricultural Land in China (GB15618-2018). The observed plant species mainly consisted of Compositae, followed by Leguminosae. Unfortunately, this investigation found that heavy metal concentrations in these plants were not extremely high and far below the standard of hyperaccumulator. Despite all this, the maximum values of bioaccumulation factor for Pb, Cd, Hg, As, Cu and Zn were Serissa japonica (Thunb.) Thunb., Rhus chinensis Mill., Potentilla sibbaldii Haller f., Erigeron canadensis L., Clerodendrum bungei var. bungei. and Rhus chinensis Mill., respectively. Regardless of the carcinogenic or noncarcinogenic risk index, the potential risk to urban children is higher. Our results suggest that heavy metal pollution was indeed relieved since their contents in soil significantly decreased in comparison with those reported in other previous studies. This finding provides a reference for the long-term treatment of heavy metal pollution in the local environment and other areas employing analogous environmental protection measures.

Community composition and correlations between bacteria and algae within epiphytic biofilms on submerged macrophytes in a plateau lake, southwest China.

Epiphytic biofilms are complex matrix-enclosed communities comprising large numbers of bacteria and algae, which play an important role in the biogeochemical cycles in aquatic systems. However, little is known about the correlations that occur between these communities or the relative impact of environmental factors on their composition. In this study, epiphytic biofilms on three different aquatic plants were sampled in a typical plateau lake (Caohai, southwest China) in July and November of 2018. Bacterial diversity was assessed using Miseq sequencing approaches and algal communities were assessed using light microscopy. Gammaproteobacteria (54.64%), Bacteroidetes (17.50%) and Firmicutes (13.99%) were the dominant bacterial taxa and Chlorophyta (47.62%), Bacillariophyta (28.57%) and Euglenophyta (19.05%) were the dominant algae. The alpha diversity values of the epiphytic bacterial and algal communities were greater during the macrophyte decline period (November) than during the growth period (July). Microbial community composition was significantly affected by abiotic factors (water temperature, NH4+, pH or TP) and biotic factors (algae or bacteria). Interestingly, in July and November, the epiphytic algal community dissimilarity was stronger than that observed for bacterial community dissimilarity, suggesting that bacterial community dissimilarity may increase more slowly with environmental change than algal community dissimilarity. Furthermore, association network analysis revealed complex correlations between algal biomass and bacteria phylotype, and that 67.83% of correlations were positive and 32.17% were negative. This may indicate that facilitative correlations between algae and bacteria are predominant in epiphytic biofilms. These results provide new information on algal-bacterial correlations as well as the possible mechanisms that drive variations in the microbial community in epiphytic biofilms in freshwater lakes.

Contamination features and ecological risks of heavy metals in the farmland along shoreline of Caohai plateau wetland, China.

Spatial distribution characteristics of heavy metal (Cd, Pb, Cr, Cu, Zn, and Hg) contents and their ecological risks in the farmland along the shoreline of the Caohai wetland were investigated. Incubation experiments were also conducted to characterize the emission of heavy metals across soil-water interface if the farmland was reclaimed to wetland. The results showed that spatial distribution characteristics of these heavy metal contents were significantly different. Concentrations of Cd, Zn, and Hg were higher than the corresponding geochemical background levels. Ecological risk assessment suggested that the farmland along the shoreline of Caohai wetland were characterized by non-pollution or slight pollution of Pb, Cr, and Cu, moderate pollution of Cd, slight to moderate pollution of Hg, and slight pollution of Zn. Emission rates of Cd, Zn, and Hg across soil/sediment-water interface first increased, then decreased and finally reached equilibrium after the farmland soil was submersed. The contribution-rates of Cd, Zn, and Hg transferring from sediment to overlying water were calculated to be 12.7%, 14.8%, and 10.4%, respectively. We conclude that environmental issues caused by heavy metals, especially by Cd, Zn, and Hg, in the farmland along the shoreline of the Caohai wetland should be paid great attention.

Evaluation of potential ecological risk, possible sources and controlling factors of heavy metals in surface sediment of Caohai Wetland, China.

Caohai, a plateau wetland in Southwest China, is a national nature reserve providing protection for a variety of threatened and endangered species of migrant birds (e.g., the black-necked crane Grus nigricollis). It has been experiencing the increasing environmental problems with heavy metals due to anthropogenic activities. However, the contamination of heavy metals in different habitats is unclear. Surface sediment samples from these habitats were thus collected to analyze the distribution characteristics, potential risk and possible sources for heavy metals (Cd, Hg, Pb, Zn, Ni, Cr, Cu, Be, and V). The results showed that all of Cd, Hg, Pb, Zn, Cr, and Ni concentrations exceeded the background values, and these elements (except Cr and Ni) presented comparatively high levels in habitat adjacent to urban in comparison with the other habitats. Based on the regression analysis, we found that metals with higher EFs (Cd, Hg, Pb, and Zn) were mainly controlled by anthropogenic loadings, while metals with lower EFs (Cr, Ni, Be, and V) were mainly associated with sediment properties (pH or NOM). In addition, the results from geo-accumulation index, Hakanson potential ecological risk assessment, and risk assessment code showed that Cd and Hg posed a medium to high environmental risk to the ecosystem, and the other heavy metals posed no or low risk. Therefore, to protect this wetland ecosystem and to supply a well habitat for migratory birds, greater efforts aimed at reducing anthropogenic discharges and remediating sediment contaminated with heavy metals should be pursued.

[Eutrophication control in local area by physic-ecological engineering].

An integrated physical and ecological engineering experiment for ecological remediation was performed at the Maixi River bay in Baihua Reservoir Guizhou Province, China. The results show that eutrophic parameters, such as total nitrogen, total phosphorus, chlorophyll a and chemical oxygen demand from the experimental site (enclosed water) were significantly lower than those of the reference site. The largest differences between the sites were 0.61 mg x L(-1), 0.041 mg x L(-1), 23.06 microg x L(-1), 8.4 mg x L(-1) respectively; experimental site transparency was > 1.50 m which was significantly higher than that of the reference site. The eutrophic index of the experimental site was oligo-trophic and mid-trophic, while the control site was mid-trophic state and eutrophic state. Phytoplankton abundance was 2 125.5 x 10(4) cells x L(-1) in June, 2011 at the control site,but phytoplankton abundance was lower at the experimental site with 33 x 10(4) cells x L(-1). Cyanobacteria dominated phytoplankton biomass at both sites, however the experimental site consisted of a higher proportion of diatoms and dinoflagellates. After more than one year of operation, the ecological engineering technology effectively controlled the occurrence of algae blooms, changed phytoplankton community structure, and controlled the negative impacts of eutrophication. Integrating physical and ecological engineering technology could improve water quality for reservoirs on the Guizhou plateau.