Phenol is one of the major organic pollutants in wastewater, discharged from many industrial processes such as textile processing, coal, resin synthesis, perfume production, chemical spills, and pharmaceutical manufacture (Sarika et al. 2020). Simple phenol (Fig. 1) has an aromatic ring with six carbons. It has a hydroxyl functional group with electron pairs of the oxygen atom that the hydroxyl functional group conjugated with the aromatic system that causes partial transfer of negative charge from oxygen to the ring and delocalization of the charge, and it additionally strengthens polarization of the O-H bond. Consequently, phenol gains an acidic character and can form toxic molecules (Anku et al. 2017).
The fundamental factor that determines phenol toxicity is their reaction with a cell’s biomolecules and is related to the easiness of donation of free electrons by them from an oxidized substrate. Phenols undergo active transformation by oxidases within cytochrome P450 when they enter after the cell penetration. The effect of their practice is the organization of phenoxy radicals and intermediate metabolites, like semiquinone and quinone methides which interact with bio-molecules in the cell (Fig. 2). Also, phenols can produce ROS, like superoxide radicals or hydrogen peroxide in these reactions. Transformation of phenols leads to the enhancement of toxicity compounds by the organization of electrophilic metabolites that may damage DNA, proteins, lipids, and enzymes (Harris et al. 2020; Perillo et al. 2020). The harmful influence of phenols and their derivatives and their entry into nature leads to acute toxicity, histopathological changes, mutagenicity, carcinogenicity, and death of cells (Schweigert et al. 2001).
Introduced phenol into the surroundings or nature may be an ecological hazard because of its high toxicity, even at low concentrations. The essential factor that determines phenol toxicity is the reactivity of the compound with a cell’s biomolecules and is related with easiness of donation of free electrons by phenol from oxidized substrate. A fish or algae constantly ingests organic particles suspended in water. Some pollutants, such as phenol, may be simultaneously ingested, and these substances can cause poisoning (Anku et al. 2017).
Some species can degrade phenol and toxic compounds (Dhatwalia et al. 2016; Tan et al. 2017; Mansouri et al. 2021) Many aerobic and anaerobic microorganisms in nature can use phenol as a carbon and energy source (Margesin et al. 2004; Pradeep et al. 2015). Essentially, biodegradation of phenol is its biotransformation into simpler metabolites such as H2O, CO2 (aerobic), or CH4 (anaerobic) (Al-Khalid & El-Naas 2012), which this factor occurs in some organisms, but if the content of phenol increases, the oxidation also increases; thus, the growth will be gradually inhibited, and even mortality occurs (Duan et al. 2017).
The oxidation process is harmful to most living organisms, and some organisms produce antioxidants to reduce the harm of free radicals in the cells of animals and plants ( Kapoor et al. 2019; Hasanuzzaman et al. 2020). It is necessary to generate, utilize and develop effective and safer antioxidants, protecting living organisms from the hazardous effects of ROS (Spinedi et al. 2021).
H. pluvialis is a microalga that contains the pigment astaxanthin. Astaxanthin (Fig. 3) has high antioxidant properties that even show antioxidant activities greater than Vitamin C (Rao et al. 2015; Zhao et al. 2019). It has functional groups with two β rings and a hydroxyl at each end (carotenoids - OH) which confers antioxidant properties (Ambati et al. 2014). The bioactive pigments can potentially improve metabolites and growth of alga in different environments (Nakano et al. 2020; Rashidian et al. 2021); Thus, algae probably have due to the pigments which have antioxidant properties an ability to counteract harmful effects of the environment and improve metabolites. For this reason, this research is to investigate the toxic effects of phenol in two time periods (7 and 14 days) to improve metabolites and astaxanthin.