Contribution of arsenic species in unicellular algae to the cycling of arsenic in marine ecosystems

Microbial arsenic methylation in soil-water systems and its environmental significance

In this review, we have summarized the current knowledge about the environmental importance, relevance, and consequences of microbial arsenic (As) methylation in various ecosystems. In this regard, we have presented As biomethylation in terrestrial and aquatic ecosystems particularly in rice paddy soils and wetlands. The functions of As biomethylation by microbial consortia in anaerobic and aerobic conditions are extensively discussed. In addition, we have tried to explain the interconnections between As transformation and carbon (C), such as microbial degradation of organic compounds and methane (CH4) emission. These processes can cause As release because of the reduction of arsenate (As(V)) to the more mobile arsenite (As(III)) as well as As methylation and the formation of toxic trivalent methylated As species in anaerobic conditions. Furthermore, the sulfur (S) transformation can form highly toxic thiolated As species owing to its interference with As biomethylation. Besides, we have focused on many other mutual interlinks that remain elusive between As and C, including As biomethylation, thiolation, and CH4 emission, in the soil-water systems. Recent developments have clarified the significant and complex interactions between the coupled microbial process in anoxic and submerged soils. These processes, performed by little-known/unknown microbial taxa or well-known members of microbial communities with unrecognized metabolic pathways, conducted several concurrent reactions that contributed to global warming on our planet and have unfavorable impacts on water quality and human food resources. Finally, some environmental implications in rice production and arsenic removal from soil-water systems are discussed. Generally, our understanding of the ecological and metabolic evidence for the coupling and synchronous processes of As, C, and S are involved in environmental contamination-caused toxicity in human food, including high As content in rice grain, water resources, and global warming through methanogenesis elucidate combating global rice safety, drinking water, and climate changes.

Lipophilic arsenic compounds in the cultured green alga Chlamydomonas reinhardtii

In this study, arsenic (As) speciation was investigated in the freshwater alga Chlamydomonas reinhardtii treated with 20 μg/L arsenate using fractionation as well as ICP-MS/ESI-MS analyses and was compared with the known As metabolite profile of wild-grown Saccharina latissima. While the total As accumulation in C. reinhardtii was about 85% lower than in S. latissima, the relative percentage of arsenolipids was significantly higher in C. reinhardtii (57.0% vs. 5.01%). As-containing hydrocarbons and phospholipids dominated the hydrophobic As profile in S. latissima, but no As-containing hydrocarbons were detectable in C. reinhardtii. Instead for the first time, an arsenoriboside-containing phytol (AsSugPhytol) was found to dominate the hydrophobic arsenicals of C. reinhardtii. Interestingly, this compound and its relatives had so far been only found in green marine microalgae, open sea plankton (mixed assemblage), and sediments but not in brown or red macroalgae. This compound family might therefore relate to differences in the arsenic metabolism between the algae phyla.

Strategies for the analysis of arsenolipids in marine foods: A review

Arsenic-containing lipids, also named arsenolipids (AsLs), are a group of organic compounds usually found in a variety of marine organisms such as fish, algae, shellfish, marine oils, and microorganisms. Numerous AsLs have been recognised so far, from simple compounds such as arsenic fatty acids (AsFAs), arsenic hydrocarbons (AsHCs), and trimethylarsenio fatty alcohols (TMAsFOHs) to more complex arsenic-containing species, of which arsenophospholipids (AsPLs) are a case in point. Mass spectrometry, both as inductively coupled plasma (ICP-MS) and liquid chromatography coupled by an electrospray source (LC-ESI-MS), was applied to organic arsenicals playing a key role in extending and refining the characterisation of arsenic-containing lipids in marine organisms. Herein, upon the introduction of a systematic notation for AsLs and a brief examination of their toxicity and biological role, the most relevant literature concerning the characterisation of AsLs in marine organisms, including edible ones, is reviewed. The use of both ICP-MS and ESI-MS coupled with reversed-phase liquid chromatography (RPLC) has brought significant advancements in the field. In the case of ESI-MS, the employment of negative polarity and tandem MS analyses has further enhanced these advancements. One notable development is the identification of the m/z 389.0 ion ([AsC10H19O9P]-) as a diagnostic product ion of AsPLs, which is obtained from the fragmentation of the deprotonated forms of AsPLs ([M - H]-). The pinpointing product ions offer the possibility of determining the identity and regiochemistry of AsPL side chains. Advanced MS-based analytical methods may contribute remarkably to the understanding of the chemical diversity characterising the metalloid As in natural organic compounds of marine organisms.

Enhancement of arsenic uptake and accumulation in green microalga Chlamydomonas reinhardtii through heterologous expression of the phosphate transporter DsPht1

Arsenate (AsV) is a predominant arsenic contaminant in aerobic water. Microalgae have been recently used in the phytoremediation of arsenic-contaminated water. However, the amount of AsV uptake in microalgae is limited, which hinders the application of microalgae in arsenic-contaminated water treatment. Here, we found that the expression of a novel phosphate transporter DsPht1 in Dunaliella salina was highly upregulated after AsV exposure. Fluorescent protein-tagging analysis showed the plasma membrane location of DsPht1. Furthermore, DsPht1 was overexpressed in a model microalga Chlamydomonas reinhardtii. The DsPht1 transgenetic lines accumulated up to 6.4-fold higher total arsenic than the untransformed line, and the AsV amount in total arsenic increased by 8.3-fold. Moreover, the organoarsenic content was also higher in the transgenetic lines. Overall, the DsPht1 transformants generated in this study increased arsenate uptake and transformation, which are promising for the effective phytoremediation of arsenic-contaminated water.

Arsenic speciation in low-trophic marine food chain - An arsenic exposure study on microalgae (Diacronema lutheri) and blue mussels (Mytilus edulis L.)

