In situ arsenic speciation at the soil/water interface of saline-alkaline lakes of the Pantanal, Brazil: A DGT-based approach

Bioaccumulation and speciation of arsenic in plankton from tropical soda lakes along a salinity gradient

Uptake and transformation of arsenic (As) by living organisms can alter its distribution and biogeochemical cycles in the environment. Although well known for its toxicity, several aspects of As accumulation and biological transformation by field species are still little explored. In this study, the bioaccumulation and speciation of As in phytoplankton and zooplankton from five soda lakes in the Brazilian Pantanal wetland were studied. Such lakes exhibited contrasting biogeochemical characteristics along an environmental gradient. Additionally, the influence of contrasting climatic events was assessed by collecting samples during an exceptional drought in 2017 and a flood in 2018. Total As (AsTot) content and speciation were determined using spectrometric techniques, while a suspect screening of organoarsenicals in plankton samples was carried out by high-resolution mass spectrometry. Results showed that AsTot content ranged from 16.9 to 62.0 mg kg-1 during the dry period and from 2.4 to 12.3 mg kg-1 during the wet period. The bioconcentration and bioaccumulation factors (BCF and BAF) in phytoplankton and zooplankton were found to be highly dependent on the lake typology, which is influenced by an ongoing evapoconcentration process in the region. Eutrophic and As-enriched lakes exhibited the lowest BCF and BAF values, possibly due to the formation of non-labile As complexes with organic matter or limited uptake of As by plankton caused by high salinity stress. The season played a decisive role in the results, as significantly higher BCF and BAF values were observed during the flooding event when the concentration of dissolved As in water was low. The diversity of As species was found to be dependent on the lake typology and on the resident biological community, cyanobacteria being responsible for a significant portion of As metabolism. Arsenosugars and their degradation products were detected in both phytoplankton and zooplankton, providing evidence for previously reported detoxification pathways. Although no biomagnification pattern was observed, the diet seemed to be an important exposure pathway for zooplankton.

Arbuscular Mycorrhizal Fungi Alter Arsenic Translocation Characteristics of Iris tectorum Maxim

Arsenic (As) pollution in wetlands, mainly as As(III) and As(V), has threatened wetland plant growth. It has been well documented that arbuscular mycorrhizal (AM) fungi can alleviate As stress in terrestrial plants. However, whether AM fungi can protect natural wetland plants from As stress remains largely unknown. Therefore, three hydroponic experiments were conducted in which Iris tectorum Maxim. (I. tectorum) plants were exposed to As(III) or As(V) stresses, to investigate the effects of mycorrhizal inoculation on As uptake, efflux, and accumulation. The results suggested that short-term kinetics of As influx in I. tectorum followed the Michaelis-Menten function. Mycorrhizal inoculation decreased the maximum uptake rate (Vmax) and Michaelis constant (Km) of plants for As(III) influx, while yielding no significant difference in As(V) influx. Generally, mycorrhizal plants released more As into environments after 72 h efflux, especially under As(V) exposure. Moreover, mycorrhizal plants exhibited potential higher As accumulation capacity, probably due to more active As reduction, which was one of the mechanisms through which AM fungi mitigate As phytotoxicity. Our study has revealed the role of aerobic microorganism AM fungi in regulating As translocation in wetland plants and supports the involvement of AM fungi in alleviating plant As stress in anaerobic wetlands.

Spatial and seasonal variation of arsenic speciation in Pantanal soda lakes

The occurrence of high arsenic concentrations (up to 3000 μg L^-1) in water of soda lakes of the Pantanal wetland is a remarkable case of natural arsenic contamination in South America. However, little is known about arsenic speciation in this environment, particularly regarding speciation changes related to lake trophic status and seasonal variations. To fill this gap, arsenic speciation analysis was carried out in surface (SW) and subsurface (SSW) waters sampled in five soda lakes with different eutrophication status, in two dry and one wet season. As(V) was the dominant species in these waters, while As(III), DMA, MMA and likely complex organic species were present in lower amounts. The results allow to conclude that the arsenic speciation in SW and SSW varies seasonally according to the regional wet or dry periods and lake water levels. In eutrophic turbid and in oligotrophic vegetated soda lakes, arsenic speciation was also characterized by spatial differences between edge and center or between the SW and SSW. Cyanobacteria or macrophytes/algae are involved in arsenic biotransformation in soda lakes through its metabolic and detoxification processes. Significant variation in surface water arsenic speciation occurs as a result of seasonal primary production fluctuation or water arsenic concentration changes in the soda lakes, increasing organoarsenics in dry periods, whereas in flood periods, As(V) prevails. Spatial distribution of arsenic species is significantly impacted by biogeochemical conditions at the water/sediment interface in soda lakes.

