This study focuses on the characterization of four bacterial isolates from heavy metal-polluted rhizosphere in order to examine their plant growth promoting (PGP) activity. The PGP activity on the canola ( Brassica napus ) of the strains which showed cadmium resistance and multiple PGP traits was assessed in the presence and in the absence of Cd 2+ . The strains, Pseudomonas tolaasii ACC23, Pseudomonas fluorescens ACC9, Alcaligenes sp. ZN4 and Mycobacterium sp. ACC14 showed 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity. They also synthesized ACCD enzyme in vitro when 0.4 mM Cd 2+ was added to the growth medium. The presence of the metal, however, reduced the ACCD activity in Alcaligenes sp. ZN4 and Mycobacterium sp. ACC14, while it did not affect the ACCD activity of P. tolaasii ACC23 and P. fluorescens ACC9. ACC9 and ACC23 produced indole acetic acid (IAA) and siderophores, while ACC14 produced only IAA. IAA and siderophores were produced more actively under Cd-stress. Root elongation assays conducted on B. napus under gnotobiotic conditions demonstrated increases (from 34% up to 97%) in root elongation of inoculated canola seedlings compared to the control plants. Subsequently, the effect of inoculation with these strains on growth and uptake of Cd 2+ in roots and shoots of canola was studied in pot experiments using Cd-free and Cd-treated (15 μg Cd 2+  g −1  dw) soil. Inoculation with P. tolaasii ACC23, P. fluorescens ACC9 and Mycobacterium sp. ACC14 promoted the growth of plants at concentrations of 0 and 15 μg Cd 2+  g −1  soil. The maximum growth was observed in the plants inoculated with P. tolaasii ACC23. The strains did not influence the specific accumulation of cadmium in the root and shoot systems, but all increased the plant biomass and consequently the total cadmium accumulation. The present observations showed that the bacterial strains used in this study protect the plants against the inhibitory effects of cadmium, probably due to the production of IAA, siderophores and ACCD activity.

Improvement of Brassica napus growth under cadmium stress by cadmium-resistant rhizobacteria

Environmental problems have always received immense attention from scientists. Toxicants pollution is a critical environmental concern that has posed serious threats to human health and agricultural production. Heavy metals and pesticides are top of the list of environmental toxicants endangering nature. This review focuses on the toxic effect of heavy metals (cadmium (Cd), lead (Pb), copper (Cu), and zinc (Zn)) and pesticides (insecticides, herbicides, and fungicides) adversely influencing the agricultural ecosystem (plant and soil) and human health. Furthermore, heavy metals accumulation and pesticide residues in soils and plants have been discussed in detail. In addition, the characteristics of contaminated soil and plant physiological parameters have been reviewed. Moreover, human diseases caused by exposure to heavy metals and pesticides were also reported. The bioaccumulation, mechanism of action, and transmission pathways of both heavy metals and pesticides are emphasized. In addition, the bioavailability in soil and plant uptake of these contaminants has also been considered. Meanwhile, the synergistic and antagonistic interactions between heavy metals and pesticides and their combined toxic effects have been discussed. Previous relevant studies are included to cover all aspects of this review. The information in this review provides deep insights into the understanding of environmental toxicants and their hazardous effects.

Heavy Metals and Pesticides Toxicity in Agricultural Soil and Plants: Ecological Risks and Human Health Implications.

Heavy metals when present in amounts equal to the geochemical background do not interfere with the soil metabolism, which is associated with the growth and development of soil microorganisms as well as the processes of synthesis and re-synthesis, governed by intraand extracellular enzymes. In the said concentrations, heavy metals do not cause undesirable changes in the development of plants. On the contrary, such elements as copper and zinc are essential constituents of physiological processes in all living organisms, including microorganisms and plants. Some soils suffer from zinc and copper deficits, which is why they are enriched with fertilizers containing copper or zinc to satisfy the nutritional requirements of crops. Cadmium is different in that its essential role in the proper functioning of living organisms has not been proven yet. In Poland, soils contaminated with heavy metals, including cadmium, copper and zinc, occur only locally. The purpose of this study has been to discuss the characteristics of these elements in terms of the chemical properties and the role in the natural environment, the effect they produce on plants when present in excessive concentrations in soil and the response of soil microbes and enzymes to such contaminants. Crops cultivated on soil with an elevated content of heavy metals typically present inhibited growth, reduced transpiration, chlorosis of leaves, limited germination of seeds and deformations of the root system. The effect induced by heavy metals is more pronounced in the early development of plants. Mobility and plant availability of heavy metals depend on a series of factors, for example the soil pH, content of organic matter, grainsize composition of soil, content of iron and manganese oxides, soil sorption capacity and the type of metal. Higher bioavailability of heavy metals is observed in soils with a low content of humic acids. As the soil pH increases (within 6.5-7.5), metals, especially zinc and – to a lesser degree – copper become less toxic to plants.

http://yadda.icm.edu.pl/yadda/element/bwmeta1.element.agro-e1fd2c35-5b09-4442-9051-c70272c71ace/c/769-796.pdf

Effect of cadmium, copper and zinc on plants, soil microorganisms and soil enzymes.