Microalgae and blue mussels are known to accumulate undesirable substances from the environment, including arsenic (As). Microalgae can biotransform inorganic As (iAs) to organoarsenic species, which can be transferred to blue mussels. Knowledge on As uptake, biotransformation, and trophic transfer is important with regards to feed and food safety since As species have varying toxicities. In the current work, experiments were conducted in two parts: (1) exposure of the microalgae Diacronema lutheri to 5 and 10 μg/L As(V) in seawater for 4 days, and (2) dietary As exposure where blue mussels (Mytilus edulis L.) were fed with D. lutheri exposed to 5 and 10 μg/L As(V), or by aquatic exposure to 5 μg/L As(V) in seawater, for a total of 25 days. The results showed that D. lutheri can take up As from seawater and transform it to methylated As species and arsenosugars (AsSug). However, exposure to 10 μg/L As(V) resulted in accumulation of iAs in D. lutheri and lower production of methylated As species, which may suggest that detoxification mechanisms were overwhelmed. Blue mussels exposed to As via the diet and seawater showed no accumulation of As. Use of linear mixed models revealed that the blue mussels were gradually losing As instead, which may be due to As concentration differences in the mussels' natural environment and the experimental setup. Both D. lutheri and blue mussels contained notable proportions of simple methylated As species and AsSug. Arsenobetaine (AB) was not detected in D. lutheri but present in minor fraction in mussels. The findings suggest that low-trophic marine organisms mainly contain methylated As species and AsSug. The use of low-trophic marine organisms as feed ingredients requires further studies since AsSug are regarded as potentially toxic, which may introduce new risks to feed and food safety.

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.

Carbohydrate-Containing Low Molecular Weight Metabolites of Microalgae

Microalgae are abundant components of the biosphere rich in low molecular weight carbohydrate-containing natural products (glycoconjugates). Glycoconjugates take part in the processes of photosynthesis, provide producers with important biological molecules, influence other organisms and are known by their biological activities. Some of them, for example, glycosylated toxins and arsenicals, are detrimental and can be transferred via food chains into higher organisms, including humans. So far, the studies on a series of particular groups of microalgal glycoconjugates were not comprehensively discussed in special reviews. In this review, a special focus is given to glycoconjugates' isolation, structure determination, properties and approaches to search for new bioactive metabolites. Analysis of literature data concerning structures, functions and biological activities of ribosylated arsenicals, galactosylated and sulfoquinovosylated lipids, phosphoglycolipids, glycoside derivatives of toxins, and other groups of glycoconjugates was carried out and discussed. Recent studies were fundamental in the discovery of a great variety of new carbohydrate-containing metabolites and their biological activities in defining the role of microalgal viral infections in regulating microalgal blooms as well as in the detection of glycoconjugates with potent immunomodulatory properties. Those discoveries support growing interest in these molecules.

The influences of dissolved inorganic and organic phosphorus on arsenate toxicity in marine diatom Skeletonema costatum and dinoflagellate Amphidinium carterae

In this study, arsenate (As(V)) uptake, bioaccumulation, subcellular distribution and biotransformation were assessed in the marine diatom Skeletonema costatum and dinoflagellate Amphidinium carterae cultured in dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP). The results of 3-days As(V) exposure showed that As(V) was more toxic in DOP cultures than in DIP counterparts. The higher As accumulation contributed to more severe As(V) toxicity. The 4-h As(V) uptake kinetics followed Michaelis-Menten kinetics. The maximum uptake rates were higher in DOP cultures than those in DIP counterparts. After P addition, the half-saturation constants remained constant in S. costatum (2.42-3.07 μM) but decreased in A. carterae (from 10.9 to 3.8 μM) compared with that in the respective P-depleted counterparts. During long-term As(V) exposure, A. carterae accumulated more As than S. costatum. Simultaneously, As(V) was reduced and transformed into organic As species in DIP-cultured S. costatum, which was severely inhibited in their DOP counterparts. Only As(V) reduction occurred in A. carterae. Overall, this study demonstrated species-specific effects of DOP on As(V) toxicity, and thus provide a new insight into the relationship between As contamination and eutrophication on the basis of marine microalgae.

Assessing indicators of arsenic toxicity using variable fluorescence in a commercially valuable microalgae: Physiological and toxicological aspects

Indicators signaling Arsenic (As) stress through physiology of microalgae using non-destructive methods like variable fluorescence are rare but requisite. This study reports stress markers indicating arsenic (As) toxicity (in two concentrations 11.25 µg/L and 22.5 µg/L compared to a control) exposed to a microalga (Diacronema lutheri), using fast repetition rate fluorometry (FRRf). Growth and physiological parameters such as cell density, chl a and the maximum quantum yield F_v/F_m showed coherence and impeded after the exponential phase (day 9 - day 12) in As treatments compared to the control (p < 0.05). On contrary photo-physiological constants were elevated showing higher optical (a_LHII) and functional [Sigma (σ_PSII)] absorption cross-section for the As treatments (p < 0.05) further implying the lack of biomass production yet an increase in light absorption. In addition, As exposure increased the energy dissipation by heat (NPQ-NSV) showing a strong relationship with the de-epoxidation ratio (DR) involving photoprotective pigments. Total As bioaccumulation by D. lutheri showed a strong affinity with Fe adsorption throughout the algal growth curve. This study suggests some prompt photo-physiological proxies signaling As contamination and endorsing its usefulness in risk assessments, given the high toxicity and ubiquitous presence of As in the ecosystem.

Arsenic in Lake Geneva (Switzerland, France): long term monitoring, and redox and methylation speciation in an As unpolluted, oligo-mesotrophic lake

Arsenic speciation was followed monthly along the spring productivity period (January-June 2021) in the Petit Lac (76 m deep) and in April and June 2021 in the Grand Lac (309.7 m deep) of Lake Geneva (Switzerland/France). Lake Geneva is presently an oligo-mesotrophic lake, and As-unpolluted. The water column never becomes anoxic but the oxygen saturation at the bottom of the Grand Lac is now below 30% owing to lack of water column mixing since 2012. Thus, this lake offers excellent conditions to study As behaviour in an unpolluted, oxic freshwater body. The following 'dissolved' As species: iAs(III), iAs(III + V), MA(III), MA(III + V), DMA(III + V), and TMAO were analysed by HG-CT-ICP-MS/MS. Water column measurements were complemented with occasional sampling in the main rivers feeding the lake and in the interstitial waters of a sediment core. The presence of MA(III) and TMAO and the predominance of iAs(V) in lake and river samples has been confirmed as well as the key role of algae in the formation of organic species. While the total 'dissolved' As concentrations showed nearly vertical profiles in the Petit Lac, As concentrations steadily increase at deeper depths in the Grand Lac due to the lack of mixing and build up in bottom waters. The evaluation of 25 years of monthly data of 'dissolved' As concentrations showed no significant temporal trends between 1997 and 2021. The observed seasonal character of the 'dissolved' As along this period coincides with a lack of seasonality in As mass inventories, pointing to a seasonal internal cycling of As species in the water column with exchanges between the 'dissolved' and 'particulate' (i.e., algae) fractions.

