INTRODUCTION Apart from this, two evolutionarily distinct groups of bacteria have been capable of biomineralization: 1) Several species of the cyanobacterium phyla, 2) Some Achromatium species in the Gammaproteobacterial class.Achromatism oxaliferum - a sulfur-producing bacteria, was the first bacteria found to have biomineralization properties and was describedby Schewiakoff in 1893. In nature, two pathways of the biomineralization process have been described: Specific gene-mediated biomineralization pathways in which genes, various cell structures, and various transcriptomes are involved (Dhami et al., 2013; Take et al., 2019). The second is biologically induced mineralization, in which mineral precipitation outside the body of organisms occurs due to the chemical shift in the environment of the microorganisms (Marin et al., 2004; Anbu et al., 2016; Görgen et al., 2021). The process of biomineralization is focused not only on prokaryotes but also on eukaryotic organisms that also secrete large amounts of minerals (Vaghela and Pitroda 2019). However, the molecular mechanisms involved in that process remain a mystery among the investigators. Apart from that, Microbial induced calcium carbonate precipitation (MICP) which is an extension of the biomineralization process, has a wide range of potential applications in various fields, and some of the applications are listed below: 1. Removal of heavy metals and radionucleotide from the groundwater 2. Sequestration of atmospheric carbon dioxide through chemical fixation of CO2 in the form of calcite, aragonite, dolomite, and magnesite 3. Biomineralized carbonate is used in construction materials 4. Removal of calcium and polychlorinated biphenyls 5. MICCP technique has been recently reported toproduce a material used as filler in rubber and plastics, fluorescent particles in stationery ink, and a fluorescentmarker (Yoshida et al., 2010). Biomineralization is a widespread phenomenon occurring in many organisms, including marine invertebrates, echinoderms, various species of molluscs, and magnetotactic bacteria (MTB) (Lefèvre and Bazylinski, 2013). The biominerals synthesized by these organisms have the process of accumulation of multiple minerals inside and outside of the organism. Secreted biominerals have various functions like body structure maintenance, which plays crucial roles in magnetic and gravity sensing and the storage of biominerals(Aggarwal et al., 2021). The process of biomineralization is an enzyme-mediated metabolic pathway that involves a large set of genes in different organisms. PhoK, PhoN, Salmonella Enterica, Urease gene of the Sporosarcina pasteurii, Isotig 02195, and Isotig00817 gene of Pinctada fucata are the names of a few genes that involve in biomineralization. Various bioinformatics tools have been employed to analyze the sequence of all these genes. Tools used are Blast, KEGG pathway database, Tremble (UniprotKB), Clustalw, and the Go annotations.Go Annotations of the uniport are used to analyze the protein transcriptomes of different cellular functions, molecular interaction, and subcellular localization. CaCO3 crystal accumulation in MTB has been further analyzed using microscopic techniques like TEM and SEM(Liu et al., 2021). The sequence analysis of the genome is done by using various sequencing techniques, like the shotgun technique and next-generation sequencing technique. Both eukaryotes and prokaryotes synthesize a wide range of biominerals, and each molecular crystal has its specific function in the organisms. The functions of a few organic crystals can be summarized in Table 1. Apart from the organisms mentioned above in the table, mollusks and arthropods also secrete various chemicals. Molluscs exoskeleton has almost 95% calcite or carbonate and argonauts, providing toughness and strength. All members of arthropods which make the largest phylum of the animal kingdom, also have a tough and rigid exoskeleton. This hard covering protects against harsh environmental conditions and resistance to desiccation (Colwell et al., 2005). Chitin, polysaccharides, structural proteins, and inorganic minerals such as carbonate and calcium (Bachmeier et al., 2002). Therefore, the current eview intends to comprehend the information about biomineralization and metagenomic approach. And also discusses about the potential usage of metagenomics in biomineralization and bioremediation. FUNDAMENTAL UNDERSTANDING OF METAGENOMICS Metagenomics is a new emerging domain of environmental biotechnology that allows us to extract DNA from non-culturable microbes and further use it to study and express novel genes, enzymes, and its application in medicine, environmental biodegradation preparation of metagenomic libraries, and biosurfactants. In 1985, Pace and his colleagues were the first individuals who proposed the idea of cloning DNA samples directly from the environment. The DNA obtained from Environmental samples is identified as a metagenome and is widely used for discovering new species, Studying various cellular and metabolic pathways (Alves et al., 2018). Multiple studies suggest that the microbial metagenomic community can act as a potential biomarker for identifyingvarious air, water, and soil pollution (Kisand et al., 2012). It has been observed that the accumulation of certain pollutants in the environment triggers the expression of specific microbial genes. For example, if any niche has cyclo- hexane accumulation, then it induces the expression of the cyclohexane degrading enzyme (Techtmann et al., 2016). Apart from this, various other bioactive compounds, bioremediation bacteria, and other enzymes with potential environmental pollution degradation activity have been identified and analyzed through a metagenomic approach. Carboxylesterase, monooxygenase, laccases, Esterases, phenol degrading enzymes, polyaromatichydrolyzing enzymes, etc. are a few enzymes that have been discovered and analyzed through a metagenomic approach. (Ufarte et al., 2015) The gene content and organization of gene clusters in MTB bacteria have also been identified and analyzed through the environmental DNA sample and metagenomic approach applying the shotgun DNA sequencing technique. 1. The sequential steps one should follow during handling non-culturable microorganisms. 2. Selection of metagenomic samples directly from the environment containing a mix of microbes. 