Pelagic and estuarine birds as sentinels of metal(loid)s in the South Atlantic Ocean: Ecological niches as main factors acting on bioaccumulation

Activities related to the offshore exploration and production of oil and natural gas provide economic development and an essential energy source. However, besides the risk of petroleum hydrocarbon contamination, these activities can also be sources of metals and metalloids for marine organism contamination. In this research, we evaluated the potential use of two pelagic (black-browed albatross Thalassarche melanophris and yellow-nosed albatross T. chlororhynchos) and one estuarine bird species (neotropical cormorant Nannopterum brasilianus) as sentinels of contamination of As, Cd, Cr, Cu, Pb, Mn, Mo, Zn, Ni, Ba, V, and Hg in an area under influence of oil and gas activities. The analyses were carried out in samples collected from 2015 to 2022 from 97 individuals. A factor alert; an adaptation from the contamination factor is proposed to identify individuals with high concentrations that possibly suffered contamination by anthropogenic origin. Grouping all species, the metal(loid)s with the highest concentrations were in decreasing order: Zn > Cu > Mn > Hg > As > Cd > Mo > V > Cr > Ba > Ni > Pb. Similar concentrations were observed for V, Mn, Cr and Pb among the three species. Pelagic birds showed higher levels of concentrations for Hg, As and Cd. Based on the correlations and multivariate analysis performed, the results indicate that the ecological niche factor has greater relevance in the bioaccumulation of these elements compared to the habitat. Although some individuals showed high concentrations in part of the trace elements, suggesting exposure to anthropic sources, the direct influence of oil production and exploration activities was not observed, suggesting that activities on the continent are the primary contamination source. The results of this work highlight the role of seabirds as sentinels for metal(loid)s, contributing to the knowledge of the occurrence of contaminants in the South Atlantic Ocean.

Arsenic pools in soils under native vegetation on a steatite outcrop in Brazil

Pristine soils under native vegetation can present high levels of potentially toxic elements when developed from the weathering of some unusual parent materials, especially ultramafic rocks and some metal ores. Here, we used various selective extractions in order to study the partition and potential availability of As in eight soils developed from steatite (a talc-rich rock) on an ultramafic hill in Brazil. Soils varied from shallow Entisols on the summit to Inceptisols and Oxisols on slopes and footslopes, where total As contents (determined by X-ray fluorescence) reached levels as high as 225 mg kg^-1, which might raise concerns about their potential agricultural use and occupation. Despite these high values for pristine soils, water- and Mehlich-available As were nil or negligible in all soils, whereas oxalate-extractable As reached a maximum 4.2 mg kg^-1, and the highest semi-total (nitric acid digestion) was 9.3 mg kg^-1. However, As relative availability (compared to total As) varied widely among soils, with one Inceptisol (with a total 11-19 mg kg^-1) reaching 100% of its total As extractable by nitric acid, whereas an Oxisol showed <0.1% in nitric acid extract. Generally, we can conclude that, in soils with the highest total As concentrations, most As is contained within resistant, coarse phases such as primary magnetite, chromite and others, and a minor but still considerable part is bound to secondary Fe oxides. Thus, despite the unusually high As contents for soils under pristine savannic and forest native vegetations, the different As pools assessed here apparently do not raise immediate concerns where ultramafic rocks rich in Fe oxides give rise to soils under tropical climate. However, it is theoretically possible that subsoil saturation and Fe oxide reduction release some As in ground- and surface waters, which deserves further investigation.