Effects of phosphate on the toxicity and bioaccumulation of arsenate in marine diatom Skeletonema costatum

The effects of nutrient phosphate (P) at environmentally relevant levels on the toxicity of arsenic (As) in marine microalgae have been rarely known. In the present study, we explored the toxicity and bioaccumulation of As in a globally distributed diatom species Skeletonema costatum at different ambient P concentrations. The results showed that As toxicity was suppressed at elevated P concentrations. Intracellular As content ([As]_intra) and the molar ratio of intracellular As to P ([As:P]) were negatively correlated with the ambient P concentrations. The trends of As bioaccumulation were substantially different between the relatively low (0, 0.5 and 1.5 μM) and high P concentrations (7.5 and 37.5 μM). Both [As]_intra and [As:P] suggested that As bioaccumulation was a better factor to explain the As toxicity comparing to the ambient As concentration. The 4 h As uptake kinetics at different P concentrations followed Michaelis-Menten kinetic pattern. The maximum uptake rates (V_max) decreased with the increase in P concentrations, and the half-saturation constants (K_d) remained constant except for that at extremely high P concentration (37.5 μM-P), suggesting the depression of P on As uptake was mainly due to the non-competitive effect. Overall, these results demonstrate that the P concentration in seawater is an important factor affecting As toxicity and bioaccumulation in the marine diatom. This study therefore helps us better understand the effects of eutrophication on the toxicity and biogeochemistry of As in the marine environment.

The speciation of arsenic in the muscle tissue of inland and coastal freshwater fish from a remote boreal region

Elevated concentrations of total arsenic (As) have been reported in boreal freshwater fish in both human-impacted and relatively pristine areas. We assessed the arsenic speciation profiles in muscle tissue of six fish species (n = 300) sampled from nine locations across a remote freshwater watershed in northern Ontario, Canada, extending from inland headwater lakes to the coastal marine confluence. Of the five arsenic species measured, only arsenobetaine (AsB) and dimethylarsinic acid (DMA) were detected in these fish. Riverine fish had up to 10-fold higher total [As] when compared to lacustrine fish. On average, these riverine fish also had higher percentages of AsB (%AsB, 60 ± 26%) and lower percentages of unmeasured arsenic (%UNM, 20 ± 21%), compared to lacustrine fish (28 ± 18% and 52 ± 21% %AsB and %UNM, respectively). DMA percentages (%DMA) were relatively consistent across the watershed, averaging 20 ± 21% across all fish. We examined ecological drivers of As speciation and found that %AsB increased slightly with fish weight in large-body predatory fish, but not in forage fish or insectivores. Furthermore, %AsB was positively related to trophic elevation (inferred from δ¹⁵N) in lacustrine fish across 3 out of 4 communities and within some populations. Lastly, riverine fish with a more marine-based diet had markedly higher %AsB when compared to fish with more freshwater-based diets, indicating an effect of anadromy on arsenic speciation. Overall, knowledge on arsenic speciation in freshwater fish has been limited and these results indicate potential drivers that can be considered in future studies. Furthermore, the absence of toxic inorganic As species in these boreal fish is an important consideration for future environmental monitoring practices and risk assessments, some of which assume 10-20% of total [As] in fish is present as toxic inorganic As. Additional studies on As bioaccumulation and biotransformation are needed in freshwater systems, particularly at the base of aquatic food webs.

Rethinking quantified methods for arsenic speciation and risk in a biowaste hydrothermal liquefaction system

Controversy exists to quantify the fate and speciation of Arsenic (As). We investigated its characteristics by As-containing algae in various pH hydrothermal liquefaction (HTL) system, specifically via two classical methods, i.e. the European Community Bureau of Reference (BCR) and Wenzel's method. Solid residue immobilized 11.23-16.55% of As, and 88.07-82.44% was in aqueous by the pH regulators (e.g., CH₃COOH, HCl, and KOH). ICP-MS and XRD analysis revealed that As (V) was converted into As (III) and As (0) in the solid residue, while the As (V) was mainly converted into As (III) in the aqueous phase during HTL. When the classified forms of As in solid residue are compared, Wenzel's method was more appropriate for dividing the bio-availability forms of As, whereas BCR was better for estimating the toxic-potential forms of As. Subsequently, pH regulators raised the risk of As in solid residue associated with the increasing of unstable forms. The amide was hydrolyzed to carboxylic acid with acidic additives, which weakened the reducing environment in the HTL process. In contrast, the amide was hydrolyzed to ammonia with the alkaline additives, which enhanced the reducing environment and increased the risk of As in products. This work provided a new insight in systematically evaluating the risk and speciation of As in HTL.

Arsenic species and their health risks in edible seaweeds collected along the Chinese coastline

Edible seaweeds with a relatively high total arsenic concentration have been a global concern. As the largest seaweed producer, China contributes about 60 % of the global seaweed production. The present study investigated 20 seaweed species collected from representative seaweed farming sites in the six provinces along the Chinese coastline, of which Saccharina japonica, Undaria pinnatifida, Neopyropia spp., Gracilaria spp., Sargassum fusiforme were listed as the most consumed seaweeds in Food and Agriculture Organization of the United Nations (FAO). The inorganic arsenic (iAs) concentration in most of the seaweeds was below maximum limits (0.3 mg iAs/kg) as seaweed additives for infant food in the National Food Safety Standard of Pollutants in China (GB2762-2017, 2017), except for the species Sargassum, in which the iAs concentration significantly exceeded the limit and ranged from 15.1 to 83.7 mg/kg. Arsenic speciation in 4 cultivated seaweeds grown in both temperate and subtropical zones is reported for the first time. No significant differences in total As and iAs concentration were identified, except slightly higher total As concentration were found in Saccharina japonica growing in the temperate zone. The estimated daily intake (EDI) of toxic iAs via seaweed consumption was generally below the EFSA CONTAM Panel benchmark dose lower confidence limit (0.3 μg/kg bw/day) except for all Sargassum species where the EDI was significantly higher than 0.3 μg/kg bw/day. Moreover, the first-ever reported data on As speciation indicated very high iAs concentrations in Sargassum hemiphyllum and Sargassum henslowianum. To minimize the food chain iAs exposure, reducing both human intake of Sargassum spp. and the used of Sargassum spp. for animal feed is highly recommended. CAPSULE: This study showed that edible seaweed Sargassum spp. consumption may pose a health risk related to inorganic arsenic (iAs) exposure. The risk of iAs exposure via seaweed consumption or livestock is a concern that needs to be monitored. The arsenic accumulation and speciation may be predominantly species-dependent rather than environmental-dependent.

