Element pattern in two dominant species of seaweed from Betsukari coastline - Mashike, Hokkaido, Japan.

The present study investigated the monthly of element accumulation in seaweeds. Patterns of As, Ba, Cd, Cu, Fe, Mn, Pb, and Zn concentrations in dominant species of Phaeophyceae and Rhodophyceae, namely Saccharina japonica and Pterocladiella tenuis respectively, collected from the Betsukari coastline-Mashike, Hokkaido, Japan, were investigated. Our results indicated that element accumulation was more related to specific seaweed species than to their supply in seawater concentration. S. japonica was found to be an accumulator of As, whereas P. tenuis notably accumulated Mn. The accumulation of specific elements also affects the coupled patterns between closely related elements. The monthly pattern of Cd was similar to that of As in S. japonica, and it is an element with unknown biological function in the seaweed. The monthly accumulation pattern of Fe and Mn, a well-known closely related element that forms the extracellular surface in seaweed, was found to be similar in P. tenuis. A similar transport mechanism affected the antagonistic pattern of Cd and Zn accumulation in S. japonica. Our data can be employed in the assessment of biomonitoring of element cycles in the environment.

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: Salinity stress on arsenic biotransformation.

Salinity stress affects aquatic microalgal growth and their physiological responses have been studied extensively. However, arsenic (As) accumulation and biotransformation by freshwater phytoplankton under a salinity gradient have never been addressed. This study reports a distinctive pattern of As uptake, accumulation, and biotransformation by four axenic freshwater phytoplankton species, i.e., Scenedesmus acutus, Closterium aciculare, Staurastrum paradoxum, and Pediastrum duplex. Phytoplankton cells were incubated in sterilised C medium modified with varying salinity levels (0-5‰) in association with arsenate and phosphate concentrations. The biotransformation of arsenate (i.e., As(V)) to arsenite (As(III)) and to further methylated species decreased with increasing salinity in the culture medium whereas As accumulation increased. Among the four strains, only S. acutus and S. paradoxum converted As(V) to As(III), with no detected methylated species. In contrast, C. aciculare and P. duplex biotransformed As(V) to As(III) and further to methyl arsenic species, such as DMAA. S. acutus and S. paradoxum exhibited higher accumulation tendency than the other two species. S. paradoxum showed the lowest As reduction rate (i.e., As(V) to As(III)) compared to other species, although, without significant variations. The morphological changes were observed in phytoplankton cells in response to increased salinity stress. Moreover, As(V) concentrations in the culture medium significantly decreased by day 7-14. Thus, this study presents a conceptual model of the As biotransformation pattern by axenic freshwater phytoplankton.

Integrated effects of important environmental factors on arsenic biotransformation and photosynthetic efficiency by marine microalgae.

Microalgae play an important role in arsenic (As) bioaccumulation and biogeochemical cycling in marine ecosystems. Marine microalgal growth and As biotransformation processes depend on environmental factors, including salinity, temperature, and nutrient concentrations, and data in this regard are available in the literature. However, research on the integrated effects of environmental factors on marine diatom species remains scarce and unclear. Herein, salinity and temperature are both considered in combination to investigate their influence on As uptake, biotransformation, and photosynthetic efficiency (PE). Two strains of marine diatom species, Asteroplanus karianus and Skeletonema sp., were cultured in an f/2-based nutrient medium. Microalgae were cultured under various temperatures (5.0, 20, and 35 °C) and salinities (1.0‰, 10‰, 25‰, and 40‰) in association with As and phosphate-enriched (1.0 µmol L-1 of As(V) + 10 µmol L-1 of PO43-) or deficient (20 nmol L-1 of As(V) + 1.0 µmol L-1 of PO43-) conditions. For both species, maximum growth, As accumulation, biotransformation, and PE were recorded at 10 and 14 day of culture. Microalgal growth, As accumulation, biotransformation, and PE were maximum at 20 °C with salinities of 10‰ and 20‰. Cell shape was also observed to be good at optimal at this temperature (20 °C) and range of salinity (10‰ and 20‰). A conceptual model of integrated effects of environmental factors on growth and As accumulation and biotransformation activities by these marine microalgae has been proposed. This study contributed to the elucidation of the relationship between environmental factors and As biotransformation mechanisms, which may further provide significant insight about As remediation processes.