Claudia S. Benimeli

Actinobacteria: Current research and perspectives for bioremediation of pesticides and heavy metals

Actinobacteria exhibit cosmopolitan distribution since their members are widely distributed in aquatic and terrestrial ecosystems. In the environment they play relevant ecological roles including recycling of substances, degradation of complex polymers, and production of bioactive molecules. Biotechnological potential of actinobacteria in the environment was demonstrated by their ability to remove organic and inorganic pollutants. This ability is the reason why actinobacteria have received special attention as candidates for bioremediation, which has gained importance because of the widespread release of contaminants into the environment. Among organic contaminants, pesticides are widely used for pest control, although the negative impact of these chemicals in the environmental balance is increasingly becoming apparent. Similarly, the extensive application of heavy metals in industrial processes lead to highly contaminated areas worldwide. Several studies focused in the use of actinobacteria for cleaning up the environment were performed in the last 15 years. Strategies such as bioaugmentation, biostimulation, cell immobilization, production of biosurfactants, design of defined mixed cultures and the use of plant-microbe systems were developed to enhance the capabilities of actinobacteria in bioremediation. In this review, we compiled and discussed works focused in the study of different bioremediation strategies using actinobacteria and how they contributed to the improvement of the already existing strategies. In addition, we discuss the importance of omic studies to elucidate mechanisms and regulations that bacteria use to cope with pollutant toxicity, since they are still little known in actinobacteria. A brief account of sources and harmful effects of pesticides and heavy metals is also given.

Lindane removal by pure and mixed cultures of immobilized actinobacteria

Lindane (γ-HCH) is an organochlorine insecticide that has been widely used in developing countries. It is known to persist in the environment and can cause serious health problems. One of the strategies adopted to remove lindane from the environment is bioremediation using microorganisms. Immobilized cells present advantages over free suspended cells, like their high degradation efficiency and protection against toxins. The aims of this work were: (1) To evaluate the ability of Streptomyces strains immobilized in four different matrices to remove lindane, (2) To select the support with optimum lindane removal by pure cultures, (3) To assay the selected support with consortia and (4) To evaluate the reusability of the immobilized cells. Four Streptomyces sp. strains had previously shown their ability to grow in the presence of lindane. Lindane removal by microorganisms immobilized was significantly higher than in free cells. Specifically immobilized cells in cloth sachets showed an improvement of around 25% in lindane removal compared to the abiotic control. Three strains showed significantly higher microbial growth when they were entrapped in silicone tubes. Strains immobilized in PVA-alginate demonstrated lowest growth. Mixed cultures immobilized inside cloth sachets showed no significant enhancement compared to pure cultures, reaching a maximum removal of 81% after 96 h for consortium I, consisting of the four immobilized strains together. Nevertheless, the cells could be reused for two additional cycles of 96 h each, obtaining a maximum removal efficiency of 71.5% when each of the four strains was immobilized in a separate bag (consortium III).

Versatility of Streptomyces sp. M7 to bioremediate soils co-contaminated with Cr(VI) and lindane.

The aim of this work was to study the impact of environmental factors on the bioremediation of Cr(VI) and lindane contaminated soil, by an actinobacterium, Streptomyces sp. M7, in order to optimize the process. Soil samples were contaminated with 25 µg kg(-1) of lindane and 50 mg kg(-1) of Cr(VI) and inoculated with Streptomyces sp. M7. The lowest inoculum concentration which simultaneously produced highest removal of Cr(VI) and lindane was 1 g kg(-1). The influence of physical and chemical parameters was assessed using a full factorial design. The factors and levels tested were: Temperature: 25, 30, 35°C; Humidity: 10%, 20%, 30%; Initial Cr(VI) concentration: 20, 50, 80 mg kg(-1); Initial lindane concentration: 10, 25, 40 µg kg(-1). Streptomyces sp. M7 exhibited strong versatility, showing the ability to bioremediate co-contaminated soil samples at several physicochemical conditions. Streptomyces sp. M7 inoculum size was optimized. Also, it was fitted a model to study this process, and it was possible to predict the system performance, knowing the initial conditions. Moreover, optimum temperature and humidity conditions for the bioremediation of soil with different concentrations of Cr(VI) and lindane were determined. Lettuce seedlings were a suitable biomarker to evaluate the contaminants mixture toxicity. Streptomyces sp. M7 carried out a successful bioremediation, which was demonstrated through ecotoxicity test with Lactuca sativa.

