with transcription termination) are curtail for cell death. Molecular docking studies confirm that interaction of butachlor with GroES and NusB is responsible for its toxicity [59].
Agrawal et al. [60] demonstrated the molecular basis of butachlor toxicity/tolerance in three Anabaena species using comparative proteomics. The study showed that 75 proteins involved in photosynthesis, C, N and protein metabolism, redox homeostasis, and signal transduction were differentially expressed in each Anabaena sp. Agrawal et al. [61] reported that a novel aldo-keto reductase (AKR17A1) from Anabaena sp.7120 has the capacity to degrade chloroacetanilide herbicide butachlor. The study demonstrated that, in addition to combating multiple stresses, aldo-keto reductase encoding open reading frame all 2,316 plays a significant role in butachlor degradation. The gene can be used to develop transgenics with butachlor degradation and stress tolerance capabilities [61].
For evaluation of biodegradation and biotransformation of pesticide by microalgae, time-dependent environmental risk assessment is very essential [62]. Esperanza et al. [63] evaluated the toxicity of the widespread herbicide atrazine to the green alga C. reinhardtii by the transcriptomic and proteomic approach. They found that exposure of the microalgae to sublethal concentration of atrazine (0.25 μM) for 3 h resulted in differential expression of 185 genes, of this 124 showed upregulation and 61 genes showed downregulation. These genes belonged to 13 different categories of function such as photosynthesis, metabolism, gene expression, energy, amino acids, cell cycle, redox, lipid, regulation, ROS and stress, proteases, other and unknown [64]. They also noted that nine genes related to photosynthesis were differentially expressed, of which three genes (HLA3, LCIA, and ELI3) showed significant upregulation and six genes (LHCBM8, LHCSR3, LI818R-1, PTOX2, CAH4, and CAH5) showed significant downregulation. In a recent study Tiwari et al. [65] demonstrated the tolerance strategy of cyanobacteria Fischerella sp. exposed to organophosphorus insecticide MP by analyses of proteome and transcriptome. Proteome analysis revealed a differential expression of proteins connected to various metabolic activities such as photosynthesis, energy and protein metabolism, redox homeostasis, signal transduction, and cellular defense. Transcript analyses showed differential expression of genes such as phycocyanin α subunit (cpcA), ribulose bisphosphate carboxylase (rbcl), F0F1 ATP synthase subunit α, F0F1 ATP synthase subunit β, SOD (sod), NifH (nif H), DnaK (dnaK), and Peptidase S8 in Fischerella sp. exposed to MP. In addition, some hypothetical proteins related to signaling and carbohydrate metabolism were also found to be upregulated in the cyanobacterium exposed to MP stress. One hypothetical protein was found to be homologous to lectin with an MP binding pocket. The author suggests that this carbohydrate binding protein might have been involved in metabolism and degradation of the pesticide.
1.6 Factor Affecting Phycoremediation of Pesticides
Microalgae have the capacity to degrade a wide range of pesticides owing to their robust metabolic machinery. However, several factors influence pesticide degradation by microalgae. Some of the key factors are discussed below.
1.6.1 Biological Factor
Phycoremediation of pollutants such as pesticides by a selected microalgal strain depends on its physiology, survival and growth behaviors, species density, tolerance, and previous exposure to the specific pollutant. Moreover, a good synergy and compatibility of the organism with the existing microbiota play a key role is phycoremediation [66–68]. According to previous reports, a consortium of algae and bacteria performs better as a bioremediating candidate than individual algal or bacterial strain [67, 69, 70].
1.6.2 Chemical Factor
The characteristic features of the xenobiotic compounds such as physical and chemical properties (properties, i.e., hydrophobicity, solubility, and volatility) and concentration play a key role in phycoremediation [70–72]. For instance, light aromatic and saturated compounds are more easily degraded than polar and high molecular weight compounds [73].
1.6.3 Environment Factor
Environmental factors such as temperature, pH, light duration and intensity, and oxidation-reduction potential, salinity, and dissolved oxygen of the medium are key players in the process of phycoremediation of pollutants such as pesticides. These factors may limit the growth and survivability of the microalgae and may influence the media geochemistry and consequently affecting the efficacy of the process [71, 70, 74].
1.7 Benefit and Shortcomings of Phycoremediation
The major benefits [49] and shortcomings of phycoremediation are discussed below.
1.7.1 Benefits
1 1. Phycoremediation technology is a cost-effective technology. There is no requirement of sophisticated instruments and expensive chemicals. Microalgae can efficiently remediate environmental contamination without any extra cost.
2 2. The biomass generated during the process of remediation can act as a potential feedstock for the production of various products such as bio-chemicals (e.g., pharmaceuticals), bio-fertilizer, and bio-fuel.
3 3. Microalgae are photosynthetic creatures; thus, they consume the CO2 generated during the phycoremediation process and help in maintaining CO2 balance.
4 4. Conventional remedial methods generate a large amount of sludge which may be hazardous for the environment. But the sludge generated after phycoremediation contains algal biomass which can be used for energy generation and production of other value-added products.
1.7.2 Shortcomings
1 1. Bioremediation has several shortcomings. For instance, bioremediation depends a lot on the nature of the organism. Biodegradation of xenobiotics such as pesticide is not a benign response of the microorganism; on the contrary, it is a survival strategy. Most microorganisms carry out biodegradation under conditions which fulfils its necessities. Thus, certain modification of environment might be required to enable the organism to degrade pollutant in an efficient manner [75].
2 2. Low compatibility of the microalgal strain with the existing microflora and fauna can significantly affect the phycoremediation process.
3 3. Environmental factors such as pH, temperature, and salinity may influence the feasibility and success of the phycoremediation process.
4 4. Phycoremediation of pesticide is a slow process which makes its practical feasibility questionable.
1.8 Conclusion and Future Prospects
Bioremediation has proved to be an excellent tool for environmental remediation of pesticides originating from agricultural activities. There are a number of conventional techniques which are employed for pesticide remediation. But the cost associated with these methods is huge which made humans look for alternative remediation methods such as bioremediation. Traditionally, bacteria and fungi have been exploited for bioremediation but recently scientists and researchers have given sufficient attention to microalgae as a bioremediation candidate pertaining to its low nutritional requirements and versatile metabolic activity. Further, microalgae-based remediation may be integrated with other technology such as biofuel production, making them superior to its fungal and bacterial counterparts. However, there is an urgent need of more advance studies using proteomics and genomic tools to identify key genes involved in pesticide degradation. These genes can be used for development of transgenic microalgae for an efficient bioremediation of pesticides.
References
