Lichens as bioindicators of atmospheric heavy metal pollution in Singapore.

Lichens have been used as bioindicators in various atmospheric pollution assessments in several countries. This study presents the first data on levels of heavy metals (As, Cd, Cu, Ni, Pb, and Zn) in lichens at different locations in Singapore, Southeast Asia. Singapore is a fully industrialised island nation, with a prevailing tropical climate and a population of 4 million people within a confined land area of less than 700 km2. The ubiquitous lichen species, Dirinaria picta was collected from six sample sites across Singapore and analysed for heavy metals using inductively coupled plasma mass spectrometry (ICPMS). No significant relationship existed between metal levels in lichen and soil, indicating that accumulated metals in lichen are primarily derived from the atmosphere. Peak concentrations of zinc (83.55 microg g(-1)), copper (45.13 microg g(-1)) and lead (16.59 microg g(-1)) in lichens were found at Sembawang, Jurong and the National University of Singapore campus which are locations associated with heavy petroleum and shipping industries, and road traffic respectively. The mean heavy metal levels of lichen samples in Singapore were found to be at the upper range of values reported in the literature for temperate countries.

Prevalence of microplastics in Singapore's coastal marine environment.

Microplastics have been recently identified as marine pollutants of significant concern due to their persistence, ubiquity and potential to act as vectors for the transfer and exposure of persistent organic pollutants to marine organisms. This study documents, for the first time, the presence and abundance of microplastics (>1.6 microm) in Singapore's coastal environment. An optimized sampling protocol for the collection and analysis of microplastics was developed, and beach sediments and seawater (surface microlayer and subsurface layer) samples were collected from nine different locations around the coastline. Low density microplastics were separated from sediments by flotation and polymer types were identified using Fourier transform infrared (FTIR) spectrometry. Synthetic polymer microplastics identified in beach sediments included polyethylene, polypropylene, polystyrene, nylon, polyvinyl alcohol and acrylonitrile butadiene styrene. Microplastics were detected in samples from four out of seven beach environments, with the greatest quantity found in sediments from two popular beaches in the eastern part of Singapore. Polyethylene, polypropylene and polystyrene microplastics were also found in the surface microlayer (50-60 microm) and subsurface layer (1m) of coastal waters. The presence of microplastics in sediments and seawater is likely due to on-going waste disposal practices from industries and recreational activities, and discharge from shipping.

Persistent organic pollutants in moss as bioindicators of atmospheric pollution in Singapore.

Moss samples were collected from the island of Singapore and analysed for a range of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs). Although all compounds analysed have been banned from use in Singapore, they were detected in all samples collected. Among the hexachlorocyclohexanes (HCH), beta-HCH was the most dominant isomer, while trans-chlorane was found to be the most dominant cyclodiene pesticide. High levels (79.12 ng g(-1)) of PCBs were detected in mosses collected from a nearby island due to active accumulation of pollutants associated with boat emissions. Significant spatial variations in the levels of organic pollutants in Singapore were largely absent, indicating that air masses moving over the island deposit OCPs and PCBs in a uniform pattern. A comparison of moss pollutant levels with available data from Czech Republic showed that dichlorodiphenyldichloroethylene (p,p'-DDE), dichlorodiphenyltrichloroethane (p,p'-DDT) and PCB levels in Singapore are the highest. The presence of these compounds suggests that they are still used in the region and are environmentally recalcitrant.

Organochlorine residues in odontocete species from the southeast coast of India.

Blubber from bottle-nose dolphins, spinner dolphins, humpback dolphin (Tursiops truncatus, Stenella longirostris and Sousa chinensis) were collected from the Bay of Bengal (southeast coast of India) and analyzed for the organochlorine pesticides hexachlorocyclohexane (HCHs), p,p'-dichlorodiphenyl trichloroethane (DDTs), and polychlorinated biphenyls (PCBs). All nine specimens analyzed contained considerable levels of all the three chemical classes where DDT was in the range of 3330-23330 ng/g; HCHs in the range of 95-765 ng/g; and PCBs in the range of 210-1220 ng/g (wet weight basis). The reasons for this and the variations in the isomer pattern of HCHs and DDT and its metabolites in marine mammal tissues are discussed.

Biodegradation of phenanthrene by the indigenous microbial biomass in a zinc amended soil.

