![]() ![]() MTs are currently classified in 15 evolutionarily unrelated families ( Capdevila and Atrian, 2011), and most information on phylogeny and structure of metal-binding domains has been elucidated in deep detail for plant MTs ( Leszczyszyn et al., 2013). The structure and function of both MTs and PCs in terrestrial plants, along with mechanisms for HM sequestration and compartmentalization, have been described in detail by Cobbett and Goldsbrough (2002). This includes both polypeptides directly chelating HMs, such as metallothioneins (MTs) and phytochelatins (PCs), as well as compounds biosynthesized to decrease the oxidative stress induced by HMs ( Cobbett and Goldsbrough, 2002). Massive sequencing of microalgal genomes and transcriptomes ( Keeling et al., 2014 Blaby-Haas and Merchant, 2019), carried out in the last decade, provided scientists with a plethora of information available on potential proteins and biosynthetic pathways involved in HM detoxification. Furthermore, the presence of organelles in which HMs can be enclosed, limiting their detrimental effects to cellular metabolism, makes microbial eukaryotes more suitable for HM remediation compared to photosynthetic bacteria. The far greater genetic, enzymatic, and chemical diversity found in microalgae, compared to terrestrial plants, animals, or fungi ( Keeling, 2013), coupled with the ability of microalgae to grow with sunlight and inorganic nutrients, makes photosynthetic microorganisms the best candidates for biotechnological applications and HM remediation. Many organic ligands and metal-binding proteins from microalgae are thus likely to be unknown, and some of them might reveal useful for phytoremediation or other biotechnological applications. Since most studies focus on plants, animals, yeasts, and bacteria, a significant proportion of microalgal metabolites are unknown or uncharacterized to date. ![]() This leads to a greater diversity in primary and secondary metabolites including organic ligands and metal-binding proteins for microalgae. ![]() Two main naturally occurring metal-removal processes are currently under investigation in biotechnological research: passive sorption onto algal biomass ( de-Bashan and Bashan, 2010) and active sequestration and transport by specific ligands ( Monteiro et al., 2011).Įukaryotic microalgae are present in six different supergroups (Archaeplastida, Hacrobia, Rhizaria, Excavata, Alveolata, and Heterokontophyta), resulting far more genetically diverse than either terrestrial plants (Archaeplastida), or fungi and animals (Opisthokonta). Biotechnological strategies to improve the removal of HMs from polluted waters made significant progress, although large-scale applications of microalgae-based HM-remediation are still not feasible ( Kumar et al., 2015). Eukaryotic microalgae appear to be particularly suitable to use in bioremediation since they require only sunlight and inorganic nutrients for their growth, can achieve fast growth rates, and are able to compartmentalize heavy metals (HMs) within specific organelles. The ability of some microbes to thrive in heavy metal-polluted environments is attracting the interest of the biotechnological industry. In spite of the broad metabolic and chemical diversity of microalgae that are currently receiving increasing attention by biotechnological research, knowledge on MTs and PCs from these organisms is still limited to date. ![]() The present review reports the current knowledge on microalgal MTs and PCs and describes the state of art of their use for HM bioremediation and other putative biotechnological applications, also emphasizing on techniques aimed at increasing the cellular concentrations of MTs and PCs. A number of techniques, including genetic engineering, focus on increasing the biosynthesis of MTs and PCs in microalgae. Metal-binding peptides include genetically encoded metallothioneins (MTs) and enzymatically produced phytochelatins (PCs). Strategies used by microalgae to minimize HM toxicity include the biosynthesis of metal-binding peptides that chelate metal cations inhibiting their activity. HMs enter cells through transporter proteins and can bind to enzymes and nucleic acids interfering with their functioning. The persistence of heavy metals (HMs) in the environment causes adverse effects to all living organisms HMs accumulate along the food chain affecting different levels of biological organizations, from cells to tissues. ![]()
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