pesticide chlorpyrifos. The study showed that the organism could tolerate chlorpyrifos up to 15 mg L−1. Maximum removal of chlorpyrifos was achieved at a temperature of 30°C, pH 7.0, and 100 mg protein−1 biomass. Metabolization of the pesticide by the cyanobacteria resulted in production of 3,5,6-trichloro-2-pyridinol as degradation product. The same cyanobacterial strain was later reported to degrade anilofos by Singh et al. [42]. In the study, the organism was found to tolerate high concentration of anilofos (25 mg L−1).
The influenced of the pesticide on photosynthetic pigment content was dose-dependent. The herbicide was uptaken rapidly by the organism during the first 6 hours after which there was slow uptake until 5 days. The cyanobacterium utilized anilfos as a source of phosphate with maximum removal of anilofos at temperature of 30°C, pH 8.0, and 100 mg protein L−1. In addition to cyanobacteria, microalgae are reported to degrade herbicides. For instance, the green alga C. reinhardtii was found to accumulate and biodegrade the pesticide prometryne. The study demonstrated that C. reinhardtii had the capacity to degrade prometryne at a moderate concentration of 5 g L−1. This uptake and degradation of herbicide by C. reinhardtii reflect the internal tolerance mechanism of the green algae and establish it as a potential strain for remediation of prometryne from contaminated water [43].
In a recent study, Kurade et al. [13] found that C. vulgaris has the capacity of bioremediation of diazinon (Figure 1.3). In the study, the rate constant of degradation (k) of diazinon (0.5–100 mg L−1) ranged between 0.2304 to 0.049 d−1 and the half-life (T1/2) ranged between 3.01 and 14.06 d−1. According to gas chromatography mass spectroscopic (GC-MS) study, metabolism of diazinon by microalgal strain resulted in the formation of 2-isopropyl-6-methyl-4-pyrimidinol (IMP) which is a by-product with low toxcity. In another work, Kumar et al. [44] studied the degradation of pesticide acephate and imidacloprid by the microalgae C. mexicana. They concluded that C. mexicana was able to remove 25% and 21% of acephate and imidacloprid, respectively. 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