Lindane removal using Streptomyces strains and maize plants: a biological system for reducing pesticides in soils

Background and aimsPlants and contaminant-degrading microbes are a suitable combination for the remediation of pesticides. The aim of this study was to evaluate the effectiveness of Streptomyces strains cultured with maize plants in relation to lindane removal.MethodsFour Streptomyces strains were cultured and added as both single and mixed cultures, along with maize plants, to artificially polluted hydroponic systems and soils. The effectiveness of the resulting soil bioremediation was then evaluated through phytotoxicity testing using lettuce seedlings.ResultsIn the hydroponic and soil experiments, similar levels of lindane removal were recorded in the inoculated and non-inoculated systems where maize plants were introduced. However, the vigor index (VI) of the maize plants was highest when grown in inoculated and artificially polluted soil. In the phytotoxicity assay, the VI of the lettuce seedlings increased with increasing bioremediation time for the soils, thus indicating the effectiveness of the process.ConclusionsSimilar levels of lindane removal were recorded in both inoculated and non-inoculated planted systems, indicating that pesticide removal was not significantly affected by the bacterial inoculant. However, inoculation an actinobacteria consortium led to an increase in the VI of the maize and protected the plants against the existing toxicity. Furthermore, maize plants may attenuate the transient toxic effects of microbial lindane degradation.

Selection of an actinobacteria mixed culture for chlordane remediation. Pesticide effects on microbial morphology and bioemulsifier production

Chlordane bioremediation using actinobacteria mixed culture is an attractive clean‐up technique. Their ability to produce bioemulsifiers could increase the bioavailability of this pesticide. In order to select a defined actinobacteria mixed culture for chlordane remediation, compatibility assays were performed among six Streptomyces strains. The strains did not show growth inhibition, and they were assayed for chlordane removal, either as pure or as mixed cultures. In pure cultures, all of the strains showed specific dechlorination activity (1.42–24.20 EU mg−1) and chlordane removal abilities (91.3–95.5%). The specific dechlorination activity was mainly improved with cultures of three or four microorganisms. The mixed culture consisting of Streptomyces sp. A2‐A5‐A13 was selected. Their ability to produce bioemulsifiers in the presence of glucose or chlordane was tested, but no significant differences were observed (p > 0.05). However, the stability of the emulsions formed was linked to the carbon source used. Only in chlordane presence the emulsions retained 100% of their initial height. Finally, the selected consortium showed a high degree of sporulation in the pesticide presence. This is the first study on the effects that chlordane exerts on microbe morphology and emulsifier production for a defined mixed culture of Streptomyces with ability to remediate the pesticide.

Use of Actinobacteria Consortia to Improve Methoxychlor Bioremediation in Different Contaminated Matrices

Methoxychlor (MTX) is an organochlorine pesticide which has been banned in most countries; however, it is still being used in agricultural products and against mosquito. This pesticide has estrogenic activity and mimics endocrine hormone functions. Thus, it is important to analyze its behavior in different matrices.

Bioremediation Potential of Heavy Metal–Resistant Actinobacteria and Maize Plants in Polluted Soil

The screening and characterization of metal resistant microorganisms and plants are important for developing novel bioremediation processes. Considering these, we assessed the potential of copper- and chromium-resistant actinomycetes for bioremediation activity in polluted soils. Also, we assessed the effects of copper concentrations on roots, shoots, and leaf growth of maize and the copper uptake and accumulation by the maize plants. Four chromium resistant Streptomyces strains reduced hexavalent chromium up to 85–95% after 21 days. The novel copper-resistant actinobacterium Amycolatopsis tucumanensis efficiently immobilized copper when inoculated into copper-polluted soil microcosms: bioavailable Cu was 31% lower in soil compared to non-bioaugmented soil. Maize plant was found interesting both as biomarker and bioremediation tool. The bioremediation activity of A. tucumanensis inoculated maize plants grown in polluted soil microcosms correlated well with the values obtained with chemical and physical methods: 20% and 17% lower tissue contents of copper were measured in roots and leaves, respectively. The roots, shoots, and leaves of maize plants also showed a great ability to accumulate copper, which however increased with metal concentration. The metal concentrations were 382 times more in roots, 157 in shoots, and only 16 in leaves, compared to the control (without CuSO4).

