In another recent work, Tiwari et al. [45] demonstrated that cyanobacterium Fischerella sp. isolated from paddy fields has the capacity to degrade organophosphorus pesticide MP. Based on their study, they recommend the organism as a potential candidate for pesticide bioremediation.
Figure 1.3 Schematic representation diazinon degradation by Chlorella vulgaris [38].
1.4 Strategies for Phycoremediation of Pesticides
1.4.1 Involvement of Enzymes in Phycoremediation of Pesticides
Biodegradation involves the breakdown of organic compounds into its inorganic constituents. Enzymes are one of the important biomolecules involved in the degradation of pesticides. The three main enzymes involved in pesticide degradation are hydrolases, esterases (also hydrolases), and the mixed function oxidases (MFOs). These enzyme systems are involved in the first metabolism stage of the pesticide and the glutathione S-transferase (GST) system, in the second phase [46]. In general pesticide, metabolism involves three main phases. During the Phase I of pesticide metabolism, the parent compound is converted into a more water-soluble and less toxic form by various processes such as oxidation, reduction, or hydrolysis. In the second phase, the water solubility and toxicity of the pesticide is further reduced by conjugation of the pesticide or pesticide metabolite to an amino acid or sugar. In the third phase, Phase II metabolites are converted into non-toxic secondary conjugates [46, 47]. Microalgae are photosynthetic organisms equipped with efficient enzyme system to metabolize and degrade various organic pollutants such as pesticides. Pertaining to their potential to degrade pesticides, microalgal species are recommended for remediation of the site contaminated with highly toxic pesticide like lindane [8]. Degradation of organophosphorus pesticide in presence of microbial enzymes has attracted the attention of scientist across the world. For instance, the enzyme alkaline phosphatase secreted by Spirulina platensis can hydrolyze chlorpyrifos, an organophosphorus pesticide, into 3,5,6-trichloro-2-pyridinol (TCP) [48]. Thus, immobilization of these pesticide degrading enzymes secreted form microalgae on solid matrix can be employed for remediation of pesticide contaminated sites [8].
1.4.2 Use of Genetically Engineered Microalgae
Development of genetically manipulated microalgae is a modern technology. This involves overexpression of contains proteins and enzymes which can combat the toxic effect of the contaminant. Extensive sequence information and good background knowledge about molecular, biochemical, physiological, and ecological characteristics of the microalgal species are required for the development of transgenic species to be used for bioremediation [49]. According to studies, Anabaena sp. strain PCC7120 and N. ellipsossorum are capable of degrading γ-Hexachlorocyclohexane (HCH) [37]. These two strains showed enhanced degradation of lindane when they are genetically modified using Lin A gene [37]. Thus, microorganisms can be genetically modified to develop highly efficient pesticide degradation strains which can be employed for eco-friendly remediation of pesticides.
1.5 Molecular Aspects of Pesticide Biodegradation by Microalgae
Several scientific studies are available in which algae and cyanobacteria have been reported to be highly efficient in detoxification of xenobiotics such as pesticides. Singh et al. [41] reported the degradation of the organophosphorus insecticide chlorpyrifos by the cyanobacterium Synechocystis PUPCCC. According to the author, the degradation mechanism of chlorpyrifos by cyanobacteria might be similar to bacteria. In bacteria, phosphotriesterases are the major group of enzymes involved in degradation of organophosphate pesticides [50]. These enzymes are encoded by a gene called opd (organophosphate-degrading). Mulbry and Karns [51] cloned and sequenced the gene. Phosphotriesterases are responsible for hydrolysis of phosphoester bonds, such as P–O, P–F, P–NC, and P–S [52]. The opd gene encoding organophosphorus hydrolase (the enzyme responsible for degradation of organophosphate pesticide) has 996 nucleotides, a typical promoter sequence of the promoter TTGCAA N17 TATACT from E. coli [53]. Chungjatupornchai and Fa-Aroonsawat [54] expressed opd gene from Flavobacterium sp. both on the surface and intracellularly in the cyanobacterium Synechococcus PCC7942 and used it for biodegradation of organophosphate pesticide. This reflects the importance of opd gene in biodegradation of organophosphate pesticides.
Exposure of plants to toxic organic substances provokes production of intracellular reactive oxygen species (ROS) which may adversely affect various cellular functions such as peroxidation of lipids and oxidation of proteins [55]. In order to minimize the adverse effects of ROS, plants possess an elaborate defense system consisting of antioxidant enzymes. Scavenging of ROS depends on the coordinated function of antioxidant enzymes such as Superoxide dismutase (SOD), Catalase (CAT), and Ascorbate peroxidise (APX) [56]. SOD is involved in the dismutation of superoxide anion O2− to H2O2 and O2. H2O2 is further scavenged by the catalytic activity of CAT and APX. APX is involved in the ascorbate-glutathione cycle to reduce stress [55, 57]. Jin et al. [43] reported an upregulation of the genes encoding Mn-SOD, CAT, and APX in green alga C. reinhardtii exposed to the herbicide prometryne. An efficient scavenging/detoxification system is responsible for quick accumulation and degradation of pesticide by microalgae [40]. Jin et al. [43] also noted an upregulation of the inducible gene HO-1 (Heme Oxygenase-1) in C. reinhardtii exposed to the herbicide prometryne suggesting its involvement in the tolerance of the microalgae toward the herbicide. Kumari et al. [58] evaluated butachlor toxicity in Aulosira fertilissima using a proteomic approach. They concluded that out of eight proteins altered during butachlor exposure, downregulation of GroES (associated with protein folding), and overexpression of NusB (associated 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