Characterising contaminants distribution in marine-coastal sediments through multivariate and nonparametric statistical analyses: a complementary strategy supporting environmental monitoring and control

This work investigates a statistical approach analysing data from monitoring activities on marine-coastal areas for environmental quality determination and surveillance. Analyses were performed on a database of the Environmental Protection and Prevention Agency of the Puglia Region. As, Cr, Ni, and Pb concentration values in marine sediments and biota from 2013 to 2015 and 2017 were processed to investigate different contaminant characteristics. Hierarchical cluster analysis identified three contaminant distribution classes with (1) highest Cr, Ni, and Pb concentrations, (2) highest As concentration, and (3) lowest contaminants concentration. The Kruskal-Wallis and Friedman tests showed that contaminant distributions were statistically different when considering the monitoring years and classes. However, statistical similarities resulted during the 2013-2017 and 2014-2015 periods. Spearman's coefficients displayed positive correlations among the pollutants in each matrix and mainly negative correlations for matrices comparison. This methodology aims to provide a practical support for monitoring to identify potential environmental deterioration over time and correlations with specific contamination sources.

Metabolomics Analysis of Chronic Exposure to Dimethylarsenic Acid in Mice and Toxicity Assessment of Organic Arsenic in Food

Dimethylarsenic acid is a natural organic arsenic in seafood and one of the important metabolites of inorganic arsenic, which is generally considered to have low or no toxicity. However, due to the controversy of the toxicity of organic arsenic, the food safety standard of organic arsenic has not been established until now, and the effects of organic arsenic on chronic toxicity and the overall metabolic level of animals are rarely reported. In our study, 64 female C57BL/6 mice were exposed to different concentrations of dimethylarsenic acid with water intake. Fifteen metabolites in serum were detected to be altered with the increase of arsenic concentration and exposure time. Dimethylarsenic acid exposure significantly affected the overall metabolic level of mice, and the related effects were not recovered shortly after the suspension of arsenic intake. Although arsenic was excreted largely in urine and feces, continued dimethylarsenic acid exposure could still lead to arsenic accumulation in the liver and kidneys and cause mild nephritis in mice.

Decreasing arsenic accumulation but promoting arsenate biotransformation in Microcystis aeruginosa regulated by nano-Fe2O3

Iron oxide nanoparticles (nano-Fe₂O₃) widely distribute in waters with low toxicity to aquatic organisms. But it is unclear for nano-Fe₂O₃ to affect the fate of coexisting arsenic (As) with its bioaccumulation and biotransformation. In this study, we thus mainly investigated arsenate (As(V)) toxicity, uptake kinetics, biotransformation and subcellular distribution in Microcystis aeruginosa influenced by nano-Fe₂O₃. The results showed that M. aeruginosa was more sensitive to As(V) associated with nano-Fe₂O₃. Due to the exaggerated increase of efflux rate constants of As compared with the uptake rate constants in algal cells affected by different levels of nano-Fe₂O₃, the As(V) bioconcentration factor decreased with nano-Fe₂O₃ increasing correspondingly, indicating that As bioaccumulation was diminished by nano-Fe₂O_3. The decreased As accumulation in M. aeruginosa could be supported by the evidential As(V) sequestration through high adsorption of nano-Fe₂O₃, which resulted in decreasing free As level for algae uptake in media. Meanwhile, As subcellular distribution was adjusted by nano-Fe₂O₃ with decreasing in cell walls and rising in cytoplasmic organelles compared with nano-Fe₂O₃ free. As(V) reduction and methylation were enhanced with increasing nano-Fe₂O₃, stimulating by its sensitivity to the interaction of nano-Fe₂O₃ and As(V) as well as the rising level of As in cytoplasmic organelles of this algae. It is confirmed by the higher relative gene expression levels of arsC and arsM in elevated nano-Fe₂O₃. Accordingly, it is highlighted to be deserved more attention that the changing behavior of As(V) by nano-Fe₂O₃ that reduce As bioaccumulation and accelerate its biotransformation in algae in As contaminated water.

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 PO₄^3─, 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.

Arsenic bioaccumulation and biotransformation in aquatic organisms

Arsenic exists universally in freshwater and marine environments, threatening the survival of aquatic organisms and human health. To elucidate arsenic bioaccumulation and biotransformation processes in aquatic organisms, this review evaluates the dissolved uptake, dietary assimilation, biotransformation, and elimination of arsenic in aquatic organisms and discusses the major factors influencing these processes. Environmental factors such as phosphorus concentration, pH, salinity, and dissolved organic matter influence arsenic absorption from aquatic systems, whereas ingestion rate, gut passage time, and gut environment affect the assimilation of arsenic from foodstuffs. Arsenic bioaccumulation and biotransformation mechanisms differ depending on specific arsenic species and the involved aquatic organism. Although some enzymes engaged in arsenic biotransformation are known, deciphering the complicated synthesis and degradation pathway of arsenobetaine remains a challenge. The elimination of arsenic involves many processes, such as fecal excretion, renal elimination, molting, and reproductive processes. This review facilitates our understanding of the environmental behavior and biological fate of arsenic and contributes to regulation of the environmental risk posed by arsenic pollution.

Toxic metal pollution and ecological risk assessment in water and sediment at ship breaking sites in the Bay of Bengal Coast, Bangladesh

Quantification of four toxic metals (As, Cr, Cd, and Pb) in water and sediments at the Sitakunda ship breaking area in Bangladesh was studied. Along with this, sediment quality and ecological risk were evaluated for the metal intrusion to the study area. A total sample number of 120 (water; n = 60 and sediment; n = 60) were analyzed for both winter and summer seasons using atomic absorption spectrophotometer (AAS). The trace metal concentration in both water and sediment showed decreasing trend as follows; Cr (mean-W: 0.118 mg/L; mean-S:121.87 mg/kg) > Pb (mean-W: 0.064 mg/L; mean-S: 65.31 mg/kg) > As (mean-W: 0.03 mg/L; mean-S: 32.53 mg/kg) > Cd (mean-W: 0.004 mg/L; mean-S: 4.81 mg/kg). However, in both segments, the concentrations of the toxic metals exceeded the recommended acceptable limits. As and Cd showed significant variation (water and sediment) between the seasons, while Pb and Cr had no seasonal impact. Metal pollution index (MPI) and contamination factor (CF) was evaluated and revealed that the study area exhibited the critical score of water quality (MPI > 100). The cumulative effect of the metal concentrations was high (CI > 3). The assessed mean geoaccumulaiton index (I_geo) revealed that the study area was moderate to strongly polluted except for Cr. According to the contamination factor (CF), the sediment samples were moderate to highly contaminated by Cd, Pb, and As. Moreover, the explored range of pollution load index (PLI) in all sampling sites in the ship breaking region was from 1.75 to 3.10, suggesting that the sediment in the study area was highly polluted by heavy metals (PLI > 1). The risk index and the potential ecological risk index (PERI) suggested that the study area was at high risk due to metals pollution. Therefore, it is obligatory to maintain some crucial efforts for the betterment of the surrounding environment near the investigated sites.

