Abstract
Heavy metal pollution is a worldwide problem with many associated health risks, including bone loss, kidney damage, and several forms of cancer. There is a great need of bioremediation of these toxic metals from the environment, as well as implementing a monitoring system to control the spreading pollution. This study focuses on the bioremediation potential of Rhodobacter sphaeroides in the presence of the toxic gold chloride (AuCl3). Growth characteristics of the bacterial cells exposed to a range of toxic gold concentrations were analyzed through the growth kinetics and the colony forming units under aerobic, photosynthetic, and anaerobic growth conditions. The localization of the gold particles within two cellular fractions, cytoplasm and the plasma membrane, are analyzed using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Results of this study demonstrated the photosynthetic growth condition as best suited for the metal tolerance, compared to the aerobic and anaerobic growth conditions. Results also revealed the overall accumulation and localization of gold particles, while not different between the membrane and the cytoplasmic fractions increased at different concentrations of the gold contamination. The results of the localization under photosynthetic growth condition revealed the accumulation reached the highest very quickly, and an overall shift in localization of the gold particles from an equal distribution to an increase within the membrane fraction at the highest concentrations of gold contamination. The localization of the gold particles was validated by Transmission Electron Microscopy (TEM) where the results confirmed the increase in accumulation within the membrane, and photosynthetic membranes, of R. sphaeroides.
Highlights
The definition of heavy metals has differed over the years, beginning with defining heavy metals as metals with a density of five times greater than water [1] and as metals with densities above 4 - 5 g/cm3 [2] [3]
The localization of the gold particles was validated by Transmission Electron Microscopy (TEM) where the results confirmed the increase in accumulation within the membrane, and photosynthetic membranes, of R. sphaeroides
At the 0.5 μM gold concentration, a higher accumulation of gold particles is seen within the cytoplasmic fraction for the first 72 hours (p value < 0.0001), and an equilibrium between membrane and cytoplasm is reached at 96 hours, and gold starts to get sequestered and/or reduced at higher concentration (p value < 0.05) in the membrane fraction
Summary
The definition of heavy metals has differed over the years, beginning with defining heavy metals as metals with a density of five times greater than water [1] and as metals with densities above 4 - 5 g/cm3 [2] [3]. There are about 30 metals and metalloids within the heavy metals group, including zinc (Zn), mercury (Hg), gold (Au), lead (Pb), cadmium (Cd), copper (Cu), silver (Ag), platinum (Pt), arsenic (As) and chromium (Cr), which have densities that are greater than 5 g/cm3 [4]. Heavy metals may have a nutritional benefit to the organism as cofactors, such as zinc, magnesium, copper, chromium, or nickel, while other metals, such as lead, cadmium, mercury, arsenic, and gold, are not yet identified with any beneficial attributes to the organism [5]. There is a world-wide problem of heavy metal contamination that comes from a variety of different sources These pollutants come from acid rain, the by-products of industrial effluents, gold mines, and metal ions that leach into the soil columns which all have detrimental effects worldwide [7] [8]. Since the metal toxicity is dependent on numerous factors, some metals, such as chromium, cobalt and nickel, play important roles in various in metabolic processes as essential micronutrients, stabilizing molecules [14] and catalysts in enzymatic reactions [15]; they help regulate osmotic balance [16] and are used in redox reactions [17]
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