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.

Speciation analysis and toxicity evaluation of arsenolipids-an overview focusing on sea food

Arsenic, which can be divided into inorganic and organic arsenic, is a toxic metalloid that has been identified as a human carcinogen. A common source of arsenic exposure in seafood is arsenolipid, which is a complex structure of lipid-soluble organic arsenic compounds. At present, the known arsenolipid species mainly include arsenic-containing fatty acids (AsFAs), arsenic-containing hydrocarbons (AsHCs), arsenic glycophospholipids (AsPLs), and cationic trimethyl fatty alcohols (TMAsFOHs). Furthermore, the toxicity between different species is unique. However, the mechanism underlying arsenolipid toxicity and anabolism remain unclear, as arsenolipids exhibit a complex structure, are present at low quantities, and are difficult to extract and detect. Therefore, the objective of this overview is to summarize the latest research progress on methods to evaluate the toxicity and analyze the main speciation of arsenolipids in seafood. In addition, novel insights are provided to further elucidate the speciation, toxicity, and anabolism of arsenolipids and assess the risks on human health.

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.

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.

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.

Marine natural products

This review covers the literature published between January and December in 2018 for marine natural products (MNPs), with 717 citations (706 for the period January to December 2018) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1554 in 469 papers for 2018), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. The proportion of MNPs assigned absolute configuration over the last decade is also surveyed.

The response of Pyropia haitanensis to inorganic arsenic under laboratory culture

Up to now, complicated organoarsenicals were mainly identified in marine organisms, suggesting that these organisms play a critical role in arsenic biogeochemical cycling because of low phosphate and relatively high arsenic concentration in the marine environment. However, the response of marine macroalgae to inorganic arsenic remains unknown. In this study, Pyropia haitanensis were exposed to arsenate [As(V)] (0.1, 1, 10, 100 μM) or arsenite [As(III)] (0.1, 1, 10 μM) under laboratory conditions for 3 d. The species of water-soluble arsenic, the total concentration of lipid-soluble and cell residue arsenic of the algae cells was analyzed. As(V) was mainly transformed into oxo-arsenosugar-phosphate, with other arsenic compounds such as monomethylated, As(III), demethylated arsenic and oxo-arsenosugar-glycerol being likely the intermediates of arsenosugar synthesis. When high concentration of As(III) was toxic to P. haitanensis, As(III) entered into the cells and was transformed into less toxic organoarsenicals and As(V). Transcriptome results showed genes involved in DNA replication, mismatch repair, base excision repair, and nucleotide excision repair were up-regulated in the algae cells exposed to 10 μM As(V), and multiple genes involved in glutathione metabolism and photosynthetic were up-regulated by 1 μM As(III). A large number of ABC transporters were down-regulated by As(V) while ten genes related to ABC transporters were up-regulated by As(III), indicating that ABC transporters were involved in transporting As(III) to vacuoles in algae cells. These results indicated that P. haitanensis detoxifies inorganic arsenic via transforming them into organoarsenicals and enhancing the isolation of highly toxic As(III) in vacuoles.

Arsenolipids in Cultured Picocystis Strain ML and Their Occurrence in Biota and Sediment from Mono Lake, California

