Exploring phosphate impact on arsenate uptake and distribution in freshwater phytoplankton: Insights from single-cell ICP-MS.

In this study, we investigated the interaction between arsenate (AsV) and phosphate (PO43-) in freshwater phytoplankton using single-cell inductively coupled plasma mass spectrometry (SC-ICP-MS). This study aimed to elucidate the influence of varying PO43- concentrations on arsenic (As) uptake and distribution at the single-cell level, providing insights into intraspecies diversity. Two species of freshwater phytoplanktons, Scenedesmus acutus and Pediastrum duplex, were cultured under different concentrations of PO43- and AsV in a controlled laboratory environment. Scenedesmus acutus, a species with strong salt tolerance, and Pediastrum duplex, known for its weak salt tolerance, were selected based on their contrasting behaviors in previous studies. SC-ICP-MS revealed non-uniform uptake of As by individual phytoplankton cells, with distinct variations in response to PO43- availability. Arsenic uptake by both species declined with a high PO43- level after 7 days of exposure. However, after 14 days, As uptake increased in S. acutus with higher PO43- concentrations, but decreased in P. duplex. Moreover, our findings revealed differences in cell morphology and membrane integrity between the two species in response to AsV and various PO43- concentrations. S. acutus maintained cell integrity under all experimental culture conditions, whereas P. duplex experienced cell lysis at elevated AsV and PO43- concentrations. This study highlights the varying responses of freshwater phytoplankton to changes in AsV and PO43- levels and underscores the advantages of SC-ICP-MS over conventional ICP-MS in providing detailed, cellular level insights. These findings are crucial for understanding and managing As pollution in aquatic ecosystems.

Effect of salinity on arsenic uptake, biotransformation, and time-dependent speciation pattern by Sargassum species.

The arsenic (As) content of seaweed has been extensively studied due to its toxicological concerns. As a primary producer, seaweed plays a vital role in the biochemical cycling of As in marine environments. Several studies have focused on the growth and behavior of seaweed under a salinity gradient; however, information related to the impact of salinity on As uptake, biotransformation mechanism, and time-dependent speciation patterns of these plants is limited. This study aimed to investigate the temporal effects of salinity on these factors in seaweed. Three seaweed species, Sargassum fusiforme, Sargassum thunbergii, and Sargassum horneri, were maintained in a 1% Provasoli-enriched seawater medium for 14 d under 5‰, 15‰, 25‰, and 34‰ salinities. The results revealed that the high salinity media promoted a rapid uptake of As by all three species. Arsenic accumulation inside the cell approached 100% within seven days of culture for S. thunbergii, irrespective of the salinity content of the media. In addition, As(V) biotransformation and release by S. fusiforme and S. thunbergii were time-dependent, while S. horneri released dimethylarsinic acid (DMAA) from day 3 of the culture. All seaweed species showed methylation of As(V) to DMAA during the culture period. Furthermore, S. thunbergii released DMAA when As(V) was completely depleted from the culture media, whereas the release by S. fusiforme and S. horneri was relatively earlier than that of S. thunbergii. S. horneri showed minimal tolerance to low salinity, as the cells revealed significant damage. Based on the results of this study, a conceptual model was developed that demonstrated the effects of salinity on As uptake and the biotransformation mechanism of seaweed.

Influence of Different Arsenic Species on the Bioavailability and Bioaccumulation of Arsenic by Sargassum horneri C. Agardh: Effects under Different Phosphate Conditions.

The growth response and incorporation of As into the Sargassum horneri was evaluated for up to 7 days using either arsenate (As(V)), arsenite (As(III)) or methylarsonate (MMAA(V) and DMAA(V)) at 0, 0.25, 0.5, 1, 2, and 4 µM with various phosphate (P) levels (0, 2.5, 5 and 10 µM). Except As(III), algal chlorophyll fluorescence was almost similar and insignificant, regardless of whether different concentrations of P or As(V) or MMAA(V) or DMAA(V) were provided (p > 0.05). As(III) at higher concentrations negatively affected algal growth rate, though concentrations of all As species had significant effects on growth rate (p < 0.01). Growth studies indicated that toxicity and sensitivity of As species to the algae followed the trend: As(III) > As(V) > MMAA(V) ~ DMAA(V). As bioaccumulation was varied significantly depending on the increasing concentrations of all As species and increasing P levels considerably affected As(V) uptake but no other As species uptake (p < 0.01). The algae accumulated As(V) and As(III) more efficiently than MMAA(V) and DMAA(V). At equal concentrations of As (4 µM) and P (0 µM), the alga was able to accumulate 638.2 ± 71.3, 404.1 ± 70.6, 176.7 ± 19.6, and 205.6 ± 33.2 nM g-1 dry weight of As from As(V), As(III), MMAA(V), and DMAA(V), respectively. The influence of low P levels with increased As(V) concentrations more steeply increased As uptake, but P on other As species did not display similar trends. The algae also showed passive modes for As adsorption of all As species. The maximum adsorption of As (63.7 ± 6.1 nM g-1 dry weight) was found due to 4 µM As(V) exposure, which was 2.5, 7.3, and 6.9 times higher than the adsorption amounts for the same concentration of As(III), MMAA(V), and DMAA(V) exposure, respectively. The bioavailability and accumulation behaviors of As were significantly influenced by P and As species, and this information is essential for As research on marine ecosystems.

