Environmental and Agricultural Microbiology. Группа авторов

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Environmental and Agricultural Microbiology - Группа авторов


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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.

      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

      2.3.2 Green Algae

      The smaller freshwater green algae Pseudokirchneriella subcapitata, formerly known as Selenastrum capricornutum Pintz, amplifies metal binding sites, leading to an increase in bioaccumulation and consequential increase the capacity to accumulate chromium [51]. Spirogyra sp. was found to be a cost effective and eco-friendly biosorbent while studied using different concentrations of chromium (1.0, 5.0, 15.0, and 25.0 mg L−1), different dosages of dead algal biomass (0.1, 0.2, and 0.3g) with variation time, pH, and temperature [52]. Chatterjee and Abraham, 2015 [53] observed maximum biosorption in the dried biomass of the Spirogyra sp. (2.5 g L−1) at pH 6.0 when it was treated with 10 mg L−1 chromium concentration for one hour. Sphaeroplea sp. was treated with different chromium concentration with variation in time period, in its natural and acid treated form to study the biosorption capacity. Maximum result was observed at pH 5.0 in the acid treated alga (158.9 mg g−1) than its natural form (29.85 mg g−1) [54].

      2.3.3 Diatoms

      Sbihi et al., (2012) [56] have observed maximum Cr(VI) biosorption capacity 93.45 mg g−1 of Cr(VI) in Planothidium lanceolatum at a concentration of 0.4-g dried diatoms per liter with a Cr(VI) concentration of 20 mg L−1. Hence, it was proved to be a potent microalga for biosorption of hexavalent chromium.

Schematic illustration of the mechanism of Cr(VI) reduction through micro-algal biomass.

      Except extracellular chromium reduction, intracellular reduction can also be taken as a major mechanism. Algal cells are found to be better source of Cr(VI) reduction, so there must be presence of Cr(VI) reducing enzymes in their cells like bacteria and fungi. The protoplasm of these cells contains some components such as NADH, proteins, low molecular weight carbohydrates, fatty acids, amino acids, and flavoproteins which can completely reduce Cr(VI) to Cr(III). Generally, in chromium-rich region an oxidative stress condition is created inside the cell leading to the generation of several harmful reactive


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