Bioaccumulation, genotoxicity, and risks to native fish species from inorganic contaminants in the Pantanal Sul-Mato-Grossense, Brazil

The Aquidauana River is one of the most important rivers in the Pantanal region, Brazil. However, its waters have been contaminated by nearby anthropogenic activities, threatening native fish species. In this study, our objectives were: 1) to determine the concentrations of Al, As, Cd, Cu, Fe, Mn, Ni, Pb, and Zn in water and sediment samples from the Aquidauana River and to assess the risks posed to aquatic biota; 2) to quantify the concentration of these elements in muscle and liver tissue samples from four native fish species; 3) to evaluate the potential bioaccumulation of inorganic elements in the muscles and liver; and 4) to investigate genotoxicity biomarkers and their association with the inorganic element concentrations present in the muscle tissue. Water and fish samples were collected in November 2020. The concentrations of Al, As, Cd, Cu, Fe, and Pb in the water samples were in disagreement with the Brazilian legislation and presented risks to the aquatic biota. In terms of mixtures of inorganic elements, there was a great increase in the risk to biota. The As concentration did not meet the Brazilian standard for sediments in the sample collected at sampling site 6. The concentrations of Cd and Pb in the muscle tissue of Hypostomus regani, Prochilodus lineatus, Brycon hilarii, and Mylossoma duriventre exceeded the Brazilian standards for human consumption. H. regani showed greater genotoxic damage, and the higher the Al and Fe concentrations in the muscle tissue, the higher the frequencies of lobulated nuclei and nuclear invaginations. Together, our results demonstrate the negative impacts on native fish species from the Aquidauana River contamination and indicate risks to Pantanal biodiversity.

Arsenic extraction from seriously contaminated paddy soils with ferrihydrite-loaded sand columns

Reductive dissolution of iron oxides in flooded paddy soils is the most important cause of arsenic (As) release into soil aqueous solution and thus entry into rice. From the perspective of soil cleanup, however, As release under flooded condition could facilitate labile As removal. In this study, a porous column pre-loaded with ferrihydrite (Fh) was constructed, and its efficiency of soil As extraction was investigated using a purpose-designed mesocosm coupled with diffusive gradients in thin films (DGT) for in situ visualization. With Fh-column deployed in aqueous solution, >90% removal of As(III) was achieved within 5 days at initial As (100 mg L^-1) of two orders of magnitude higher than in most paddy soil solutions (1-1538 μg L^-1). By applying Fh-column in a seriously contaminated paddy soil (102 mg As kg^-1), porewater As showed stepwise decreases from 2727 μg L^-1 to 129-1455 μg L^-1 at a distance-dependent manner over four intermittent extractions during 91 days. Soil DGT-As exhibited similar spatiotemporal changes to porewater As. After four extractions, 17.8% of total soil As was removed by Fh-column in a 10 cm radius range on average and ∼1/3 of As bound to amorphous and crystalline Fe/Al oxides was depleted, which accounted for 88.7% of decline in total soil As. With the post-extracted soil, a 48% lower As accumulation in rice seedlings and a 65% decline in bulk soil DGT-As were attained. This study provides a conceptual foundation for rapid removal of high soluble As by Fh-columns from flooded soils, improving seriously As-contaminated paddies to sustainable resources for safe food production.

In situ fractionation and redox speciation of arsenic in soda lakes of Nhecolândia (Pantanal, Brazil) using the diffusive gradients in thin films (DGT) technique

In situ fractionation and redox speciation of As in three different saline-alkaline lakes (green, black and crystalline lakes) in the Pantanal of Nhecolândia (Brazil) were performed by using Diffusive Gradients in Thin films (DGT). The results indicated that As is present mainly in dissolved form. Total As concentration was similar when using different filter membranes, demonstrating that the species adsorbed by DGT devices were <10 kDa. Higher concentrations of labile total As were observed in the center of the lakes, indicating that the nature of the organic matter influences the formation of As complexes. Total As concentrations determined by using ZrO₂ DGT were consistent with As concentration in ultrafiltered water samples collected in the black lake. However, part of the data about As(III) obtained in grab samples contrasted with DGT results. The differences observed may indicate that alterations in the species occur during the storage period before analysis by ultrafiltration. As(III) concentrations measured by DGT in the black and crystalline lakes were 1-3 μg L^-1 and 4-7 μg L^-1, respectively, accounting for only 4%-8% of the total DGT inorganic As. In the green lake, As(III) concentrations were significantly higher at the center (217 μg L^-1). Both the phytoplankton community and the dissolved organic carbon influence the As speciation and bioavailability in the lakes of Nhecolândia. The DGT approach used in the present work was able to perform As speciation and demonstrates that in situ sampling analytical techniques are essential in understanding As speciation and its behavior in complex natural aquatic systems.