María S. Fuentes

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.

Enhanced removal of a pesticides mixture by single cultures and consortia of free and immobilized Streptomyces strains.

Pesticides are normally used to control specific pests and to increase the productivity in crops; as a result, soils are contaminated with mixtures of pesticides. In this work, the ability of Streptomyces strains (either as pure or mixed cultures) to remove pentachlorophenol and chlorpyrifos was studied. The antagonism among the strains and their tolerance to the toxic mixture was evaluated. Results revealed that the strains did not have any antagonistic effects and showed tolerance against the pesticides mixture. In fact, the growth of mixed cultures was significantly higher than in pure cultures. Moreover, a pure culture (Streptomyces sp. A5) and a quadruple culture had the highest pentachlorophenol removal percentages (10.6% and 10.1%, resp.), while Streptomyces sp. M7 presented the best chlorpyrifos removal (99.2%). Mixed culture of all Streptomyces spp. when assayed either as free or immobilized cells showed chlorpyrifos removal percentages of 40.17% and 71.05%, respectively, and for pentachlorophenol 5.24% and 14.72%, respectively, suggesting better removal of both pesticides by using immobilized cells. These results reveal that environments contaminated with mixtures of xenobiotics could be successfully cleaned up by using either free or immobilized cultures of Streptomyces, through in situ or ex situ remediation techniques.

Removal of the insecticide diazinon from liquid media by free and immobilized Streptomyces sp. isolated from agricultural soil.

From an agricultural soil that had received continuous applications of organophosphorus pesticides, 30 actinobacteria strains were isolated. Two strains, identified as Streptomyces sp. AC1‐6 and Streptomyces sp. ISP4, were selected because of their tolerance to diazinon and based on the relationship between diazinon removal and microbial growth. In liquid medium with diazinon at concentrations of 25 and 50 mg L−1, both strains were able to remove approximately 40–50% and 70–90% of the initial diazinon after 24 and 96 h of incubation, respectively. This diazinon removal was accompanied by microbial growth of the strains, an initial pH decrease, and glucose consumption in the liquid medium. Evaluation of the diazinon removal achieved by the free actinobacteria and Streptomyces sp. AC1‐6 immobilized on alginate beads revealed that the immobilized cells exhibited a 60% higher diazinon removal compared with the free cells. The reusability of the encapsulated biomass was confirmed, and a diazinon removal rate of more than 50% was obtained after the second batch. This work constitutes one of the few reports that describe Streptomyces strains as diazinon degraders. Given the high diazinon removal found, the streptomycetes exhibit suitable potential as diazinon‐degrading actinobacteria for elimination of diazinon from liquid residues.

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.

An Overview on Microbial Degradation of Lindane

Lindane is a cyclic, saturated and highly chlorinated pesticide with a broad spectrum, which has been used worldwide for many decades to control a variety of pests, and also in human health and veterinary. Afterward, it has been demonstrated that lindane and its isomers may cause serious damage to health in the short and long term. Besides, lindane is known to be persistent in the environment and tends to bioaccumulate along the food chain. Thus, lindane residues remain in the environment for a long time and have been recently found in water, soil, sediments, plants, and animals all over the world, and even in human fluids and tissues. In this context, nowadays, scientists, working all over the world, are involved in developing lindane remediation technologies including physical, chemical, and biological techniques. This article provides updated information on the biologic degradation of lindane using different microorganisms such as bacteria, fungi, and algae, under both aerobic and anaerobic conditions.

Pesticides Removal Using Actinomycetes and Plants

The γ-hexachlorocyclohexane (γ-HCH, lindane) is an organochlorine pesticide used in agriculture and medicine to world level. It has a big tendency to bioaccumulation into the environment so is listed as a priority pollutant by the US EPA. Hence the development of new technologies to remediate these sites using microorganisms is every time more necessary. The actinomycetes are Gram-positive bacteria with great potential to bioremediate xenobiotics. One strain, Streptomyces sp. M7, isolated from organochlorine pesticide contaminated sediment, was selected for its capacity to grow in presence of lindane as only carbon source. This microorganism was cultured in soil extract medium added of lindane 100 μg L−1, obtaining a maximal growth of 0.065 mg mL−1, similar to the control, with a highest lindane remotion of 70.4 % at 30°C and pH 7. When different initial pesticide concentrations (100, 150, 200, and 300 μg L−1) were added in soil medium, an increment of the microbial growth was detected in all the concentrations tested. Also a diminution of the residual lindane concentration was determined in the soil samples in relation to controls without bacteria (29.1, 78.0, 38.8, and 14.4 %, respectively). Besides, it was determined the optimum Streptomyces sp. M7 inoculum when lindane 100 μg kg−1soil was added to the soil sample. The optimum inoculum was 2 g kg−1 soil for obtaining the most efficiently bioremediation process: the lindane removal in these conditions was 67.8 % at 28 days of incubation. Later it was considered necessary to know the pesticide effects on maize plants seeded in lindane-contaminated soil previously inoculated with Streptomyces sp. M7. Lindane concentrations of 100, 200, and 400 mg kg−1 soil did not affect the germination and vigor index of maize plants seeded in contaminated soils without Streptomyces sp. M7. When this microorganism was inoculated at the same conditions, a better vigor index was observed and 68 % of lindane was removed. In this connection, Streptomyces sp. M7 was grown on culture medium in presence of root exudates of maize, spiked with 1.66 mg L−1 of lindane. The highest level of pesticide removal obtained on this condition suggests that root exudates enhanced removal of lindane by the bacterium. On the other hand, little information is available on the ability of biotransformation of organochlorine pesticides by actinomycete strains. It was demonstrated that Streptomyces sp. M7 possesses the LinA enzyme that catalyzes dehydrochlorination of lindane to 1,3,4,6-tetrachloro-1,4-cyclohexadiene (1,4-TCDN) via γ-pentachlorocyclohexene (γ-PCCH). These results confirm that actinomycete strains could be considered one of the most promising bacterial groups for lindane biodegradation in contaminated environment. Particularly, Streptomyces sp. M7 could be used for this purpose.