Rhamnolipid, a metal sequestering agent produced by Pseudomonas Sp., has been effective in the removal of metals in soil washing technologies. Rhamnolipid has a strong affinity for cadmium (Cd) compared to some other metals (e.g. cobalt (Co), nickel (Ni)) and might also be useful in chelate-assisted phytoextraction. There have been many studies investigating the formation of metal–rhamnolipid complexes and the ability of rhamnolipid to remove metals from soil. However, to date, the longevity of rhamnolipid in soil has not been measured. Therefore, this study investigated the rate of rhamnolipid degradation in soils of varying physicochemical properties and contaminated with varying concentrations of Cd and zinc (Zn). The rate of rhamnolipid degradation was compared with ethylene diamine tetraacetic acid (EDTA) and citric acid. Our results indicate that citric acid was rapidly degraded, with 20% degradation occurring between 1 and 4 d depending on the level of soil contamination and 70% degradation within 20 d. EDTA was more persistent in the soils; only 14% of the EDTA was degraded after 20 d. Rhamnolipid had cumulative degradation between those of citric acid and EDTA. In most contaminated soils, cumulative degradation of the chelates and ligands were lower than in the uncontaminated soils. These results show that rhamnolipid may remain in the soil long enough to enhance metal phytoextraction, but not remain long enough to raise concerns regarding metal transport in the long-term.

Biodegradation of rhamnolipid, EDTA and citric acid in cadmium and zinc contaminated soils

The pollution of the soil with heavy metals is one of the worst legacies of our intensive agricultural–industrial activities, and it negatively affects various characteristics of the soil, including soil enzyme activities. Soil enzymes are natural molecules that catalyze soil microbial reactions and mainly originate from microorganisms and plants. Since enzyme activities play fundamental roles in soil chemical and biological reactions, their inhibition by heavy metals has received considerable attention and has been well documented by many researchers over the last few decades. The activities of soil enzymes have often been proposed as sensitive indicators of important microbial reactions involved in nutrient cycles and they respond to changes in the soil caused by natural or anthropogenic factors. In this regard, soil enzyme activities are often used to evaluate the impact of human activity on soil life. The purpose of this chapter is thus to emphasis some facts, hypotheses, and probabilities, as well as the results of research into the relationships between soil enzymes and heavy metals.

Effects of Heavy Metals on Soil Enzyme Activities

Cadmium (Cd) is ubiquitous in the human environment and has toxic effect on soil microbial biomass or its activity, including microbial biomass carbon (Cmic), dehydrogenase activity (DHA) and basal respiration (BR), etc., Cmic, DHA, BR were used as bioindicators of the toxic effect of Cd in soil. This study was conducted to determine the effects of Cd on soil microbial biomass and its activity in a paddy soil. The inhibition of microbial biomass and its activity by different Cd concentrations was described by the kinetic model (M1) and the sigmoid dose-response model (M2) in order to calculate three ecological doses of Cd: ED50, ED10 and ED5. Results showed that M2 was better fit than M1 for describing the ecological toxicity dose effect of cadmium on soil microbial biomass and its activity in a paddy soil. M2 for ED values (mg/kg soil) of Cmic, DHA, BR best fitted the measured paddy soil bioindicators. M2 showed that all ED values (mg/kg) increased in turn with increased incubation time. ED50, ED10 and ED5 of Cmic with M2 were increased in turn from 403.2, 141.1, 100.4 to 1000.7, 230.9, 144.8, respectively, after 10 d to 60 d of incubation. ED50, ED10 and ED5 of DHA with M2 increased in turn from 67.6, 6.2, 1.5 to 101.1, 50.9, 41.0, respectively, after 10 d to 60 d of incubation. ED50, ED10 and ED5 of BR with M2 increased in turn from 149.7, 6.5, 1.8 to 156.5, 50.8, 35.5, respectively, after 10 d to 60 d of incubation. So the ecological dose increased in turn with increased incubation time for M2 showed that toxicity of cadmium to soil microbial biomass and its activity was decreased with increased incubation time.

/pdf/toxicity-of-cadmium-to-soil-microbial-biomass-and-its-34mi5m6kvv.pdf

Toxicity of cadmium to soil microbial biomass and its activity: Effect of incubation time on Cd ecological dose in a paddy soil

A pot experiment with rice under submerged condition showed that with the increase of Cd concentration, soil microbial biomass carbon (Cmic) and nitrogen (Nmic) increased initially but decreased at a certain concentration, and the turning points varied with different soil types. Soil enzyme activities had the similar variation trend with soil Cmic and Nmic, and the turning points varied with different soil types and soil enzymes. The variation coefficients were in order of dehydrogenase activity > acid phophatase activity > urease activity. Soil respiration rate and metabolic quotient increased tardily with increasing cadmium concentration. The chlorophyll content of rice increased initially but decreased then with the increase of Cd contamination, and the turning points differed with different soil types. Rice proline content and peroxidase activity were enhanced gradually with increasing cadmium concentration. The variation coefficients of rice physiological indices on paddy soils derived from silty loam and clayed red earth were in order of peroxidase activity > chlorophyll content > proline content, and peroxidase activity > proline content > chlorophyll content, respectively. Correlation analysis indicated that there was a close correlation between the variations of soil microbial biomass and enzymatic activities and rice physiological indices under Cd contamination.

Yun-kuo Luo

Papers

Toxicity of cadmium to soil microbial biomass and its activity: Effect of incubation time on Cd ecological dose in a paddy soil

Effects of Cd contamination on paddy soil microbial biomass and enzyme activities and rice physiological indices