Do distribution and expansion of exotic invasive Asteraceae plants relate to leaf construction cost in a man-made wetland?

Exotic species especially Asteraceae plants severely invade wetlands in Shenzhen Bay, an important part of the coast wetland in Guangdong-Hong Kong-Macau Bay Area, China. However, the reasons causing their expansion are unclear. The leaf traits and expansion indices of six invasive Asteraceae plants from the Overseas Chinese Town (OCT) wetland were studied and the results showed that nearly 45% of the total plant species (31 out of 69 species) in the OCT wetland, belonging to 15 families and 27 genera, were exotic invasive species. The expansion indices of six Asteraceae species negatively correlated with their leaf construction cost based on mass (CCM), caloric values and carbon concentration, but their relations with ash content were positive. Multiple linear regression analysis revealed that CCM was the most important factor affecting the expansion of an exotic species, indicating CCM may be an important reason causing the expansion of exotic species in coastal wetlands.

Barnacles as potential bioindicator of microplastic pollution in Hong Kong.

Microplastic (MP) pollution is an emerging problem in the marine environment and the assessment of the presence and abundance of microplastics in wild organisms is essential for risk assessment. The occurrence of microplastics in four species of barnacles at 30 sites in Hong Kong waters was investigated. The median number of microplastics ranged between 0 and 8.63 particles g-1 wet weight, or 0 and 1.9 particles individual-1, with fibers being the most abundant type of microplastics. The chemical composition of 152 pieces out of 606 potential microplastics was analyzed using micro-Fourier Transform Infrared Spectroscopy (µ-FTIR). Fifty-two of them were synthetic polymers, 95 natural cotton fibers and five unknowns. Eight types of polymer were identified with cellophane being the most abundant (58%). Correlation analysis was conducted between the abundance of MPs in sediments obtained in our previous study and that in individual barnacle species in this study, and a positive correlation was established for the barnacle Amphibalanus amphitrite, highlighting the potential of using this species as a bioindicator of microplastics.

Is Laguncularia racemosa more invasive than Sonneratia apetala in northern Fujian, China in terms of leaf energetic cost?

Laguncularia racemosa and Sonneratia apetala are fast-growing exotic mangrove species in Southern China and widely used for afforestation. However, the invasiveness of the two exotic species is still unclear. We compared structural and physiological traits and energy-use related traits between L. racemosa and S. apetala, and with two natives (Kandelia obovata and Aegiceras corniculatum) in northern Fujian. Results showed that leaf construction cost based on mass (CCM) and caloric values of L. racemosa were significantly lower than S. apetala, and the two natives had highest CCM. Because lower CCM, L. racemosa grew faster with a taller height (4.83 m) and wider ground diameter circumference (40.03 cm) than S. apetala (4.43 m tall and 35.63 cm wide) and the two natives (2.42 m tall and 26.78 cm wide). These findings indicated that L. racemosa could be more invasive than S. apetala in mangrove forests in northern Fujian, China where it still grew well, which deserves more attention.

Effects of salinity on anatomical features and physiology of a semi-mangrove plant Myoporum bontioides.

The effect of different concentrations of NaCl, 0, 100, 200, 300 and 400 mM, on the anatomical features and physiology of Myoporum bontioides was investigated. The photosynthetic rates (Pn) were significantly reduced by salt stress, with the lowest values at 400 mM NaCl. The content of malondialdehyde (MDA), proline and soluble sugar, as well as the activities of peroxidase (POD) and catalase (CAT) increased at the beginning, but became similar to the control as the experiment proceeded. The NaCl effect on superoxide dismutase (SOD) was different from the other parameters, with a significant reduction at 400 mM NaCl at Day 7. Salt glands were found in both upper and lower epidermis, and the ratios of the thickness of palisade to spongy mesophyll tissues increased with NaCl concentrations. The medullary ray was clearly damaged by NaCl at levels of 200 and 300 mM. These results demonstrated that M. bontioides could adapt to a relatively low salinity, and was not a halophilous species.

Structure and function of soil microbial community in artificially planted Sonneratia apetala and S. caseolaris forests at different stand ages in Shenzhen Bay, China.

