Biotransformation and Volatilization of Arsenic by Three Photosynthetic Cyanobacteria

Abstract

Arsenic (As) is a pervasive and ubiquitous environmental toxin that has created worldwide human health problems. However, there are few studies about how organisms detoxify As. Cyanobacteria are capable of both photolithotrophic growth in the light and heterotrophic growth in the dark and are ubiquitous in soils, aquatic systems, and wetlands. In this study, we investigated As biotransformation in three cyanobacterial species (Microcystis sp. PCC7806, Nostoc sp. PCC7120, and Synechocystis sp. PCC6803). Each accumulated large amounts of As, up to 0.39 g kg 21 dry weight, 0.45 g kg 21 dry weight, and 0.38 g kg 21 dry weight when treated with 100 mM sodium arsenite for 14 d, respectively. Inorganic arsenate and arsenite were the predominant species, with arsenate making up .80% of total As; methylated arsenicals were detected following exposure to higher As concentrations. When treated with arsenate for 6 weeks, cells of each cyanobacterium produced volatile arsenicals. The genes encoding the As(III) S-adenosylmethionine methyltransferase (ArsM) were cloned from these three cyanobacteria. When expressed in an As-hypersensitive strain of Escherichia coli, each conferred resistance to arsenite. Two of the ArsM homologs (SsArsM from Synechocystis sp. PCC6803 and NsArsM from Nostoc sp. PCC7120) were purified and were shown to methylate arsenite in vitro with trimethylarsine as the end product. Given that ArsM homologs are widespread in cyanobacteria, we propose that they play an important role in As biogeochemistry.

Arsenic Speciation and Volatilization from Flooded Paddy Soils Amended with Different Organic Matters

“…In the present study, we found that the As flux of CT was more than 2 times lower than that of DT2, while DMAA in the soil solution of CT was several times higher than that in DT2 ( Table 1), indicating that the transformation from DMAA to TMAs may be the rate-limiting step, which is consistent with a previous finding. 27 The higher concentration of As(III) in the soil solution of CT than the other treatments may result from a higher abundance and activity of arsenatereducing bacteria harboring the arsC gene, as shown in Figure 3c.…”

Effects of microbial processes on the fate of arsenic in paddy soil

“…It has been reported that bacteria, anaerobic archaea and halophiles in soils or sludges can methylate As [60][61][62][63]. Arsenic biomethylation has been observed in numerous cyanobacteria and algae as well [14,57]. Furthermore, methylcobalamin-dependent nonenzymatic methylation of As has been reported for numerous anaerobic prokaryotes [64].…”

“…In particular, autotrophic sulfate-reducing bacteria as well as methanoarchaea, which are abundant in paddy fields [65,66], were suggested to be responsible for this process [63]. Methylated species of As(III) and volatile arsenicals were detected when the cyanobacteria were treated with As(III) or As(V) [57]. Microorganisms play a crucial role in the biogeochemical cycle of As especially for cyanobacteria which are ubiquitous in aquatic environments, including paddy soil, wetland and ocean [67].…”

“…Most living organisms have developed As resistance mechanisms or even make use of As for their ordinary physiology although As is very toxic [55]. Microbial oxidation of As(III) is believed to be a detoxification mechanism of microorganisms because As(V) is less toxic than As(III) [55,57]. On the other hand, As(III) can also be as an electron donor in the process of bacterial metabolism.…”

“…Arsenic methylation occurring in the soil greatly affects the toxicity and fate of As in paddy soils (Figure 1). Widespread ArsM homologues in prokaryotic and eukaryotic microorganisms can catalyze this process, and many methylated intermediates and trimethylarsine as the end are produced in this process [53,[57][58][59]. It has been reported that bacteria, anaerobic archaea and halophiles in soils or sludges can methylate As [60][61][62][63].…”