(2.1)
(2.2)
2.2 Effects of Hexavalent Chromium Toxicity
2.2.1 Toxicity to Microorganisms
Chronic exposure to hexavalent chromium has many deleterious effects on the structure and function of the microbial cells, and in some cases, it also causes dormancy. It leads to species loss and disturbs the diversity. Growth of Scenedesmus acutus was inhibited when it treated with more than 15 ppm of hexavalent chromium [14]. Spirogyra sp. and Mougeotia sp. were found forming Cr(V) while exposed to Cr(VI) [15]. The lag growth phase of Euglena gracilis was lengthened when treated with Cr(VI) and motility was also lost due to the modifications in the cytoskeleton induced by Cr(VI) [16]. It was reported that the photosynthesis was inhibited due to the presence of Cr in the cells of Scenedesmus sp. and Chlorella sp. [17, 18]. The sulfate transport system mediated transport of chromate ions has diverse toxic effects in the cytoplasm of Salmonella typhimurium, Alkaligenes eutrophus, Escherichia coli, and Pseudomonas fluorescens. According to Viamajala et al., (2002) [19], the minimum concentration of Cr (VI) (0.015 mM) has slowed down the growth of Shewanella oneidensis. The reduction of growth was observed in mycelium of fungi due to the toxic effect of hexavalent chromium. Interference of chromium causes gene mutation and conversion which further lead to growth inhibition in fungal cell [16].
2.2.2 Toxicity to Plant Body
Hexavalent chromium diffuses across the cell membrane due to the structural resemblance of chromate ions to phosphate or sulphate. It can easily enter inside the cell and where the reduction takes place producing Cr(V) and then Cr(III) reactive oxygen species and free radicals [20]. Cr(III) is impermeable, so unable to cross the cellular membrane and prefers to bind the protein molecules available on the membrane surface with greater affinity causing DNA damage, inhibition of DNA replication, and RNA transcription [21]. Plant growth, development, and plant physiology (mineral nutrition, water relations, and photosynthesis) are greatly affected by hexavalent chromium [22]. The amount of chlorophyll (Chl) content, nitrate reductase activity, and δ-aminolevulinic acid contents were also reduced in plants growing in chromium contaminated soil [23]. Hexavalent chromium induces the inhibition of photosynthesis rate in terms of CO2 fixation, electron transport processes, enzyme activities, and photophosphorylation in plants [24, 25]. Bishnoi et al., (1993) [26] has observed that Cr(VI) was influencing the PS I and PS II by isolating the chloroplasts from peas. The direct effect of Cr exposure has also been found on enzymes or other metabolites that may cause increased oxidative stress and lipid peroxidation [27–29]. Consequently, herein, we can conclude three key roles of Cr on plants as follows:
1 (i) Production of a new metabolites to change the metabolic pool which would providetolerance of Cr stress (e.g., phytochelatins and histidine) [30].
2 (ii) Variation of the production in several pigments (like chlorophyll and anthocyanin) for the sustenance of plants [31].
3 (iii) Cr stress induces the production of metabolites like glutathione and ascorbic acid which may cause damage to the plants [32, 33].
2.2.3 Toxicity to Animals
People those are directly exposed to chromium show nasal irritation, perforation of the nasal septum, nasal ulcers, “chrome holes” [34], and hypersensitivity reactions in the skin. But some other cases reported that the normal people who are not practically exposed to chromium but ingested chromium through food and water show deposition of chromium in different organelles like kidney, adrenals, lungs, liver, spleen, plasma, bone marrow, and red blood cells in due to low pH of the stomach. Ingestion of Cr(VI) poses a significant carcinogenic risk because of the solubility of particulate chromate at low pH which is weakly carcinogenic to the lungs [34]. Enduring exposure of low level of Cr(VI) between 4 and 25 ppm to skin can cause a long lasting sensitisation that leads allergic contact dermatitis (ACD) while 20 to 25ppm of Cr(VI) can cause inflammation, eczema, and open sores (ulcers) [35]. Similarly, there are some significant observations of Cr(VI) dusts exposure [36, 37]. According to these reports, inhalation of even only 2 μg of Cr(VI) dust leads irritation of nose, throat, and lungs along with respiratory inflammation, nosebleeds, ulceration, and perforation (holes) in the septum when come in contact with 0.09μg of Cr(VI). Some noteworthy observations were also documented in a group of women who were exposed to industrial chromium contamination showed irregularity in menstruation cycle, birth complications, and increases in post-birth haemorrhage [38, 39]. A remarkable study revealed that symptoms like mouth sores, diarrhoea, stomach pains, indigestion, vomiting, and higher levels of white blood cells were found when a group of individuals were exposed to approximately in drinking water that contaminated by a ferrochrome plant [40]. According to the survey of US EPA (Environment Protection Agency) in 1998, it was observed that the contamination of drinking water with 20,000 μg L−1 of Cr(VI) caused many diseases like mouth sores, vomiting, indigestion and diarrhoea [41]. Men exposed to chromium released from welding fumes exhibited toxicity in testes and blood, increased semen abnormalities, and reduced sperm concentrations [42]. It has explained when adult female rats take Cr(VI) contaminated drinking water; it is found to be toxic to the ovaries. It damages the ovarian tissues, reduces the number of follicles and ovum which ultimately, increases the chances of infertility. In mice, it has been observed that Cr(VI) is toxic to foetus, embryos (250, 500, and 750 mg L−1) and also increases skeletal abnormalities (250 and 500 mg L−1) [43]. Cr(VI) concentrations at 100, 200, and 400 mg L−1 was found to be toxic to reproductive organs, changed endocrine organ weight, testis enzymes levels and sperms when given to male monkeys through drinking water [44, 45].
The summary of hexavalent chromium effects optimistically made us to find out a significant bio-remediating agent to convert it to non-toxic form which would be cost-effective, easily available, and without any side effects. Herein, we can deliberate the microbes as an alternative of chemical agents. Numbers of reports are proposed basing upon the chromium removal strategy with strains of bacteria, fungi, virus, microalgae, and seaweeds. But in this present piece of work, emphasis has been given on microalgae as a potent source of bioremediation.
2.3 Chromium Bioremediation by Microalgae
Microalgae play an important role in the chromium bioremediation. Biosorption is a method of bioremediation where sorption is taking place either by using dead or living biomass, and it has various significant advantages as follows:
1 (i) High efficiency in eliminating heavy metals even from very low concentrations
2 (ii) Cost effective
3 (iii) High metal adsorbing capacity
4 (iv) The ability of recovering the important metals adsorbed
Algal cells are considered as natural ion-exchange matter as they contain various anionic groups on their surface and this allows them to eliminate heavy metal ions efficiently [46, 47]. It has been observed that various strains of algae like blue-green algae, green algae, red algae, and diatoms are able to remove hexavalent chromium from soil and water.
2.3.1 Cyanobacteria
According to Elhaddad and Mahmoud (2015) [48], a blue-green alga Spirulina platensis acts as a good biosorbent that help in reducing hexavalent chromium. When its chromium reduction efficiency was studied at different pH (1.0 to 7.0), at different time period (5, 10, 20, 30, 60,