The present study examined the relationships between soil characteristics, microbial community structure and function in the forests artificially planted with exotic Sonneratia apetala at stand ages of 1-, 2-, 7-, 10- and 14-years and Sonneratia caseolaris of 1-, 4-, 7-, 10- and 14-years in Futian National Nature Reserve, Shenzhen Bay, China. The 7-years old forests of both Sonneratia species reached peak growth and had the highest content of nitrogen and phosphorus, enzymatic activities, including dehydrogenase, cellulase, phosphatase, urease and ß-glucosidase, except arylsulphatase which increased continuously with stand ages. The microbial community structure reflected by phospholipid fatty acid (PLFA) profiles also reached the maximum value in the 7-years old forests and soil bacterial PLFAs in both forests were significantly higher than fungal PLFAs. The canonical correlation analysis revealed that differences in microbial structural variables were significantly correlated to the differences in their functional variables, and the highest correlation was found between the soil enzymatic activities and the content of carbon and nitrogen.

Removal and biodegradation of nonylphenol by immobilized Chlorella vulgaris.

The removal and biodegradation of nonylphenol (NP) by alginate-immobilized cells of Chlorella vulgaris were compared with their respective free cultures. The effects of four cell densities of 10(4) per algal bead were investigated, as were the four algal bead concentrations, with regard to the removal and biodegradation of NP. Although immobilization significantly decreased the growth rate and NP's biodegradation efficiency of C. vulgaris, NP removal over a short period was enhanced. The NP removal mechanism by immobilized cells was similar to that by free cells, including adsorption onto alginate matrix and algal cells, absorption within cells and cellular biodegradation. The optimal cell density and bead concentration for the removal and biodegradation of NP was 50-100×10(4) cells algal bead(-1) and 2-4 beads ml(-1) of wastewater, respectively. These results demonstrated that immobilized C. vulgaris cells under optimal biomass and photoautotrophic conditions are effective in removing NP from contaminated water.

Formation of iron plaque on mangrove roots receiving wastewater and its role in immobilization of wastewater-borne pollutants.

Iron (Fe) plaque formed on mangrove root increased with wastewater discharge, but the extent was species-specific. For Bruguiera gymnorrhiza, Fe plaque concentration was 0.80 mg g(-1) root d.wt at Day 0 and increased to 4.59, 6.84 and 7.52 mg g(-1) at Day 75 in the fresh water control (FW), synthetic wastewater with pollutant concentrations five times of municipal sewage (5SW) and double of 5SW (10SW) treatments, respectively; the respective increases in Excoecaria agallocha were from 0.70 to 2.37, 10.73 and 13.21 mg g(-1). For Acanthus ilicifolius, similar increase was found in 5SW, but all of the plants were dead in 10SW at Day 75. The concentrations of heavy metals and phosphorus immobilized were positively correlated with the amounts of Fe plaque formed, but the regression coefficients varied among species. The performance of mangrove plants in wastewater treatments was related to the Fe plaque formed and its immobilized wastewater-borne pollutants.

Microbial community dynamics and biodegradation of polycyclic aromatic hydrocarbons in polluted marine sediments in Hong Kong.

Dynamics of microbial community and biodegradation of polycyclic aromatic hydrocarbons (PAHs) in polluted marine sediments, artificially spiked with a mixture of PAHs (fluorene, phenanthrene, fluoranthene and pyrene), were examined for a period of 60 days. Microbial communities were characterised by bacterial counts, ester-linked fatty acid methyl ester (EL-FAME) analysis and denaturing gradient gel electrophoresis (DGGE). A noted reduction in species diversity occurred only in the high PAH level treatment at onset. Both EL-FAME and DGGE demonstrated a marked shift in microbial community, in all the PAH level treatments, afterwards, with increases in the number of fatty acid degraders, the relative abundance of fatty acid biomarkers for gram-negative bacteria and a decrease in species diversity. The shift was also accompanied by the significant decrease in PAH concentrations. By the end of the experiment, diversity indices, based on both approaches, recovered when PAH concentrations declined to their background levels, except in the high PAH level treatment.

Responses of bioaugmented ryegrass to PAH soil contamination.

The physiological and biochemical responses of ryegrass (Lolium multiflorum) to PAH induced stress in soils contaminated with phenanthrene and pyene were investigated, in the presence of PAH-degrading bacteria (Acinetobacteria junii) or arbuscular mycorrhizal fungi (AM fungi, Glomus mossae). The parameters monitored included chlorophyll content, chlorophyll a/b ratio, soluble-carbohydrate content, soluble-protein, malondialdehyde and electrolyte leakage, and superoxide dismutase (SOD) and peroxidase (POD) activities. Ryegrass showed good resistance and acclimation to PAH stress in soil, however, PAH contamination resulted in adverse effects such as damage of photosynthetic function and acceleration of shoot senescence. At PAH level of 100 mg kg(-1), chlorophyll contents were 14% lower than control (no PAH). Activities of SOD and POD were more sensitive indicators of PAH stress as compared to other parameters. However, all parameters showed trends based on either the bioaugmentation of the plants or PAH treatment level. It was concluded that the inoculation of AMF and PAH-degrading bacteria, especially the former, have a positive effect on alleviation of PAH toxicity to ryegrass plants. Furthermore, the inoculation of AMF increased the shoot and biomass of ryegrass by 11-19% and 18-78%, respectively. Bioaugmented ryegrass plants show promise as a host plants in the phytoremediation of PAH contaminated soils.

