Role of Fe plaque on arsenic biotransformation by marine macroalgae.

Macroalgae can cycle arsenic (As) in the environment. In this study, the role of iron (Fe) plaque manipulation at active sites in the As biotransformation mechanism was investigated. The strain of marine macroalgal species, Pyrophia yezoensis, was inoculated in association with arsenate (As(V)) (1.0 µmol L-1) and phosphate (10 µmol L-1) in the medium for 7 days under laboratory-controlled conditions. The Fe plaque was removed by washing the Ti(III)-citrate-EDTA solution before inoculation. The limitation of Fe plaque did not significantly (p > 0.05) affect the chlorophyll fluorescence due to cellular regeneration, which was initiated immediately after washing. However, the speciation and uptake rate of As(V) increased significantly and reduced the inhibitory effect of P on the intracellular uptake of As(V) by P. yezoensis. In the culture medium without Fe plaque, approximately 66% of As(V) was removed with Vmax = 0.32 and Km = 1.92. In the absence of Fe plaque, methylated As species, such as dimethylarsinate (DMAA(V)), was recorded 0.28 µmol L-1, while in the presence of Fe plaque, the value was 0.16 µmol L-1. Inorganic trivalent As (As(III)) was absent in the washed samples; however, 0.53 µmol L-1 concentration of As(III) was still found in the presence of Fe plaque on day 7 of incubation. The results indicated that the absence of Fe plaque promoted higher intracellular uptake of As species, reduced the inhibitory effect of P, mitigated the co-precipitation bond between AsFe plaque and enhanced the detoxification process by DMAA excretion from the cell.

Freshwater phytoplankton: biotransformation of inorganic arsenic to methylarsenic and organoarsenic.

The biotransformation and detoxification mechanisms of arsenic (As) species have been active research topics because of their significance to environmental and human health. Biotransformation of As in phytoplankton has been extensively studied. However, how different growth phases of phytoplankton impact As biotransformation in them remains uncertain. This study investigated the biotransformation of As species in freshwater phytoplankton at different growth phases to ascertain at which growth phase different types of biotransformation occur. At the logarithmic growth phase, arsenate (AsV) (>90%) and arsenite (AsIII) (>80%) predominated in culture media when phytoplankton were exposed to 20 nmol L-1 and 1.0 µmol L-1 of AsV, respectively, and methylarsenic (methylAs) species were not detected in them at all. Intracellular As was mainly present in inorganic forms (iAs) at the logarithmic phase, while substantial amounts of organoarsenic (orgAs) species were detected at the stationary phase. At the stationary phase, AsV comprised the majority of the total As in culture media, followed by AsIII and methylAs, although the methylation of AsV occurred slowly at the stationary phase. Biotransformation of AsV into AsIII and As methylation inside phytoplankton cells occurred mainly at the logarithmic phase, while the biotransformation of As into complex orgAs compounds occurred at the stationary phase. Phytoplankton rapidly released iAs and methylAs species out of their cells at the logarithmic phase, while orgAs mostly remained inside their cells.

Comparative biotransformation and detoxification potential of arsenic by three macroalgae species in seawater: Evidence from laboratory culture studies.

Algae accumulate and metabolize arsenic (As) and facilitate cycling and speciation of As in seawater. The laboratory-controlled macroalgal cultures were exposed to different molar ratios of As(V) and phosphate (P) in seawater for evaluating the uptake and metabolism of As, as a function of As(V) detoxification through biotransformation. Chlorophyll fluorescence of algal species was not significantly affected by the culture conditions (p > 0.05). Addition of 10 µM P positively reduce As stress, but different As(V)/P ratios significantly affect the growth rate (p < 0.05). Algae readily accumulated As(V) after the inoculation, transformed intracellularly, and released gradually into the medium along the incubation period, depending on As(V)/P molar ratios. Reduction and methylation were the leading processes of As(V) metabolism by Pyropia yezoensis, whereas Sargassum patens showed only the reduction. Sargassum horneri reduced As(V) under low level (0.1 µM), but both reduction and methylation were observed under a high level (1 µM). At the end of incubation, 0.17, 0.15, 0.1 µM of reduced metabolite (As[III]) were recorded from 1 µM of As(V)/P containing cultures of Sargassum horneri, Sargassum patens, and Pyropia yezoensis, respectively. On the other hand, 0.024 and 0.28 µM of methylated metabolite (DMAA[V]) were detected under the same culture conditions from Sargassum horneri and Pyropia yezoensis, respectively. The results also indicated that P in medium inhibits the intracellular uptake of As(V) and subsequent extrusion of biotransformed metabolites into the medium. These findings can help to understand the metabolic diversity of macroalgae species on As biogeochemistry in the marine environment.