Trace elements in pelagic Sargassum species in the Mexican Caribbean: Identification of key variables affecting arsenic accumulation in S. fluitans

Since 2014, the Mexican Caribbean beaches have received massive influxes of the brown seaweed Sargassum (S. fluitans III, S. natans I and S. natans VIII), causing serious ecological and economic effects. Concentrations of arsenic (As), cadmium (Cd), copper (Cu), iron (Fe), lead (Pb), and zinc (Zn) were determined over an annual cycle in pelagic Sargassum species from massive influxes into the Mexican Caribbean. The contribution of trace elements, polysaccharides (alginate and fucoidans), and their main functional groups (uronic acids and sulfate) to arsenic content in Sargassum fluitans - the most abundant species in the Sargassum influx - is discussed. Arsenic was recorded in all samples, yielding mean concentrations of 74.2 ± 2.84 mg kg-1. Significant differences were found between species for As, Cu, Fe, and Pb, but not for Cd and Zn; also, S. fluitans showed significant differences in metal content between seasons for all elements, as well as in alginate and uronic acids from fucoidan. The season of the year, copper, iron, uronic acids, and sulfate content in fucoidan were the main variables associated with arsenic accumulation in S. fluitans as evidenced with a Generalized Linear Model. Arsenic content in Sargassum biomass exceeded the maximum allowable level in the rainy season; therefore, the content of this trace element should be carefully monitored for safe usage of Sargassum biomass.

First Observation of Unicellular Organisms Concentrating Arsenic in ACC Intracellular Inclusions in Lake Waters

In unicellular organisms, intracellular inclusions of amorphous calcium carbonate (ACC) were initially described in cyanobacteria and, later, in unicellular eukaryotes from Lake Geneva (Switzerland/France). Inclusions in unicellular eukaryotes, named micropearls, consist of hydrated ACCs, frequently enriched in Sr or Ba, and displaying internal oscillatory zonations, due to variations in the Ba:Ca or Sr:Ca ratios. An analysis of our database, consisting of 1597 micropearl analyses from Lake Geneva and 34 from Lake Titicaca (Bolivia/Peru), showed that a certain number of Sr- and Ba-enriched micropearls from these lakes contain As in amounts measurable by EDXS. A Q-mode statistical analysis confirmed the existence of five chemically distinct morpho-chemical groups of As-bearing micropearls, among which was a new category identified in Lake Geneva, where As is often associated with Mg. This new type of micropearl is possibly produced in a small (7–12 μm size) bi-flagellated organism. Micropearls from Lake Titicaca, which contain Sr, were found in an organism very similar to Tetraselmis cordiformis, which was observed earlier in Lake Geneva. Lake Titicaca micropearls contain larger As amounts, which can be explained by the high As concentration in the water of this lake. The ubiquity of this observed biomineralization process points to the need for a better understanding of the role of amorphous or crystalline calcium carbonates in As cycling in surface waters.

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 V_max = 0.32 and K_m = 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.

Interactions with Arsenic: Mechanisms of Toxicity and Cellular Resistance in Eukaryotic Microorganisms

Arsenic (As) is quite an abundant metalloid, with ancient origin and ubiquitous distribution, which represents a severe environmental risk and a global problem for public health. Microbial exposure to As compounds in the environment has happened since the beginning of time. Selective pressure has induced the evolution of various genetic systems conferring useful capacities in many microorganisms to detoxify and even use arsenic, as an energy source. This review summarizes the microbial impact of the As biogeochemical cycle. Moreover, the poorly known adverse effects of this element on eukaryotic microbes, as well as the As uptake and detoxification mechanisms developed by yeast and protists, are discussed. Finally, an outlook of As microbial remediation makes evident the knowledge gaps and the necessity of new approaches to mitigate this environmental challenge.

Hydrothermal liquefaction accelerates the toxicity and solubility of arsenic in biowaste

Arsenic (As) is one of notorious metalloids due to its high toxicity to human beings and ecological system. Understanding its fate and speciation transformation mechanism during hydrothermal liquefaction (HTL) of microalgae is of crucial importance for the application of its HTL products. 80.0-96.7% of As in raw microalgae was migrated into the liquid phase (aqueous phase and biocrude oil) with the increase of reaction severity from 0.108 to 0.517. HPLC-ICPMS reveals that 67% of the As in microalgae accounted for As(V) with a concentration of 68.4 mg/kg. The other fractions in microalgae were primarily As(III) with a concentration of 36.3 mg/kg. Model compounds experiments illustrate that over 30% of the As(V) in feedstocks was unexpectedly converted into more soluble and toxic As (III). Hydrochar containing O-containing groups (e.g., aliphatic C-OH) was probably contribute to the reduction transformation of As(V) to higher toxic As(III). Meantime, the aqueous phase facilitated the reduction reaction via providing a reducing environment and serving as hydrogen donator. This study firstly revealed the speciation transformation of As(V) to As(III) during HTL of wastewater cultivated microalgae.

Toxicity mechanisms of arsenic compounds in aquatic organisms

Arsenic is a toxic metalloid that is widely distributed in the environment due to its persistence and accumulative properties. The occurrence, distribution, and biological effects of arsenic in aquatic environments have been extensively studied. Acute and chronic toxicities to arsenic are associated with fatal effects at the individual and molecular levels. The toxicity of arsenic in aquatic organisms depends on its speciation and concentration. In aquatic environments, inorganic arsenic is the dominant form. While trivalent arsenicals have greater toxicity compared with pentavalent arsenicals, inorganic arsenic can assume a variety of forms through biotransformation in aquatic organisms. Biotransformation mechanisms and speciation of arsenic have been studied, but few reports have addressed the relationships among speciation, toxicity, and bioavailability in biological systems. This paper reviews the modes of action of arsenic along with its toxic effects and distribution in an attempt to improve our understanding of the mechanisms of arsenic toxicity in aquatic organisms.