Removal and biodegradation of nonylphenol by different Chlorella species.

All four Chlorella species, including one commercially available species, Chlorella vulgaris and three local isolates, Chlorella sp. (1 uoai), Chlorella sp. (2f5aia) and Chlorellaminiata (WW1), had a rapid and high ability to remove nonylphenol (NP). Among these species, C. vulgaris had the highest NP removal (nearly all NP was removed from the medium) and degradation abilities (more than 80% of NP was degraded) after 168 h, followed by WW1 and 1 uoai; 2f5aia had the lowest NP degradation ability. The NP removal by C. vulgaris was less affected by growth conditions, but its biodegradation efficiency was significantly increased by temperature and light intensity, suggesting that the biodegradation ability was positively related to photosynthetic and metabolic activities. These results indicated that C. vulgaris was the most suitable species for effective removal and biodegradation of NP, especially under 25 °C with light illumination and initial biomass between 0.5 and 1.0 mg chlorophyll l(-1).

Growth, photosynthesis and antioxidant responses of two microalgal species, Chlorella vulgaris and Selenastrum capricornutum, to nonylphenol stress.

The effect of nonylphenol (NP) on growth, photochemistry and biochemistry of two green microalgae, Chlorella vulgaris and Selenanstrum capricornutum, and their ability to degrade NP were compared. The 96 h EC50 of C. vulgaris and S. capricornutum were greater than 4.0 and 1.0 mg L(-1) NP, respectively, suggesting that the former species was more tolerant to NP. Both microalgae acclimated to NP stress through down-regulating their photosynthetic activities, including antenna size (chlorophyll a content), maximal photochemistry (Fv/Fm) and the light absorbed by PSII (ABS/CS0), but the dissipation of energy from reaction centres (DI0/RC) increased with the increase of NP concentrations. In C. vulgaris, the changes of these parameters were more significant than in S. capricornutum and recovered completely after a 96 h exposure. The antioxidant responses, such as GSH content, CAT and POD activities in C. vulgaris increased with the increase of NP concentrations after a 24h exposure, but these changes disappeared with exposure time and recovered to the control levels after 96 h. In S. capricornutum, although GSH content, CAT and POD activities also increased when exposed to low- to moderate-NP concentrations, these values were significantly reduced at a high concentration (4 mg L(-1)) even after a 96 h exposure, indicating its antioxidant responses were significantly delayed. It is clear that the more NP-tolerant species, C. vulgaris, acclimated better with a faster recovery of its photosynthetic activity from the NP-induced damage, and exhibited more efficient and rapid responses to NP-induced oxidative stress. C. vulgaris also had a higher NP degradation ability than S. capricornutum.

Effect of wastewater discharge on root anatomy and radial oxygen loss (ROL) patterns of three mangrove species in southern China.

The effects of wastewater discharge on radial oxygen loss (ROL) and root anatomy varied among mangrove species. ROL of Bruguiera gymnorrhiza (L) increased from 22.44 ng cm(-2) min(-1) in the control (just fresh water) to 31.09 ng cm(-2) min(-1) when received normal wastewater (NW) and to 44.22 ng cm(-2) min(-1) when treated with strong wastewater (10NW). However, discharge of both NW and 10NW caused 28% decreases of ROL in the root tip of Excoecaria agallocha L., and the decreases in Acanthus ilicifolius L were even more significant, with 45% when treated by 10NW The changes of ROL were related to the root anatomy. Among three species, A. ilicifolius had the highest proportional cross-sectional area of aerenchyma air spaces, suggesting that the internal oxygen transfer to root tip was the fastest. However, the area of aerenchyma air spaces in the root tip of 10NW treated A. ilicifolius was significantly reduced while area of epidermis and hypodermis (E + H) increased leading to less oxygen supply to root tip. Compared to B. gymnorrhiza and E. agallocha, the (E + H) layer of A. ilicifolius was the thinnest, and the cells without suberized walls were loosely packed in all three treatments. These results suggested that the root anatomy and ROL of B. gymnorrhiza was least affected by wastewater discharge, followed by E. agallocha, and A. ilicifolius was the most susceptible species thus was not suitable for treating strong wastewater.

