Therefore, the systems showed greater bacterial diversity. Microorganisms gradually adapt to the environment and accumulate, resulting in an increase in bacterial community richness with the extension of culture time. At the same time, the dominant flora gradually occupy the dominant position, resulting in a decrease in the diversity of bacterial communities. The higher the concentration of LOFL was, the lower the bacterial community richness was, indicating that an increase in LOFL will inhibit the reproduction and growth of microorganisms. LOFL stimulates variation in microorganisms and the emergence of drug-resistant strains, resulting in a higher diversity of bacterial communities. Highly enriched microorganisms contribute to a high degree of pollutant removal. The bacterial community composition of the three reactors is shown in Fig. 2. Obvious differences among reactors with different LOFL concentrations, temperatures and operation times can be observed. At the phylum level, Proteobacteria was identified as the dominant phylum. Proteobacteria play a key role in biological nitrogen and phosphorus removal and the removal of other pollutants . Firmicutes and Actinobacteria are typical electrochemically active bacteria phyla . However, the content of Actinobacteria increased from 0.5% to 2.7% to 2.8− 53.9% at normal temperature in this study. Normal temperature is beneficial to the growth of Actinobacteria. Firmicutes has a stronger ability to adapt to low-temperature environments. At low temperature, the percentage of Firmicutes in the seven reactors was 1.2–10.0%, while at normal temperature, the phylum appeared only at 10 and 100 µg/L concentrations.
The relative abundance of Firmicutes and Actinobacteria in MFC-CWs was higher than that in CWs.Wetland-electrochemical systems have a stronger pollutant removal ability than CW systems. Therefore, it is speculated that Firmicutes and Actinobacteria may be the key phyla. Different types of genera are greatly affected by antibiotic concentrations. The specific proportions of different genera are shown in Table S1. Rhodocyclaceae has been reported to be capable of using oxygen or nitrate as an electron acceptor in the denitrification process,ebb flow tray and short-chain fatty acids are usually used as electron donors . Rhodocyclaceae was enriched at lower silver nanoparticle concentrations , which was similar to this research. Rhodocyclaceae was enriched at 0.1 µg/L and 0.2 µg/L . Dechloromonas, Pseudomonas and Flavobacterium contain species that have denitrification capacity . They have been found frequently in constructed wetlands . Calheiros et al. used constructed wetlands with Iris pseudacorus plants to purify winery wastewater, and the content of Pseudomonas in the plant rhizosphere reached 27.3%. Santos et al. found that the abundance of the genus Novosphingobium increased in all exposed substrates, and several strains of this genus have already been described as capable of resisting and even degrading several compounds, such as tetracycline and bisphenol A . In this research, Novosphingobium accounted for only a small proportion and was not the dominant genus. Phosphorus uptake by plants can be promoted by Rhizobacter. Secretions such as extracellular phosphatase and organic acids can convert insoluble phosphate into dissolved phosphorus, which can be absorbed by plants . From this point of view, such conversion can not only alleviate the blockage of phosphorus adsorption on the substrate surface but also generate economic value in resource recovery by CW plants and realize fully effective plant harvest and phosphorus removal. Dechloromonas has been identified as an abundant genus in many constructed wetland studies.When the silver nanoparticle concentration increased from 0 to 50 μg/L, the relative abundance of the genus Dechloromonas increased.
However, when the concentration of AgNPs reached 200 μg/L, the relative abundance of the genus Dechloromonas decreased. Therefore, the ability to reduce nitrate or nitrite to nitrogen gas under autotrophic conditions decreased. Dechloromonas have the ability to biodegrade nitrogen without organic matter and exhibit phosphate-accumulating and chlorate-reducing capabilities in reactors. Candidatus_Accumulibacter has been confirmed as the dominant polyphosphate-accumulating organism in WWTPs and laboratory-scale reactors . Candidatus Accumulibacter can store volatile fatty acids as polyhydroxyalkanoates under anaerobic conditions and use PHAs as polyphosphate stores for the growth and regeneration of glycogen in the subsequent aerobic phase . Members of the familyChromatiaceae do not have internal cell photosynthetic membranes, but maintain globules of elemental sulfur inside the cells . Chromatiaceae can be found in benthic environments beneath the surface of photosynthetic bacterial mats or in planktonic environments below the anoxic–oxic interface of stratified lacustrine to hypersaline waters . There was a layer of green plankton on the surface of the plants and device walls in the system. The discovery of Chromatiaceae may be related to the presence of phytoplankton in the system. Burkholderiaceae is an ammonia-oxidizing bacteria that has the ability to reduce ammonium and nitrite by nitrification and ammonia oxidation, and it can be found in a variety of aquatic and terrestrial environments . Burkholderiaceae plays an important role in the growth of wetland plants and is beneficial for carbon turnover and nitrogen fixation . The abundance of Burkholderiaceae was 38.8% in the research of Liyan Di, which was higher than that in this research . Acidovorax has been reported to be enriched at high concentrations of Cd stress , and it promoted Ni removal from soil . In this study, it was found that with increasing temperature, Acidovarax was inhibited. The genus Pseudomonas is a typical heterotrophic nitrifying bacterium that exhibits an efficient heterotrophic nitrification-aerobic denitrification capacity.