Marine Community Metabolomes Carry Fingerprints of Phytoplankton Community Composition

Phytoplankton transform inorganic carbon into thousands of biomolecules that represent an important pool of fixed carbon, nitrogen, and sulfur in the surface ocean. Metabolite production differs between phytoplankton, and the flux of these molecules through the microbial food web depends on compound-specific bioavailability to members of a wider microbial community. Yet relatively little is known about the diversity or concentration of metabolites within marine plankton. Here, we compare 313 polar metabolites in 21 cultured phytoplankton species and in natural planktonic communities across environmental gradients to show that bulk community metabolomes reflect the chemical composition of the phytoplankton community. We also show that groups of compounds have similar patterns across space and taxonomy, suggesting that the concentrations of these compounds in the environment are controlled by similar sources and sinks. We quantify several compounds in the surface ocean that represent substantial understudied pools of labile carbon. For example, the N-containing metabolite homarine was up to 3% of particulate carbon and is produced in high concentrations by cultured Synechococcus, and S-containing gonyol accumulated up to 2.5 nM in surface particles and likely originates from dinoflagellates or haptophytes. Our results show that phytoplankton composition directly shapes the carbon composition of the surface ocean. Our findings suggest that in order to access these pools of bioavailable carbon, the wider microbial community must be adapted to phytoplankton community composition.

IMPORTANCE Microscopic phytoplankton transform 100 million tons of inorganic carbon into thousands of different organic compounds each day. The structure of each chemical is critical to its biological and ecosystem function, yet the diversity of biomolecules produced by marine microbial communities remained mainly unexplored, especially small polar molecules which are often considered the currency of the microbial loop. Here, we explore the abundance and diversity of small biomolecules in planktonic communities across ecological gradients in the North Pacific and within 21 cultured phytoplankton species. Our work demonstrates that phytoplankton diversity is an important determinant of the chemical composition of the highly bioavailable pool of organic carbon in the ocean, and we highlight understudied yet abundant compounds in both the environment and cultured organisms. These findings add to understanding of how the chemical makeup of phytoplankton shapes marine microbial communities where the ability to sense and use biomolecules depends on the chemical structure.

Arsenolipids in Plankton from High- and Low-Nutrient Oceanic Waters Along a Transect in the North Atlantic

Although the natural occurrence of arsenic-containing lipids (arsenolipids) in marine organisms is now well established, the possible role of these unusual compounds in organisms and in the cycling of arsenic in marine systems remains largely unexplored. We report the finding of arsenolipids in 61 plankton samples collected from surface marine waters of high- and low-nutrient content along a transect spanning the Gulf Stream in the North Atlantic Ocean. Using high-performance liquid chromatography (HPLC) coupled to both elemental and molecular mass spectrometry, we show that all 61 plankton samples contained six identifiable arsenolipids, namely, three arsenosugar phospholipids (AsPL958, 10-13%; AsPL978, 13-25%; and AsPL1006, 7-10% of total arsenolipids), two arsenic-containing hydrocarbons (AsHC332, 4-10% and AsHC360, 1-2%), and a methoxy-sugar arsenolipid that contained phytol (AsSugPhytol, 1-3%). The relative amounts of the six arsenolipids showed clear dependence on the nutrient status of the ambient water with plankton collected from high-nutrient waters having less of the arsenosugar phospholipids and more of the three non-P containing arsenolipids compared to low-nutrient waters. By combining these first field data of arsenolipids in plankton with reported global phytoplankton productivity, we estimate that the oceans' phytoplankton transform per year 50 000-100 000 tons of arsenic into arsenolipids.

Characterization and Mechanistic Study of the Radical SAM Enzyme ArsS Involved in Arsenosugar Biosynthesis

Arsenosugars are a group of arsenic-containing ribosides that are found predominantly in marine algae but also in terrestrial organisms. It has been proposed that arsenosugar biosynthesis involves a key intermediate 5'-deoxy-5'-dimethylarsinoyl-adenosine (DDMAA), but how DDMAA is produced remains elusive. Now, we report characterization of ArsS as a DDMAA synthase, which catalyzes a radical S-adenosylmethionine (SAM)-mediated alkylation (adenosylation) of dimethylarsenite (DMAs^III ) to produce DDMAA. This radical-mediated reaction is redox neutral, and multiple turnover can be achieved without external reductant. Phylogenomic and biochemical analyses revealed that DDMAA synthases are widespread in distinct bacterial phyla with similar catalytic efficiencies; these enzymes likely originated from cyanobacteria. This study reveals a key step in arsenosugar biosynthesis and also a new paradigm in radical SAM chemistry, highlighting the catalytic diversity of this superfamily of enzymes.

Arsenic speciation and biotransformation pathways in the aquatic ecosystem: The significance of algae

The contamination of aquatic systems with arsenic (As) is considered to be an internationally-important health and environmental issue, affecting over 115 countries globally. Arsenic contamination of aquatic ecosystems is a global threat as it can enter the food chain from As-rich water and cause harmful impacts on the humans and other living organisms. Although different factors (e.g., pH, redox potential, iron/manganese oxides, and microbes) control As biogeochemical cycling and speciation in water systems, the significance of algal species in biotransformation of As is poorly understood. The overarching attribute of this review is to briefly elaborate various As sources and its distribution in water bodies and factors affecting As biogeochemical behavior in aqueous ecosystems. This review elucidates the intriguing role of algae in biotransformation/volatilization of As in water bodies under environmentally-relevant conditions. Also, we critically delineate As sorption, uptake, oxidation and reduction pathways of As by algae and their possible role in bioremediation of As-contaminated water (e.g., drinking water, wastewater). The current review provides the updated and useful framework for government and water treatment agencies to implement algae in As remediation programs globally.

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.