Primary production in Mono Lake, a hypersaline soda lake rich in dissolved inorganic arsenic, is dominated by Picocystis strain ML. We set out to determine if this photoautotrophic picoplankter could metabolize inorganic arsenic and in doing so form unusual arsenolipids (e.g., arsenic bound to 2-O-methyl ribosides) as reported in other saline ecosystems and by halophilic algae. We cultivated Picocystis strain ML on a seawater-based medium with either low (37 µM) or high (1000 µM) phosphate in the presence of arsenite (400 µM), arsenate (800 µM), or without arsenic additions (ca 0.025 µM). Cultivars formed a variety of organoarsenic compounds, including a phytyl 2-O-methyl arsenosugar, depending upon the cultivation conditions and arsenic exposure. When the cells were grown at low P, the organoarsenicals they produced when exposed to both arsenite and arsenate were primarily arsenolipids (~88%) with only a modest content of water-soluble organoarsenic compounds (e.g., arsenosugars). When grown at high P, sequestration shifted to primarily water-soluble, simple methylated arsenicals such as dimethylarsinate; arsenolipids still constituted ~32% of organoarsenic incorporated into cells exposed to arsenate but < 1% when exposed to arsenite. Curiously, Picocystis strain ML grown at low P and exposed to arsenate sequestered huge amounts of arsenic into the cells accounting for 13.3% of the dry biomass; cells grown at low P and arsenite exposure sequestered much lower amounts, equivalent to 0.35% of dry biomass. Extraction of a resistant phase with trifluoroacetate recovered most of the sequestered arsenic in the form of arsenate. Uptake of arsenate into low P-cultivated cells was confirmed by X-ray fluorescence, while XANES/EXAFS spectra indicated the sequestered arsenic was retained as an inorganic iron precipitate, similar to scorodite, rather than as an As-containing macromolecule. Samples from Mono Lake demonstrated the presence of a wide variety of organoarsenic compounds, including arsenosugar phospholipids, most prevalent in zooplankton (Artemia) and phytoplankton samples, with much lower amounts detected in the bottom sediments. These observations suggest a trophic transfer of organoarsenicals from the phytoplankton (Picocystis) to the zooplankton (Artemia) community, with efficient bacterial mineralization of any lysis-released organoarsenicals back to inorganic oxyanions before they sink to the sediments.

Cellular toxicological characterization of a thioxolated arsenic-containing hydrocarbon

Arsenolipids, especially arsenic-containing hydrocarbons (AsHC), are an emerging class of seafood originating contaminants. Here we toxicologically characterize a recently identified oxo-AsHC 332 metabolite, thioxo-AsHC 348 in cultured human liver (HepG2) cells. Compared to results of previous studies of the parent compound oxo-AsHC 332, thioxo-AsHC 348 substantially affected cell viability in the same concentration range but exerted about 10-fold lower cellular bioavailability. Similar to oxo-AsHC 332, thioxo-AsHC 348 did not substantially induce oxidative stress nor DNA damage. Moreover, in contrast to oxo-AsHC 332 mitochondria seem not to be a primary subcellular toxicity target for thioxo-AsHC 348. This study indicates that thioxo-AsHC 348 is at least as toxic as its parent compound oxo-AsHC 332 but very likely acts via a different mode of toxic action, which still needs to be identified.

Arsenic Toxicity: Molecular Targets and Therapeutic Agents

High arsenic (As) levels in food and drinking water, or under some occupational conditions, can precipitate chronic toxicity and in some cases cancer. Millions of people are exposed to unacceptable amounts of As through drinking water and food. Highly exposed individuals may develop acute, subacute, or chronic signs of poisoning, characterized by skin lesions, cardiovascular symptoms, and in some cases, multi-organ failure. Inorganic arsenite(III) and organic arsenicals with the general formula R-As²⁺ are bound tightly to thiol groups, particularly to vicinal dithiols such as dihydrolipoic acid (DHLA), which together with some seleno-enzymes constitute vulnerable targets for the toxic action of As. In addition, R-As²⁺-compounds have even higher affinity to selenol groups, e.g., in thioredoxin reductase that also possesses a thiol group vicinal to the selenol. Inhibition of this and other ROS scavenging seleno-enzymes explain the oxidative stress associated with arsenic poisoning. The development of chelating agents, such as the dithiols BAL (dimercaptopropanol), DMPS (dimercapto-propanesulfonate) and DMSA (dimercaptosuccinic acid), took advantage of the fact that As had high affinity towards vicinal dithiols. Primary prevention by reducing exposure of the millions of people exposed to unacceptable As levels should be the prioritized strategy. However, in acute and subacute and even some cases with chronic As poisonings chelation treatment with therapeutic dithiols, in particular DMPS appears promising as regards alleviation of symptoms. In acute cases, initial treatment with BAL combined with DMPS should be considered.