Dark Reduction of Mercury by Microalgae-Associated Aerobic Bacteria in Marine Environments

Abstract

Redox transformation of mercury (Hg) is critical for Hg exchange at the air−sea interface and it can also affect the methylation of Hg in marine environments. However, the contributions of microalgae and aerobic bacteria in oxic seawater to Hg 2+ reduction are largely unknown. Here, we studied the reduction of Hg 2+ mediated by microalgae and aerobic bacteria in surface marine water and microalgae cultures under dark and sunlight conditions. The comparable reduction rates of Hg 2+ with and without light suggest that dark reduction by biological processes is as important as photochemical reduction in the tested surface marine water and microalgae cultures. The contributions of microalgae, associated free-living aerobic bacteria, and extracellular substances to dark reduction were distinguished and quantified in 7 model microalgae cultures, demonstrating that the associated aerobic bacteria are directly involved in dark Hg 2+ reduction. The aerobic bacteria in the microalgae cultures were isolated and a rapid dark reduction of Hg 2+ followed by a decrease of Hg 0 was observed. The reduction of Hg 2+ and re-oxidation of Hg 0 were demonstrated in aerobic bacteria Alteromonas spp. using double isotope tracing ( 199 Hg 2+ and 201 Hg 0 ). These findings highlight the importance of algae-associated aerobic bacteria in Hg transformation in oxic marine water.

Superoxide-Mediated Extracellular Mercury Reduction by Aerobic Marine Bacterium Alteromonas sp. KD01

“…According to our previous study, the decrease of purgeable Hg(0) during Alteromonas sp. KD01 incubation is mediated by nonenzymatic components (i.e., extracellular polymeric substances) of cells . Additionally, previous reports proposed that Hg(0) oxidation may be controlled by thiol functional groups via oxidative complexation. , For comparison, Cu 2+ and Zn 2+ were further added to probe the role of superoxide.…”

“…Immediately after the addition of Hg­(II), the solution was purged with N 2 for 10 min, and the sample was analyzed for Hg(0) to obtain a reference value for the sample at 0 h. Hg­(II) reduction is expressed as a percentage of purgeable Hg(0) to the spiked total Hg­(II). In the case of mass balance analysis, the total Hg remaining in the sample was determined by CVAFS following oxidation by aqua regia . The recoveries for mass balance ranged from 87 to 125%, which were within the acceptable ranges of the EPA 1631 method (70–130% for total Hg (THg)).…”

“…(named Alteromonas sp. KD01 in this work), 19 which genus is abundant in diverse marine environments, including sediments 23 and seawater. 19,24 The observed rapid reduction of Hg(II) by Alteromonas sp.…”

“…Reduction of divalent mercury (Hg­(II)) plays a crucial role in the exchange of elemental mercury (Hg(0)) at the air–water and air-soil interfaces, which also controls microbial uptake and methylation of Hg. − Other than the chemical reduction processes (e.g., organic matter-mediated photochemical process, − Fe­(II)-mediated Hg­(II) reduction , ), diverse microorganisms are also capable of promoting the reduction of Hg­(II). , Microbial Hg­(II) dark reduction is known to be equally or more important than photochemical Hg­(II) reduction in surface freshwater and seawater. − …”

Soil Microbial Community Composition and Tolerance to Contaminants in an Urban Brownfield Site

“…Alteromonas sp. from marine environments can uptake mercury and two species from the Red Sea can remove a range of heavy metals from aqueous solution [83][84][85].…”