Actinobacteria consortium as an efficient biotechnological tool for mixed polluted soil reclamation: Experimental factorial design for bioremediation process optimization

The objective of the present work was to establish optimal biological and physicochemical parameters in order to remove simultaneously lindane and Cr(VI) at high and/or low pollutants concentrations from the soil by an actinobacteria consortium formed by Streptomyces sp. M7, MC1, A5, and Amycolatopsis tucumanensis AB0. Also, the final aim was to treat real soils contaminated with Cr(VI) and/or lindane from the Northwest of Argentina employing the optimal biological and physicochemical conditions. In this sense, after determining the optimal inoculum concentration (2gkg-1), an experimental design model with four factors (temperature, moisture, initial concentration of Cr(VI) and lindane) was employed for predicting the system behavior during bioremediation process. According to response optimizer, the optimal moisture level was 30% for all bioremediation processes. However, the optimal temperature was different for each situation: for low initial concentrations of both pollutants, the optimal temperature was 25°C; for low initial concentrations of Cr(VI) and high initial concentrations of lindane, the optimal temperature was 30°C; and for high initial concentrations of Cr(VI), the optimal temperature was 35°C. In order to confirm the model adequacy and the validity of the optimization procedure, experiments were performed in six real contaminated soils samples. The defined actinobacteria consortium reduced the contaminants concentrations in five of the six samples, by working at laboratory scale and employing the optimal conditions obtained through the factorial design.

Improvement of lindane removal by Streptomyces sp. M7 by using stable microemulsions

Lindane is an organochlorine pesticide which persists in the environment and can cause serious health problems due to its chlorinated and hydrophobic nature. Microemulsions are isotropic and macroscopically homogeneous systems with high solubilization capacity of hydrophilic and hydrophobic compounds. The aim of this study was to evaluate the removal of high concentrations of lindane by the actinobacterium Streptomyces sp. M7 in aqueous and soil systems in the presence of stable microemulsions. Three stable microemulsions were successfully formed with Tween 80, 1-pentanol and three vegetable oils. In most cases, an increase in the cosurfactant/surfactant ratio in the microemulsions favored the solubilization of lindane, while an increase in the oil/surfactant ratio negatively affected the stability of the system. The microemulsion prepared with soybean oil allowed the solubilization of 66% of lindane added to the aqueous medium and 4.5 times more than the surfactant solution at the same concentration. This microemulsion increased the bioavailability of lindane in the aqueous medium and hence enhanced its removal by Streptomyces sp. M7 almost two times respect to the achieved with the surfactant solution. In loam soil system, the addition of the microemulsion allowed an 87% of lindane removal by Streptomyces sp. M7, increasing almost 50% the removal respect to the obtained without the addition of surfactant agents, although it did not present significant difference respect to the obtained with the surfactant solution. This is the first report on enhanced lindane removal by actinobacteria by using direct microemulsions as bioremediation tools.

Cell Immobilization Technique for the Enhanced Removal of Lindane Using Streptomyces Strains Isolated from Northwestern Argentina

Lindane (γ-HCH) is an organochlorine insecticide which has a negative effect as a pollutant agent of soil, water, and sediments. Nowadays it has been banned in almost all countries of the world, but its residues still remain in the environment. In this context, bioremediation, involving the use of microorganisms to degrade environmental contaminants, has received much attention as an effective biotechnological approach to clean up this kind of pollutants. Moreover, cell immobilization has been shown to present diverse advantages over conventional systems using free cells, such as the possibility of employing higher cell density, easier separation of cells from the system, repeated use of cells, and better protection of cells from harsh environments.