Stoichiometry and kinetics of mercury uptake by photosynthetic bacteria

Abstract

Mercury adsorption on the cell surface and intracellular uptake by bacteria represent the key first step in the production and accumulation of highly toxic mercury in living organisms. In this work, the biophysical characteristics of the mercury bioaccumulation are studied in intact cells of photosynthetic bacteria by use of analytical (dithizone) assay and physiological photosynthetic markers (pigment content, fluorescence induction and membrane potential) to determine the amount of mercury ions bound to the cell surface and taken up by the cell. It is shown that the Hg(II) uptake mechanism 1) has two kinetically distinguishable components, 2) includes co-opted influx through heavy metal transporters since the slow component is inhibited by Ca 2+ channel blockers, 3) shows complex pH-dependence demonstrating the competition of ligand binding of Hg(II) ions with H + ions (low pH) and high tendency of complex formation of Hg(II) with hydroxyl ions (high pH) and 4) is not a passive but an energy-dependent process as evidenced by light-activation and inhibition by protonophore. Photosynthetic bacteria can accumulate Hg(II) in amounts much (about 10 5 ) greater than their own masses by well defined strong and weak binding sites with equilibrium binding constants in the range of 1 (μM) -1 and 1 (mM) -1 , respectively. The strong binding sites are attributed to sulfhydryl groups as the uptake is blocked by use of sulfhydryl modifying agents and their number is much (two orders of magnitude) smaller than the number of weak binding sites. Biofilms developed by some bacteria (e.g. Rvx. gelatinosus) increase the mercury binding capacity further by a factor of about five. Photosynthetic bacteria in the light act as sponge of Hg(II) and can be potentially used for biomonitoring and bioremediation of mercury contaminated aqueous cultures.2

Mercury exposure and its effects on fertility and pregnancy outcome

“…Contamination of the biosphere by Hg is mainly caused by anthropogenic factors, including coal combustion, mining, cement production and chemical industry . When Hg is released into the natural environment, and in groundwater, microorganisms including bacteria are often responsible for the biotransformation of the metal leading to the formation of methylmercury (MeHg) . Methylmercury and ethylmercury (EtHg) are highly hazardous forms that accumulate in freshwaters, ecosystems and food chains, leading to Hg exposure in people and other natural living organisms .…”

“…[3][4][5] When Hg is released into the natural environment, and in groundwater, microorganisms including bacteria are often responsible for the biotransformation of the metal leading to the formation of methylmercury (MeHg). 6,7 Methylmercury and ethylmercury (EtHg) are highly hazardous forms that accumulate in freshwaters, ecosystems and food chains, leading to Hg exposure in people and other natural living organisms. [8][9][10] Worldwide, global recognition of the great concern caused by Hg pollution led to the adoption of the Minamata Convention on Mercury in 2017.…”

Characterization of mercury(II)-induced inhibition of photochemistry in the reaction center of photosynthetic bacteria

“…Among the possible targets of mercury(II) attack to whole cells (Kis et al 2017) and membranes (chromatophores) of photosynthetic bacteria (Asztalos et al 2012), the photochemistry of the RC protein has proved the most sensitive. The dimer and the secondary quinone are the most vulnerable sites marked by inhibition of the proton uptake and the interquinone electron transfer.…”

“…The amount of Hg 2+ bound to the RC was determined by use of indirect spectrophotometric measurement of the Hg 2+ -dithizone complex (Theraulaz and Thomas 1994;Kis et al 2017). First, all metal ions that were able to form complex with the dithizone and could disturb the mercury(II) assay were removed from the RC solution.…”

“…These studies highlight mercury(II) as a global priority contaminant whose importance as a chemical of major concern to the phototrophs (bacteria and algae) and plants has been underscored by a number of recent publications. They describe the influence of Hg stress on agroecosystem (Zhao et al 2014), the cellular and molecular targets of Hg toxicity in phototrophs (Asztalos et al 2012;Deng et al 2013;Kis et al 2015Kis et al , 2017, and how phototrophs affect Hg cycling (i.e., alteration of Hg redox state, Hg scavenging, potential for methylation) (Gregoire and Poulain 2014). The mercury(II) ion is one of the most toxic agents to photosynthetic bacteria (Giotta et al 2006) and primarily attacks the RC protein (Asztalos et al 2010(Asztalos et al , 2012.…”