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Agrochemicals find widespread application in modern day agricultural practices to control pests and weeds to accelerate crop productivity. But environmental deterioration created by these chemicals has compelled human beings to look for an eco-friendly technology such as bioremediation. With the establishment of microalgae as an ideal bioremediation candidate, isolation and selection of strains which are resistant as well as have biodegrading potential received sufficient scientific attention. There are number of scientific investigations which reveal the pesticide degradation capabilities of cyanobacteria and algae (Table 1.2). According to Megharaj et al. [31] cyanobacteria Nostoc linckia, Phormidium tenue, and Synechococcus elongatus and green algae Scenedesmus bijugatus and Chlorella vulgaris had the capability to metabolise two organophosphorus insecticide monocrotophos and quinalphos. They concluded that both cyanobacteria and algae had similar biodegradation potential. In another work Megharaj et al. [32] also showed the biodegradation of the pesticide methyl parathion (MP) by cyanobacteria P. foveolarum, N. muscorum, N. linckia, and Oscillatoria animalis and green algae S. bijugatus and C. vulgaris. The study showed that they were capable of hydrolyzing the insecticide in 20 days while C. vulgaris, N. linckia, and S. bijugatus could hydrolyze the same in 30 days. Thus, it concluded that the biodegradation capabilities of selected microalgal and cyanobacterial strain followed the following order: C. vulgaris < S. bijugatus < N. linckia < N. muscorum < O. animalis < P. foveolarum.

Table 1.2 Cyanobacterial/microalgal strains involved in biodegradation of pesticide.

Chemical	Microalgae/Cyanobacteria	Reference
Monocrotophos and Quinalphos	Chlorella vulgaris, Scenedesmus bijugatus, Synechococcus elongatus, Phormidium tenue, Nostoc linckia	[31]
Methyl parathion	C. vulgaris, S. bijugatus, N. linckia, N. muscorum, Oscillatoria animalis, P. foveolarum	[32]
DDT	Chlorococcum sp., Anabaena sp., Nostoc sp.	[77]
α-Endosulfan	Scenedesmus sp., Chlorococcum sp.,	[76]
Fenamiphos	Pseudokirchneriella subcapitata, Chlorococcum sp.	[33]
Dimethomorph and Pyrimethanil	S. quadricauda	[39]
Fluroxypyr	Chlamydomonas reinhardtii	[40]
Chlorpyrifos	Synechocystis sp. strain PUPCCC 64	[41]
Prometryne	C. reinhardtii	[43]
Anilofos	Synechocystis sp. strain PUPCCC 64	[42]
Acephate, Imidaclorpid	C. mexicana	[44]
Diazinon	C. vulgaris	[13]
Methyl parathion	Fischerella sp.	[45]

In addition to this, five green algae (Chlorella sp., Scenedesmus sp. MM1, Stichococcus sp., Scenedesmus sp. MM2, and Chlamydomonas sp.) and five cyanobacteria (Anabaena sp., Nostoc sp. MM1, N. muscorum, Nostoc sp. MM3, and Nostoc sp. MM2) have been reported to degrade fenamiphos which is an organophosphorus pesticide [33].

2,4-dichlorophenol (2,4-DCP) is often used as an intermediate in synthesis of insecticides and herbicides such as 2,4-D. Thus, the release of chlorophenols as industrial waste or by degradation of chlorinated pesticides have cause serious environmental threat [34]. Yang et al. [35] reported biotransformation and enzymatic responses of 2,4-dichlorophenol in Skeletonema costatum (diatom). They demonstrated that Cytochrome P-450, a key enzyme in biotransformation and metabolization, did not play an important role in 2,4-DCP detoxification.

Popular pest control agents such as chlorinated agrochemicals cause serious environmental problems such as accumulation in non-target organisms as well as in water and soil. Considering the high persistence and toxicity of chlorinated pesticide like lindane, many countries have prohibited its direct application [36]. Thus, there is a requirement of potential microalgal strain for eco-friendly remediation of chlorinated pesticides. Kuritz and Wolk [37] evaluated the lindane degrading potential of cyanobacteria N. ellipsosporum and Anabaena sp. genetically manipulated to biodegrade another contaminant 4-chlorobenzoate. Biodegradation of the pesticide lindane by the cyanobacterial strains Synechococcus sp., Oscillatoria sp., Cyanothece sp., Nodularia sp., Synechococcus sp., Nostoc sp., Microcystis aeruginosa, A. cylindrical, M. aeruginosa, A. spiroides, and A. flos-aquae has been reported [38].

Dosnon-Olette [39] demonstrated the removal of fungicides dimethomorph and pyrimethanil and herbicide isoproturon by the microalgae S. quadricauda and S. obliquus. The study showed that S. quadricauda removed dimethomorph and pyrimethanil more effectively than S. obliquus. Fluroxypyr (pesticide) accumulation and degradation by green alga C. reinhardtii was reported by Zhang [40]. They noted that C. reinhardtii had the potential to degrade more than 57% of bioaccumulated fluroxypyr within 5 days.

Singh et al. [41] demonstrated the potential of the cyanobacterium Synechocystis sp. to biodegrade the organophosphorus 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 (T_1/2) 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].