AIMS: To study the effect of zinc on the biodegradation of phenanthrene by the microbial biomass in soil. METHODS AND RESULTS: Uncontaminated soil was amended with zinc and phenanthrene as single or co-contaminants, and microbial metabolic activity was measured using an intracellular dehydrogenase enzyme bioassay over 37 days. Contaminants were amended at optimum, action and double the action level specified in 'The New Dutch List' (Ministry of Housing, Spatial Planning and Environment, the Netherlands, 2000). Microbial activity in soils with zinc or phenanthrene alone indicated the presence of tolerant, albeit inhibited soil micro-organisms. A zinc concentration at the optimum level of 140 mg kg(-1) in the co-contaminated soil (phenanthrene at 40 mg kg(-1)) resulted in marginal stimulation of the rate of phenanthrene biodegradation. However, Zn2+ concentrations at the action and double the action level of zinc (720 and 1440 mg kg(-1)) inhibited phenanthrene degradation. CONCLUSIONS: Biodegradation of phenanthrene in soils co-contaminated with zinc at concentrations above the action value is impeded. SIGNIFICANCE AND IMPACT OF THE STUDY: Bioremediation efforts to remove polycyclic aromatic hydrocarbon in zinc co-contaminated soils are likely to be constrained.

Application of a slow-release fertilizer for oil bioremediation in beach sediment.

A 105-d field experiment was conducted to determine the potential of the slow-release fertilizer, Osmocote (Scotts, Marysville, OH), to stimulate the indigenous microbial biodegradation of petroleum hydrocarbons in an oil-spiked beach sediment on an intertidal foreshore in Singapore. Triplicate microcosms containing 80 kg of weathered sediment, spiked with 5% (w/w) Arabian light crude oil and 1.2% (w/w) Osmocote pellets, were established, together with control microcosms minus Osmocote. Relative to the control, the presence of the Osmocote sustained a significantly higher level of nutrients (NH(4)(+)-N, NO(3)(-)-N, and PO(4)(3-)-P) in the sediment pore water over the duration of the experiment. The metabolic activity of the indigenous microbial biomass, as measured using an intracellular dehydrogenase enzyme assay, was also significantly enhanced over the duration of the experiment in amended sediments. The loss of total recoverable petroleum hydrocarbons (TRPH) and biodegradation of total n-alkanes (C(10)-C(33)), branched alkanes (pristane and phytane), as well as total target polycyclic aromatic hydrocarbons (PAHs) (two- to six-ring), in both the control and Osmocote-amended sediments, followed a first-order biodegradation model. The first-order loss rate of total recoverable petroleum hydrocarbons was 2.57 times greater than that of the control. The hopane-normalized rate constants for total n-alkane, branched alkane, and total target PAH biodegradation in the Osmocote-treated sediments were 3.95-, 5.50-, and 2.45-fold higher than the control, respectively. Overall, the presence of Osmocote was able to significantly enhance and accelerate the biodegradation of aliphatics and PAHs in oil-contaminated sediments under natural field conditions in an intertidal foreshore environment.

Measurement of symbiotic nitrogen-fixation in leguminous host-plants grown in heavy metal-contaminated soils amended with sewage sludge.

Rates of nitrogen fixation by Rhizobium in symbiosis with leguminous host-plants including white clover, broad bean and peas have been established in soils that have been amended experimentally with heavy metal-contaminated sewage sludges. Results from 15N-dilution experiments for the measurement of N2 fixation have shown that adverse heavy metal effects are apparent on symbiotic N2 fixation rates for white clover grown in inter-specific competition with ryegrass under mixed sward conditions, compared to white clover grown in pure sward. Further experiments on broad bean and pea indicated a significant, but minor-inhibitory metal-related effect on the rate of N2 fixation compared to untreated soils and soils amended with a relatively uncontaminated sludge. The implications of the results with respect to sludge utilisation in agriculture are discussed.

Microbial cellulose decomposition in soils from a rifle range contaminated with heavy metals.

The objective of this study was to assess the effects of heavy metals on microbial decomposition of cellulose in heavy metal-contaminated soils using a cotton strip assay. The assay is a measure of the potential of soil microorganisms to decompose the plant polymer, cellulose. Cellulolytic activity in soil was assessed by determining the reduction in tensile strength of the buried cotton strips over a 25- and 45-day period. Soils were obtained from a rifle range that contain high levels of lead, copper and zinc. The site has been used for approximately 50 years, resulting in metal levels of up to 30,000 mg/kg of lead, 4000 mg/kg of copper and 600 mg/kg of zinc in the most contaminated soils. All the metal-contaminated soils had lower degradation rates than the uncontaminated soils tested. Among the contaminated soils, however, the heavy metal concentration was not the major factor in determining the loss in tensile strength of the cotton strips, where cellulose decomposition was governed by other soil physicochemical properties. Soil with a higher cation exchange capacity, readily oxidisable material and volatile solids content had the greatest loss in tensile strength of cotton strips. Microbial adaptation to the presence of high concentrations of soil heavy metals and reduced bioavailability of metals is the likely explanation for this phenomenon.

Ammonia removal from prawn aquaculture water using immobilized nitrifying bacteria.