Analysis and occurrence of typical endocrine-disrupting chemicals in three sewage treatment plants.

Seven typical endocrine-disrupting chemicals (EDCs), including bisphenol A (BPA), 4-tert-octylphenol (OP), estrone (E1), estradiol (E2), 17α-estradiol (17α-E2), estriol (E3) and 17α-ethinylestradiol (EE2) in wastewater, were simultaneously determined with gas chromatography-mass spectrometry (GC-MS). Samples, including influents, effluents and wastewater of different unit processes, were taken seasonally from three different sewage treatment plants. The result showed that BPA and EE2 were the two main types of EDCs in all the samples. The average concentration of BPA were in the range of 268.1-2,588.5 ng l⁻¹ in influents and 34.0-3,099.6 ng l⁻¹ in effluents, while EE2 ranging from 133.1 to 403.2 ng l⁻¹ and from 35.3 to 269.1 ng l⁻¹, respectively. Seasonal change of EDCs levels in effluents was obvious between wet season and dry season. Besides, BPA and E3 could be effectively removed by the biological treatment processes (oxidation ditch and A²/O) with the unit removal of 64-91% and 63-100% for each compound, while other five EDCs had moderate or low removal rates. The study also proved that physical treatment processes, including screening, primary sedimentation and pure aeration, had no or little effect on EDCs removal.

Summer fluxes of atmospheric greenhouse gases N2O, CH4 and CO2 from mangrove soil in South China.

The atmospheric fluxes of N(2)O, CH(4) and CO(2) from the soil in four mangrove swamps in Shenzhen and Hong Kong, South China were investigated in the summer of 2008. The fluxes ranged from 0.14 to 23.83 micromol m(-2)h(-1), 11.9 to 5168.6 micromol m(-2)h(-1) and 0.69 to 20.56 mmol m(-2)h(-1) for N(2)O, CH(4) and CO(2), respectively. Futian mangrove swamp in Shenzhen had the highest greenhouse gas fluxes, followed by Mai Po mangrove in Hong Kong. Sha Kong Tsuen and Yung Shue O mangroves in Hong Kong had similar, low fluxes. The differences in both N(2)O and CH(4) fluxes among different tidal positions, the landward, seaward and bare mudflat, in each swamp were insignificant. The N(2)O and CO(2) fluxes were positively correlated with the soil organic carbon, total nitrogen, total phosphate, total iron and NH(4)(+)-N contents, as well as the soil porosity. However, only soil NH(4)(+)-N concentration had significant effects on CH(4) fluxes.

The role of radial oxygen loss and root anatomy on zinc uptake and tolerance in mangrove seedlings.

Root anatomy, radial oxygen loss (ROL) and zinc (Zn) uptake and tolerance in mangrove plants were investigated using seedlings of Aegiceras corniculatum, Bruguiera gymnorrhiza and Rhizophora stylosa. The results revealed that B. gymnorrhiza, which possessed the 'tightest barrier' in ROL spatial patterns among the three species studied, took up the least Zn and showed the highest Zn tolerance. Furthermore, zinc significantly decreased the ROL of all three plants by inhibition of root permeability, which included an obvious thickening of outer cortex and significant increases of lignification in cell walls. The results of SEM X-ray microanalysis further confirmed that such an inducible, low permeability of roots was likely an adaptive strategy to metal stress by direct prevention of excessive Zn entering into the root. The present study proposes new evidence of structural adaptive strategy on metal tolerance by mangrove seedlings.

Effects of wastewater discharge on formation of Fe plaque on root surface and radial oxygen loss of mangrove roots.

Effects of wastewater discharge on radial oxygen loss (ROL), formation of iron (Fe) plaque on root surface, and their correlations in Bruguiera gymnorrhiza (L.) Poir and Excoecaria agallocha L. were investigated. ROL along a lateral root increased more rapidly in control than that in strong wastewater (with pollutant concentrations ten times of that in municipal sewage, 10NW) treatment, but less Fe plaque was formed in control for both plants. For B. gymnorrhiza receiving 10NW, Fe plaque formation was more at basal and mature zones than at root tip, while opposite trend was shown in E. agallocha. At day 0, the correlation between ROL and Fe plaque was insignificant, but negative and positive correlations were found in 10NW and control, respectively, at day 105, suggesting that more ROL was induced leading to more Fe plaque. However, excess Fe plaque also served as a 'barrier' to prevent excessive ROL in 10NW plants.

Mixed heavy metals tolerance and radial oxygen loss in mangrove seedlings.