Inorganic nitrogen or organic-N can both be growth substrates for Pseudomonas. The path of Pseudomonas denitrification is that it ammonifies organic-N to NH4+-N and then converts NH4+-N to NO2− -N or even directly converts organic-N to NO3—N . Nitrate reduction is driven by narG, nirK+ nirS and nosZ. Nitrification was the rate-limiting step for denitrification due to substrate limitations . Ammoxidation is an important step in nitrification for NH4+-N removal . The heat map indicates that there were obvious differences in the bacterial communities among the different conditions. The meaning of each letter is explained in Fig. 5. The relationship between the functions of microorganisms in the system and LOFL concentration, temperature, and contact time is shown in Fig. 5 . In terms of system function, culture time and the concentration of LOFL were the main influencing factors, while temperature was the secondary factor. Changes in conditions will lead to changes in the cellular functions of the bacterial community structure. For example, with increasing LOFL concentration, the translation ability, ribosomal structure, biogenesis, intracellular trafficking, secretion, vesicular transport, energy production and conversion, amino acid transport and metabolic ability of microorganisms were enhanced. However, transcription, replication, recombination and repair, cell cycle control, cell division, chromosome partitioning, coenzyme transport and metabolism and other abilities were inhibited. With the large-scale production and widespread applications of plastic products, increasing amounts of plastic are entering the natural environment . Plastic debris becomes brittle and fragmented under the exposure of ultraviolet light and oxygen, as well as physical action from wind and waves . These plastics gradually degrade into micrometer- and nanometer-scale particles. Many studies confirm the worldwide occurrence of microplastics in marine environments , sediments , freshwater , groundwater , and soil . In freshwater, the main microplastics found are polypropylene , polyethylene , polystyrene , and polyethylene terephthalate.
Microplastics affect plants in the environment . Qi et al. found that both low-density PE and biodegradable plastic mulch films of macro- and micro-size were harmful to wheat growth. One of the toxicity mechanisms of microplastics to plants is the blockage of the root surface pores caused by adhesion to plant roots, which affects adsorption of nutrients . For nanoplastics, the uptake and accumulation of nanoplastics may affect the transport of nutrients. For instance, 100-nm PS nanoplastics block cell connections or cell wall pores, causing oxidative damage and genotoxicity in fava beans . The adsorption and aggregation between microplastics and microalgae are also considered as the reasonable explanations for toxic effects of microplastic on marine microalgae . Meanwhile, PE and biodegradable plastic mulches improve raspberry plant growth and yield . These findings suggest that the toxicity of microplastics may be related to their type, particle size and concentration. Many experiments have demonstrated the adsorption of metals on microplastics. For example, Cu2+ and Zn2+ can adsorb on the surface of PET microplastics . The adsorption isotherm of Cd2+ onto PE microplastics follow Langmuir model , while Freundlich model is more suited for that of Cd2+ onto PET microplastics . It has been found that the adsorption of cadmium onto microplastics is closely related to the functional groups on the surface of microplastics, the solution pH and the ionic strength . Polycyclic aromatic hydrocarbons , generally produced by the partial burning of organic substances, resistant to degradation, ubiquitously present in the environment, pose a concern due to the toxicity and are potentially carcinogenic to humans . Soils contaminated by PAHs are mainly responsible for the continuous formation of environmentally persistent and relatively stable free radicals . It has been reported that these chemicals could enter the plant roots from the polluted soils and be transferred to the aerial parts subsequently . Two-photon excitation microscopy was employed for tracking the acquisition of phenanthrene from the polluted growth medium into the roots of maize and wheat for two months . The study revealed the binding of Phe to the epidermis in the elongation zone, flood and drain tray then traversed the epidermal cells radially to reach the cortex of the root hair and branching zones of the root, and subsequently mobilized slowly and laterally to the shoot.In China, people’s exposure to PAHs and the related cancer risk due to the consumption of rice is much higher compared to that in other countries.
Cultivation of rice augmented thedissipation and transport of PAHs . In rice, PAHs were more readily absorbed by the root than adsorbed on the surface, and their level was relatively higher in the lateral root than in the nodal root . The analysis for a sampling of various rice tissues revealed a higher accumulation in the roots compared with the aerial tissues of rice cultivated in the PAHs-contaminated field . Among the macronutrients, phosphorus is a vital component of various organic molecules , plays a key role in signal transduction and metabolic pathways, and is thus vital for the growth and development of plants . Membrane-localized phosphate transporters belonging to the PHOSPHATE TRANSPORTER1 family facilitate the acquisition of Pi, which is the available form of P in the rhizosphere . Intricate crosstalk was demonstrated between the essential macronutrients and the uptake and translocation of Phe in wheat . The crosstalk between P and PAHs was also evident in rice as indicated by an adverse effect of PAHs on the phospholipid metabolism, which triggered damage to the cell membrane and consequent growth inhibition . The activities of the PAH-biodegrading bacteria are also influenced by the carbon /P/N ratio in the contaminated soils . These studies highlighted the prevalence of crosstalk between P and PAHs in taxonomically diverse bacteria and crop species. Therefore, it was intriguing to assume a likely role of the molecular mechanism involved in the acquisition and mobilization of Pi from the roots to shoots, and its crosstalk effect on the tissue-specific accumulation of Phe in rice grown in the Phe-contaminated rhizosphere. In model plants , phosphate overaccumulator2 is a ubiquitin-conjugating E2 enzyme that has been implicated in regulating the Pi transporters during the mobilization of Pi from the roots to shoots . The concentration of Pi in the leaves of OsPHO2 knockout mutants was significantly higher compared with the wild-type , which provided evidence of OsPHO2-mediated mobilization of Pi from the roots to shoots . Therefore, the likely role of OsPHO2 in the translocation and accumulation of Phe under different Pi regimes was assumed. For empirical evidence, the effects of mutation in OsPHO2 on the temporal translocation and accumulation of Phe were investigated during growth under different conditions . By comparing the Phe and P distribution in the WT and the mutants, the results provided evidence that Phe translocated and redistributed through the OsPHO2-regulated P translocation pathway in rice.