Introduction
The rapidly growing industrialization of the world has an impact on the quality of water, food, feed, and weather as results of high amounts of toxic substances released by various industries to the environment (1, 2). Also, pesticides and chemical composts in agriculture as well as vehicles for transportation cause to separate large quantities of pollutants in atmosphere (3); The heavy metals as a main kind of these pollutants considerably threaten the food safety because of their accumulation in water and food (4-9), and consequently, affecting human organs and tissues (1-3).
Heavy metals are classified into three categories including: toxic metals, precious metals and radionuclides. One of the toxic metals, with a significant role in water pollution is zinc (6, 10, 11). This essential element is used in many growing economic sectors, and thereby, large quantities of this heavy metal discharge into the industrial effluents (12-15). Zinc as an essential element, has many important biological impact such as: being as a major ion in structure of motifs, act as a catalytic factor of enzymes (16), essential role in the structure and function of nucleic acid and protein along with accompanying in gene expression and immune system development(17, 18). So, deficiency of zinc contributes to the impaired function of innate immune response such as phagocytosis, cytokine secretion, neutrophils and natural killer cells functions. Also, zinc deficiency impacts on impaired antibody secretion, thymic atrophy and lymphopenia (19).
Microorganisms, like humans, are dependent on appropriate levels of zinc for their vital reactions. For example; in yeast about 8% of the proteome is thought to bind zinc (19, 20) and about 400 genes involved in growth are zinc-dependent (20, 21). Several investigations have been conducted mainly, in Saccharomyces cerevisiae, as a yeast, suggesting the requirement of zinc homeostasis in growth and metabolism (20). There are two main classes of eukaryotic zinc transporters in S. cerevisiae . The Zrt1, Zrt2 and Zrt3 are classified in ZIP proteins (ZRT-IRT-like protein) (19, 22, as well as, Zrc1, Cot1, Msc2 and Zrg17 that classified in cation diffusion facilitator (23, 24). In the case of severe zinc limitation, extracellular uptake of zinc in S. cerevisiae mediated by the high-affinity Zrt1 protein which contributes to increasing of the ZRT1 expression up to 30-fold. Moreover, this microorganism can partially adopt with conditions of low zinc by expression of Fet4, a low-affinity metal transporter, to import zinc, iron, and copper into the cell (19).
Biosorption strategies have been considered for many years to solve the problem of heavy metal pollution and consequently environmental remediation. Among all living organisms, applied to absorb toxic elements, microorganisms have attracted increasing attention, because they have safety in human aspects and are easier to work with (25-27). Despite its mediocre capacity, the S. cerevisiae is a unique biomaterial in heavy metals biosorption (28) and due to easy cultivation in a large scale and manipulation at the molecular level as well as high biomass production, makes the S. cerevisiae to be administered in biomaterial approaches (11). Based on these considerations, our investigation was conducted for study of yeasts with emphasis onS. cerevisiae as an industrially important strain, isolated from industrial effluents, for biosorption of zinc and the applied the produced biomass as zinc-enriched single cell protein (SCP) in feed and food production.