3. Genomic DNA Isolation 4. Construction of the metagenomic library containing a large no of short DNA sequence 5. Sequence information from these libraries 6. Identification of a metabolic pathway 7. Species identification through comparative analysis. 8. Identification of the new coding sequence 9. Genome Annotation 10. Development of a bioinformatics tool and server for storing these sequences and analyzing them. The schematic representation of different metagenomic approaches used to understand microbial niches is illustrated in Fig. 1. LIMITATIONS OF THE METAGENOMIC APPROACH IN THE LIGHT OF BIOREMEDIATION & BIOMINERALIZATION There are various disadvantages to using the metagenomic approach for studying genes and their synthesized enzymes. (Alves et al., 2018). In metagenomic DNA, there are fewer genes available that synthesize enzymes. And these Enzymes are not worked very well in environmental conditions, which further makes it difficult to understand their potential environmental pollution marker and their application in biomineralization and bioremediation. Various enzymes do not show any expression in the express expression vector like E. coli. CONCEPT OF THE METAGENOMIC LIBRARY There have been many studies done in the areas of aquatic systems, soil microbiota, and plant microbiota to understand the significance of the construction of the metagenomic library and its screening of a particular gene or an enzyme. Apart from this metagenomic DNA library from the soil or aquatic microbiota have many the functional gene from the unculturable bacteria, further opening the way for discovering various novel metabolites (Daniel et al., 2007). 99% of the soil bacteria cannot be cultivated under laboratory conditions which further indicates microbial diversity and the Nobel functional insert in the metagenomic library (Courtois et al., 2003). The construction of metagenomic libraries plays an essential role in functional genomics, allowingus to study the function of a particular gene involving DNA extraction from the soil sample, expressing the genes in the host, and further screening and checking its enzymatic activity. GENOMIC DNA ISOLATION PROTOCOL The reagent's requirements and their respective concentrations will differ for the different organisms. The source from which extract genomic DNA mainly includes soil bacteria, plants, fungi, and mammalian cell lines. — Bacterial Genomic DNA isolation: 0.2% of SDS used along with lysozyme enzyme responsible for the breakdown of the peptidoglycan layer of the cell wall. — Animal Genomic DNA isolation: Macrozymes are used as cell lysis and tissue buffer along with 0.1% of SDS detergent. — Fungi genomic DNA isolation- 1% of SDS along with Zymolyase T, the mixture of different types of chitinase enzymes. — Plant genomic DNA Extraction - Cellulase enzyme is used to break the cellulose component of the cell wall. — Humic Acid Removal methods used in Metagenomic DNA extraction protocol. Various contaminants like urea, humic acid, and polysaccharides are present in the soil with similar solubility propertiesto DNA. All these contaminants interfere with DNA isolation and analysis. Humic acids are not easily removed through classical DNA extraction protocols such as Phenol extraction. Various methods are applied to remove the humic acid from the soil sample,including microwave-based methods (Orsisni et al., 2001) and bead beading lysis (Miller et al., 1999). Besides these methods, cesium chloride density centrifugation (Leff et al., 1995) and hexadecyltrimethylammonium bromide (Cho et al., 1996) also shows a very good amount of DNA on the agarose gel, which is free from the humic acid. Moreover, Aluminum sulfate is also used to remove the humic acid from the soil. CONNECTING THE LINK BETWEEN BIOMINERALIZATION AND METAGENOMIC The metagenomic approach has been studied that in modern microbialites the non-cyanobacterial lineage plays a crucial role in Calcium carbonate precipitation. The 16s and 18s rRNA genes collected from metagenomic samples and sequencing through Illumina suggest the presence of broad diversity of bacteria and eukaryotes and a minor presence of archaea (Couradeau et al., 2013). Also, the Cyanobacteria belonging to order pleurocapsales are present in abundant numbers in deeper sections of microbialites (Couradeau et al., 2013). The metagenomic libraries of the Magnetotactic bacteria such as Magnetospirillum gryphiswaldense and freshwater magneto spirillum suggest that genes controlling the Magento tactic property are located within four significant operons (mamAB, mamGFDC, mms6, and mamXY). (Fukuda et al., 2006). In the recent study, deletion of the operon region of the mamGFDC and mms6 responsible for the reduction and various types of size and morphological defects of magnetite crystals, and further if anyhow the Magnetotactic bacteria get devoid of mamAB operon then it will have resulted in the loss entire magnetite crystals. (Muruat et al., 2010). HEAVY METAL TOXICITY AND BIOREMEDIATION THROUGH BIOMINERALIZATION Heavy metals are defined as metallic elements having a high molecular weight and having a specific density that is five times greater than water (Fergusson et al., 1999). The excessive use of heavy metals in numerous areas such as the industry, domestic, agriculture, and medical application have created an adverse impact on human health and metabolisms and environmental implications. (Bradl et al., 2000). There are various heavy metals such as copper (Cu), nickel (Ni), iron (Fe), chromium (Cr), magnesium (Mg), and Selenium (Se) are very useful and essential for numerous metabolic pathways, but exposure to some other heavy metal such as cadmium (Cd), lead (Pb), Chromium (Cr) and arsenic (As) has a very negative impact on the health of living organisms (Jen et al., 2017; Silva et al., 2005). After understanding the harmful effect of heavy metals on public health and the environment, removing these toxic elements from the environment becomes critical and essential. Remediation of the heavy metals from the groundwater through radionuclides in recent times has become a very effective measure to remove the various heavy metals from the groundwater. (Fujita et al., 2004). Through biomineralization, the radionuclide and the contaminant metals are incorporated into the calcite lattice via a competitive co-precipitation reaction. 90Sr2+ + OH- + HCO-3 = SrCO3 (s) + H2O The calcium ions are substituted into the mineral precipitate, and cations and radionuclides integrate into the calcite structure, which further forms the strontium carbonate with low solubility in the water.