The effects of a mixture of heavy metals (Pb, Zn and Cu) on growth, radial oxygen loss (ROL) and the spatial pattern of ROL were investigated in mangrove seedlings of three species: Aegiceras corniculatum, Avicennia marina and Bruguiera gymnorrhiza. Heavy metals inhibited the growth of seedlings and led to decreased ROL and changes in the "tight" barrier spatial pattern of ROL. There was a significant positive correlation between the amount of ROL from the roots of seedlings and metal tolerance. The species with the highest ROL amount, B. gymnorrhiza, were also the most tolerant to heavy metals. The "tight" barrier spatial ROL pattern was also related to metal tolerance in the seedlings. Therefore, we conclude that both ROL amount and "tight" barrier spatial ROL pattern in the roots of the mangrove seedlings play an important role in resistance to heavy metal toxicity.

Effects of salinity on treatment of municipal wastewater by constructed mangrove wetland microcosms.

The effects of salinity on the removal of dissolved organic carbon and nutrients from municipal wastewater by constructed mangrove microcosms planted with Aegiceras corniculatum were investigated. During the four-month wastewater treatment, the treatment efficiency was reduced by high salinity, and the removal percentages of dissolved organic carbon, ammonia-N and inorganic N dropped from 91% to 71%, from 98% to 83% and from 78% to 56%, respectively, with salinity increasing from 0 to 30 parts per thousands (ppt). In spite of such inhibition at high salinity, 100% of the effluents discharge from the constructed mangrove microcosms still complied with the discharge standards set by the Hong Kong Government for Coastal Water Control Zones. These results suggested that constructed mangrove wetland treatment systems were promising to effectively treat municipal wastewater, even those with high salinity. In addition, the denitrification potential in soil was found to be retarded by the high salinity while mangrove plants grew best at 15 ppt salinity condition.

Effectiveness of bacterial inoculum and mangrove plants on remediation of sediment contaminated with polycyclic aromatic hydrocarbons.

The remediation of mangrove sediment contaminated with mixed polycyclic aromatic hydrocarbons (PAHs) having 3-, 4- and 5-rings by natural attenuation, bioaugmentation, phytoremediation and its combination was compared by greenhouse microcosm studies. At Days 90 and 154, the decreases of PAHs in contaminated mangrove sediment by phytoremediation, planted with one-year old Aegiceras corniculatum, and bioaugmentation, the inoculation of PAH-degrading bacterial strains isolated from mangrove sediment, either SCSH (Mycobacterium parafortuitum) or SAFY (Sphingobium yanoikuyae), were not better than that by natural attenuation (the non-vegetated and un-inoculated microcosms). The populations of SCSH and SAFY in sediment could not be maintained even with repeated inoculation, suggesting that the two isolates were not able to compete with the indigenous microbes and had little enhancement effect. Although some PAHs were accumulated in roots, root uptake only accounted for <15% of the spiked PAHs and the effect of plants on remediation were also insignificant. At the end of the 154-day experiment, the mass balance calculation revealed that the overall losses of PAHs by phytoremediation were comparable to that by bioaugmentation but were lower than that by natural attenuation, especially for the high molecular weight PAHs. Under natural attenuation, around 90% fluorene, 80% phenanthrene, 70% fluoranthene, 68% pyrene and 32% benzo[a]pyrene in contaminated sediment were removed. These results demonstrated that the mangrove sediment itself had sufficient indigenous microorganisms capable of naturally remedying PAH contamination.

Potential use of mangroves as constructed wetland for municipal sewage treatment in Futian, Shenzhen, China.

A pilot-scale mangrove wetland was constructed in Futian, Shenzhen for municipal sewage treatment. Three identical belts (length: 33m, width: 3m, depth: 0.5m) were filled with stone (bottom), gravel and mangrove sand (surface). Seedlings of two native mangrove species (Kandelia candel, Aegiceras corniculatum) and one exotic species (Sonneratia caseolaris) were transplanted to the belts with one species for each belt. The hydraulic loading was 5m(3)d(-1) and hydraulic retention time 3d. High levels of removal of COD, BOD(5), TN, TP and NH(3)-N were obtained. The treatment efficiency of S. caseolaris and A. corniculatum was higher than that of K. candel. Faster plant growth was obtained for S. caseolaris. The substrate in the S. caseolaris belt also showed higher enzyme activities including dehydrogenase, cellulase, phosphatase, urease and beta-glucosidase. The removal rates of organic matter and nutrients were positively correlated with plant growth. The results indicated that mangroves could be used in a constructed wetland for municipal sewage treatment, providing post-treatment to remove coliforms was also included.