A new evaluation system of iron bioavailability in seaweed.

In marine ecosystems, the avid binding of iron (Fe) to organic ligands influences Fe bioavailability in seaweed. This study aimed to elucidate Fe's biological availability to seaweed and develop a simple and rapid bioassay method as a new evaluation system. Undaria pinnatifida was used as a model seaweed species and the actual seaweed samples were collected using the 0.5 m × 0.5 m quadrat from the Mashike Bay area of Hokkaido, Japan. Chlorophyll fluorescence measurements were utilized as an index to evaluate the biological -effectiveness of Fe and compared with the results of culture tests based on growth. The effect of Fe content on media, pre-culture, concentrations and types of chelating and reducing agents in clearing solutions, cleaning time, Fe removal effect, and resistance to seaweed were systematically optimized to obtain the maximum efficacy of the washing solution. A bioassay was developed to evaluate the Fe environment by combining chlorophyll fluorescence measurements. The findings suggest that the tolerance of seaweeds to the wash solution is strongly influenced by the concentrations of the chelating and reducing agents than their types. Washing with 0.02 M Ti-Citrate/EDTA solution for 80 s was the most effective in terms of maximum Fe removal with minimum cell damage. The application of pre-culture and chemical pre-treatment methods under Fe deficiency to the culture strain confirmed the maximum reproducibility in the culture test. Finally, the developed method was applied to actual seaweed samples and was found to be applicable to many seaweed species. However, the method was less robust for some seaweed species and depended on the seaweed growth stage.

Integrated environmental factor-dependent growth and arsenic biotransformation by aquatic microalgae: A review.

Arsenic (As) is a toxic metalloid posing harming the human food chain through trophic transfer. Microalgae are primary producers, ensuring bioaccumulation and biogeochemical cycling of As in water environment. They are highly efficient at removing As from the environment, making these microscopic organisms eco-friendly and money saving method in As remediation process. However, microalgal growth and As biotransformation potential relies greatly on individual and integrated environmental factors. This review scrutinizes the available literature on the As biotransformation potentials of various marine and freshwater microalgae under individual and integrated stresses of such factors. Various combinations of important factors such as temperature, salinity, concentrations of As (V) and PO43─, pH, light intensity, and length of exposure period are summarized along with the optimum conditions for different microalgae. The effects of environmental factors on microalgal growth, changes in cell shape, and the relationship between As biotransformation and other activities are discussed in detail. Time-dependent As speciation pattern by aquatic microalgae are reviewed. Conceptual models highlighting the microalgal species particularly linked with environmental factor-dependent As biotransformation mechanisms are also summarized. This review will contribute to an in depth understanding of the connection between environmental factors, As uptake, and the biotransformation mechanism of marine and freshwater microalgae from the perspective of As remediation process.

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.

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.

Arsenic biotransformation potential of six marine diatom species: effect of temperature and salinity.

Temperature and salinity effects on marine diatom species growth has been studied extensively; however, their effect on arsenic (As) biotransformation has been imprecise. This study reports the growth, and As biotransformation and speciation patterns at various temperatures and salinities of six marine diatom species: Asteroplanus karianus, Thalassionema nitzschioides, Nitzschia longissima, Skeletonema sp., Ditylum brightwellii, and Chaetoceros didymus. The growth rate and As biotransformation potentials of these species during three weeks of culture in f/2 based medium were significantly affected by wide temperature (0-35 °C) and salinity (0.3-50‰) ranges. Growth and As biotransformation were higher at optimum temperatures of 10-25 °C, and salinity of 10-35‰, whereas growth and arsenic biotransformation were lower at <5 °C and 5‰ and >25 °C and 35‰, respectively. The results showed that As(V) to As(III) biotransformation differed significantly (p < 0.05) between day 10 and 17. At optimum temperature and salinity levels, the cell size and As biotransformation were higher for all the species. A conceptual model on temperature and salinity effects on growth and As uptake and biotransformation mechanisms by these species has been proposed based on the findings of this study.

Membrane Fouling Potentials of an Exoelectrogenic Fouling-Causing Bacterium Cultured With Different External Electron Acceptors.

Integrated microbial fuel cell (MFC) and membrane bioreactor (MBR) systems are a promising cost-effective and energy-saving technology for wastewater treatment. Membrane fouling is still an important issue of such integrated systems in which aeration (oxygen) is replaced with anode electrodes (anodic respiration). Here, we investigated the effect of culture conditions on the membrane fouling potential of fouling-causing bacteria (FCB). In the present study, Klebsiella quasipneumoniae strain S05, which is an exoelectrogenic FCB isolated from a MBR treating municipal wastewater, was cultured with different external electron acceptors (oxygen, nitrate, and solid-state anode electrode). As results, the fouling potential of S05 was lowest when cultured with anode electrode and highest without any external electron acceptor (p < 0.05, respectively). The composition of soluble microbial products (SMP) and extracellular polymeric substances (EPS) was also dependent on the type of electron acceptor. Protein and biopolymer contents in SMP were highly correlated with the fouling potential (R 2 = 0.73 and 0.81, respectively). Both the fouling potential and yield of protein and biopolymer production were significantly mitigated by supplying electron acceptors sufficiently regardless of its types. Taken together, the aeration of MBR could be replaced with solid-state anode electrodes without enhancement of membrane fouling, and the anode electrodes must be placed sufficiently to prevent the dead spaces in the integrated reactor.