New insights into toxic effects of arsenate on four Microcystis species under different phosphorus regimes

Very little information is available on the stressed growth of Microcystis imposed by arsenate (As(V)) under different phosphorus (P) regimes. In this study, we examined the growth characteristics and arsenic transformation of four Microcystis species exposed under As(V) with two P sources involving dissolved inorganic phosphorus (IP) and organophosphate (D-glucose-6-phosphate disodium salt, GP). Results showed that all the four chosen Microcystis species could grow and reproduce with GP as the only P source, and the difference was insignificant when compared with IP. From optical density (OD), chlorophyll a (Chla), and actual quantum yield (Yield), the tolerance to As(V) of the chosen species was following as FACHB 905 > FACHB 1028 > FACHB 1334 > FACHB 912. Specifically, the 96 h EC₅₀ of As(V) for FACHB 905 in IP was approx. 4 orders of magnitude higher than that in GP, but for other three algal species, the 96 h EC₅₀ values were similar under the two given different P conditions. Furthermore, all antioxidant enzyme activities of superoxide dismutase (SOD), peroxide dismutase (POD), glutathione S-transferases (GSTs), and metalloproteinase (MTs) in algal cells were significantly increased in GP conditions. Moreover, the enzyme activities of AKP, GSTs, and MTs were inhibited with increasing As(V) levels under both IP and GP conditions. In addition, arsenite (As(III)) and methylated As of monomethylarsonic acid (MMA) and dimethylthioarsinic acid (DMA) were found in FACHB 912 and FACHB 1334 media, indicating that these Microcystis could detoxify As(V) by As biotransformation under IP and GP conditions. Specifically, As(V) reduction was elevated in media of FACHB 1334 and FACHB 905, but was decreased in media of FACHB 912 under GP conditions. Our results highlight the different P sources that impact the toxic effects of arsenate exposure on Microcystis and subsequent As biotransformation.

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 PO₄^3-) or deficient (20 nmol L^-1 of As(V) + 1.0 μmol L^-1 of PO₄^3-) 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.

Effect of Light Intensity on the Mechanism of Inorganic Arsenic Accumulation and Patterns in the Red Macroalga, Sarcodia suiae

Arsenic is a noted dangerous metalloid found in many organisms, including humans, that accumulate via food consumption of aquatic products such as macroalgae, particularly where they are a major component of the human diet. The mechanism of accumulation of inorganic arsenic (iAs) as the most toxic form of arsenic (As) was investigated under three different light intensities (LI) (30, 55, and 80 μmol photons m^-2 s^-1) at varied arsenite (As (III)) concentrations (conc) (0, 125, 250, and 500 μg L^-1) using Sarcodia suiae, a red marine macroalga. The depigmentation of the algal fronds from deep red to slightly pinkish-orange and solid green has been confirmed as a form of developmental acclimation, and the direct toxic effects of conc and LI were manifested by the degree of severity of this symptom. Two-way ANOVA showed a significant difference in iAs accumulation which depended upon conc and LI. Stepwise regression analysis showed LI as the second most important variable after conc in all treatments. S. suiae did not appear to intracellularly transform As (III) to arsenate (As (V)); hence, As (III) oxidation seemed to be a preliminary biotransformation as reflected by the dominance and increased accumulation of toxic As (III) by the alga. These findings may render it a prospective environmentally friendly candidate for reducing toxic As hazard risk and as a biological component in the treatment of wastewater. Moreover, these results also suggest that a more concerted effort is required in developing protocols for public health concerns regarding food safety and quality regulations in seafood and products sourced from macroalgae including S. suiae.

Analytical Methodologies for the Determination of Organoarsenicals in Edible Marine Species: A Review

Setting regulatory limits for arsenic in food is complicated, owing to the enormous diversity of arsenic metabolism in humans, lack of knowledge about the toxicity of these chemicals, and lack of accurate arsenic speciation data on foodstuffs. Identification and quantification of the toxic arsenic compounds are imperative to understanding the risk associated with exposure to arsenic from dietary intake, which, in turn, underscores the need for speciation analysis of the food. Arsenic speciation in seafood is challenging, owing to its existence in myriads of chemical forms and oxidation states. Interconversions occurring between chemical forms, matrix complexity, lack of standards and certified reference materials, and lack of widely accepted measurement protocols present additional challenges. This review covers the current analytical techniques for diverse arsenic species. The requirement for high-quality arsenic speciation data that is essential for establishing legislation and setting regulatory limits for arsenic in food is explored.

Environment-enhancing process for algal wastewater treatment, heavy metal control and hydrothermal biofuel production: A critical review

Coupling algae growth on wastewater with hydrothermal liquefaction (HTL) is regarded as an environment-enhancing pathway for wastewater management, biomass amplification, sustainable energy generation and value-added products generation. Through this integrated pathway, microalgae can not only recover nitrogen and phosphorus, but also absorb heavy metals from the wastewater. The migration and transformation of heavy metals need to be specifically assessed and considered due to the environmental concerns associated with metal toxicity. This work reviewed recent advances with respect to bioremediation mechanisms. Particular emphasis was placed on the heavy metal migration, transformation, and the key factors involved in algal wastewater treatment and biomass conversion. Additionally, the challenges of coupling algae wastewater treatment, hydrothermal conversion, and heavy metal control were addressed. Finally, a paradigm involving enhanced algal wastewater treatment and bioenergy production for field application was proposed.

Arsenolipids in the green alga Coccomyxa (Trebouxiophyceae, Chlorophyta)

Lipid-like compounds containing a dimethylarsinoyl group, i.e. Me₂As(O)-, have been identified by liquid chromatography/inductively coupled plasma mass spectrometry (LC/ICP-MS) and non-aqueous reversed-phase high-performance liquid chromatography (positive and/or negative high-resolution tandem electrospray ionization mass spectrometry (NARP-HPLC/HR-ESI^+(-)-MS/MS) from three strains of green algae of the genus Coccomyxa (Trebouxiophyceae, Chlorophyta). The algae were cultivated in a medium containing 10 g arsenic/L, i.e. 133.5 mmol/L of Na₂HAsO₄.7H₂O. After extraction by methyl-tert-butyl ether (MTBE), total lipids were analyzed by ICP-MS or ESI-MS without any further separation or fractionation. A total of 39 molecular species of arsenic triacylglycerols (AsTAG), 15 arsenic phosphatidylcholines (AsPC), 8 arsenic phosphatidylethanolamines (AsPE), 6 arsenic phosphatidylinositols (AsPI), 2 arsenic phosphatidylglycerols (AsPG) and 5 unknown lipids (probably ceramides) were identified. The structures of all molecular species were confirmed by tandem MS. Dry matter of the individual strains contained different amounts of total arsenolipids, i.e. C. elongata CCALA 427 (0.32 mg/g), C. onubensis (1.48 mg/g), C. elongata S3 (2.13 mg/g). On the other hand, there were only slight differences between strains in the relative abundances of individual molecular species. Possible biosynthesis of long-chain lipids with the end group Me₂As(O) has also been suggested.

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.