Intensive prawn aquaculture in tropical regions is associated with high concentrations of total ammoniacal nitrogen (TAN) as a result of high rates of prawn excretion and feed loading. Excessive TAN can adversely effect productivity and result in adverse impacts on coastal waters. Cultures of indigenous nitrifying bacteria were enriched from intensive prawn aquaculture pond water using continuous and batch enrichment techniques. Cultures were capable of TAN removal over a wide range of initial TAN concentrations - up to 200 mg/l. Cultures were immobilized onto porous clay pellets to enhance cell density and applied to culture medium and TAN-augmented pond water under aerobic conditions to determine TAN removal proficiency. Immobilized cultures were able to achieve a high TAN removal proficiency in pond water--even at a low density of 0.1 pellet per liter. A concentration of less than 0.5 mg TAN/l could be maintained under a fed-batch condition of 3.2 mg TAN/l per day, after an initial 2-day lag phase. A simplified and effective culture enrichment process was developed for culture immobilization onto pellets using TAN-augmented pond water. Overall, pellet immobilization of indigenous nitrifying bacteria represents a potentially effective TAN control system for prawn aquaculture in low-cost, but intensive tropical prawn farms.

Dehydrogenase activity of the microbial biomass in soils from a field experiment amended with heavy metal contaminated sewage sludges.

Dehydrogenase activity (DHA) of the microbial biomass was measured in sewage sludge amended soil samples collected from the Braunschweig experimental site, Germany. The site had received additions of sludge with or without heavy metals at two application rates (100 m3/ha per year and 300 m3/ha per year) on soils of 'low' (4.8-5.8) and 'high' (5.4-7.0) pH since 1980. DHA was found to be a sensitive and precise assay for determining the effect of heavy metals on substrate-induced (glucose) microbial biomass in sewage sludge amended soils. Effects on DHA were determined in relation to heavy metal concentrations and other soil factors. Addition of relatively uncontaminated sludge enhanced DHA, but this was dependent on the level and type of sludge addition. Adverse metal effects were only significant in the most contaminated soils where sludge had been added to the 'high' and 'low' pH treatments at Braunschweig. However, these effects were small compared to the effects of high rates of sludge addition alone, despite exceeding statutory limits for Zn and Cu, where concentrations reached 341 and 99 micrograms/g, respectively.

The use of the cotton-strip assay to assess cellulose decomposition in heavy metal-contaminated sewage sludge-amended soils.

Cellulose decomposition in soils amended over twenty year ago with heavy metal-contaminated sewage sludges was assessed by using the cotton-strip assay. The soils of the Luddington Experiment now contain concentrations of Cu, Ni and Zn in selected plots that approximate to or exceed the statutory limits for these elements in sewage sludge-amended soils. The rates of cellulose decomposition were generally lower in the plots with elevated metal concentrations, relative to uncontaminated sludge-amended and unsludged controls. Generally, the metal-rich plots showed reductions in the time taken to reach 50% cotton-tensile-strength loss (CTSL). However, the reductions could not be consistently related to any one metal. The difference in decomposition rates between treatments was systematically reduced over the duration of a time-course experiment. A lower initial population of the appropriate decomposer community of micro-organisms may account for the observed short-term lag in decomposition rates.

The effect of heavy metals on dinitrogen fixation by Rhizobium-white clover in a range of long-term sewage sludge amended and metal-contaminated soils.

An investigation was conducted to determine whether effective strains of Rhizobium leguminosarum biovar. trifolii capable of symbiotic N2 fixation with white clover (Trifolium repens) were present in a range of metal-contaminated soils. A number of historically sewage-amended sites (including experimental, pasture grassland and arable sites) were selected and compared with highly contaminated samples from abandoned heavy metal mines. Many sites had metal concentrations above the limits established by the UK Government, based on those developed by the European Commission (EC) for sludge-amended soils. Acetylene reduction activity (ARA) was used to screen the samples for effective N2 fixation. When the host plant was indigenous to the sward, rhizobia were found in the nodules and in the soil rhizosphere at all the sites tested. They were shown to be capable of effective symbiosis and N2 fixation, even though metal concentrations greatly exceeded the soil metal limits in some cases. However, nodulation failed to occur in some cases where T. repens was not indigenous to metal-contaminated soils. This indicated either that an ineffective rhizobial population was present, or that effective cells were absent from the soil. The influence of individual metals on ARA could not be determined conclusively because of the confounding effects of soil physicochemical variability and the presence of different metals at high concentrations together in the soil. However, Cd concentrations appeared to be particularly important in determining the presence of effective ARA in soils with no indigenous clover. In contrast to previous studies, the results presented here suggest that heavy metals may have had a quantitative effect on the free-living population of rhizobia, rather than a genetic effect.