Identification of Steps in the Pathway of Arsenosugar Biosynthesis

Arsenosugars are arsenic-containing ribosides that play a substantial role in arsenic biogeochemical cycles. Arsenosugars were identified more than 30 years ago, and yet their mechanism of biosynthesis remains unknown. In this study we report identification of the arsS gene from the cyanobacterium Synechocystis sp. PCC 6803 and show that it is involved in arsenosugar biosynthesis. In the Synechocystis sp. PCC 6803 ars operon, arsS is adjacent to the arsM gene that encodes an As(III) S-adenosylmethionine (SAM) methyltransferase. The gene product, ArsS, contains a characteristic CX3CX2C motif which is typical for the radical SAM superfamily. The function of ArsS was identified from a combination of arsS disruption in Synechocystis sp. PCC 6803 and heterologous expression of arsM and arsS in Escherichia coli. Both genes are necessary, indicating a multistep pathway of arsenosugar biosynthesis. In addition, we demonstrate that ArsS orthologs from three other freshwater cyanobacteria and one picocyanobacterium are involved in arsenosugar biosynthesis in those microbes. This study represents the identification of the first two steps in the pathway of arsenosugar biosynthesis. Our discovery expands the catalytic repertoire of the diverse radical SAM enzyme superfamily and provides a basis for studying the biogeochemistry of complex organoarsenicals.

Arsenic speciation in wild marine organisms and a health risk assessment in a subtropical bay of China

The total arsenic (As) and As species were analyzed in 19 species of wild marine organisms collected from 12 locations in Daya Bay, China; additionally, both the levels of As in the seawater and sediments and the salinity were investigated. The greatest level of As was found in crabs (13.4-35.1 μg/g), followed by shrimps (8.52-27.6 μg/g), benthic fish (3.45-28.6 μg/g), and pelagic fish (1.22-5.23 μg/g). There were significantly positive correlations between the As concentrations in the benthic fish Callionymus richardsonii/shrimp Metapenaeopsis palmensis and those in sediments, indicating that As levels in them were highly dependent on those in the sediments. Arsenobetaine (AsB) (87.3-99.8%) was the most dominant form of As found in all marine organisms. In benthic fish and shrimp, the bioaccumulation of As, especially AsB, was positively correlated with the salinity of the seawater, indicating that the salinity could control the transfer of As. The calculated hazard quotients (HQs) of the inorganic As in the marine organisms were all <1; thus, there was no apparent health hazard through the consumption of wild marine organisms.

Arsenolipid biosynthesis by the unicellular alga Dunaliella tertiolecta is influenced by As/P ratio in culture experiments

Culture experiments exposing unicellular algae to varying arsenate/phosphate regimes and determining their arsenometallomes by HPLC–MS shows the interconnection of arsenolipids and water-soluble arsenicals.

Impacts of environmental factors on arsenate biotransformation and release in Microcystis aeruginosa using the Taguchi experimental design approach

Very limited information is available on how and to what extent environmental factors influence arsenic (As) biotransformation and release in freshwater algae. These factors include concentrations of arsenate (As(V)), dissolved inorganic nitrogen (N), phosphate (P), and ambient pH. This study conducted a series of experiments using Taguchi methods to determine optimum conditions for As biotransformation. We assessed principal effective factors of As(V), N, P, and pH and determined that As biotransformation and release actuate at 10.0 μM As(V) in dead alga cells, the As efflux ratio and organic As efflux content actuate at 1.0 mg/L P, algal growth and intracellular arsenite (As(III)) content actuate at 10.0 mg/L N, and the total sum of As(III) efflux from dead alga cells actuates at a pH level of 10. Moreover, N is the critical component for As(V) biotransformation in M. aeruginosa, specifically for As(III) transformation, because N can accelerate algal growth, subsequently improving As(III) accumulation and its efflux, which results in an As(V) to As(III) reduction. Furthermore, low P concentrations in combination with high N concentrations promote As accumulation. Following As(V), P was the primary impacting factor for As accumulation. In addition, small amounts of As accumulation under low concentrations of As and high P were securely stored in living algal cells and were easily released after cell death. Results from this study will help to assess practical applications and the overall control of key environmental factors, particularly those associated with algal bioremediation in As polluted water.

Marine Primary Productivity as a Potential Indirect Source of Selenium and Other Trace Elements in Atmospheric Deposition

Atmospheric processes play an important role in the supply of the trace element selenium (Se) as well as other essential trace elements to terrestrial environments, mainly via wet deposition. Here we investigate whether the marine biosphere can be identified as a source of Se and of other trace elements in precipitation samples. We used artificial neural network (ANN) modeling and other statistical methods to analyze relationships between a high-resolution atmospheric deposition chemistry time series (March 2007-January 2009) from Plynlimon (UK) and exposure of air masses to marine chlorophyll a and to other source proxies. Using ANN sensitivity analyses, we found that higher air mass exposure to marine productivity leads to higher concentrations of dissolved organic carbon (DOC) in rainfall. Furthermore, marine productivity was found to be an important but indirect factor in controlling Se as well as vanadium (V), cobalt (Co), nickel (Ni), zinc (Zn), and aluminum (Al) concentrations in atmospheric deposition, likely via scavenging by organic compounds derived from marine organisms. Marine organisms may thus play an indirect but important role in the delivery of trace elements to terrestrial environments and food chains.

A symbiotic bacterium differentially influences arsenate absorption and transformation in Dunaliella salina under different phosphate regimes

In this study, we investigated the effects of a symbiotic bacterium and phosphate (PO4(3-)) nutrition on the toxicity and metabolism of arsenate (As(V)) in Dunaliella salina. The bacterium was identified as Alteromonas macleodii based on analysis of its 16S rRNA gene sequence. When no As(V) was added, A. macleodii significantly enhanced the growth of D. salina, irrespective of PO4(3-) nutrition levels, but this effect was reversed after As(V)+PO4(3-) treatment (1.12mgL(-1)) for 3 days. Arsenic (As) absorption by the non-axenic D. salina was significantly higher than that by its axenic counterpart during incubation with 1.12mgL(-1) PO4(3-). However, when the culture was treated with 0.112mgL(-1) PO4(3-), As(V) reduction and its subsequent arsenite (As(III)) excretion by non-axenic D. salina were remarkably enhanced, which, in turn, contributed to lower As absorption in non-axenic algal cells from days 7 to 9. Moreover, dimethylarsinic acid was synthesized by D. salina alone, and the rates of its production and excretion were accelerated when the PO4(3-) concentration was 0.112mgL(-1). Our data demonstrate that A. macleodii strongly affected As toxicity, uptake, and speciation in D. salina, and these impacts were mediated by PO4(3-) in the cultures.