Microbial population genomes from the Amazon River reveal possible modulation of the organic matter degradation process in tropical freshwaters.

Rivers connect the carbon cycle in land with that in aquatic ecosystems by transporting and transforming terrestrial organic matter (TeOM). The Amazon River receives huge loads of TeOM from the surrounding rainforest, promoting a substantial microbial heterotrophic activity and consequently, CO2 outgassing. In the Amazon River, microbes degrade up to 55% of the lignin present in the TeOM. Yet, the main microbial genomes involved in TeOM degradation were unknown. Here, we characterize 51 population genomes (PGs) representing some of the most abundant microbes in the Amazon River deriving from 106 metagenomes. The 51 reconstructed PGs are among the most abundant microbes in the Amazon River, and 53% of them are not able to degrade TeOM. Among the PGs capable of degrading TeOM, 20% were exclusively cellulolytic, while the others could also oxidize lignin. The transport and consumption of lignin oxidation byproducts seemed to be decoupled from the oxidation process, being apparently performed by different groups of microorganisms. By connecting the genomic features of abundant microbes in the Amazon River with the degradation machinery of TeOM, we suggest that a complex microbial consortium could explain the quick turnover of TeOM previously observed in this ecosystem.

Egg microbiota is the starting point of hatchling gut microbiota in the endangered yellow-spotted Amazon river turtle.

Establishment and development of gut microbiota during vertebrates' early life are likely to be important predictors of health and fitness. Host-parental and host-environment interactions are essential to these processes. In oviparous reptiles whose nests represent a source of the parent's microbial inocula, the relative role of host-selection and stochastic environmental factors during gut microbial assemblage remains unknown. We sampled eggs incubated in artificial nests as well as hatchlings and juveniles (up to 30 days old) of the yellow-spotted Amazon river turtle (Podocnemis unifilis) developing in tubs filled with river water. We examined the relative role of the internal egg microbiota and the abiotic environment on hatchling and juvenile turtle's cloacal microbiota assemblages during the first 30 days of development. A mean of 71% of ASVs in hatched eggs could be traced to the nest environmental microbiota and in turn a mean of 77% of hatchlings' cloacal ASVs were traced to hatched eggs. Between day 5 and 20 of juvenile turtle's development, the river water environment plays a key role in the establishment of the gut microbiota (accounting for a mean of 13%-34.6% of cloacal ASVs) and strongly influences shifts in microbial diversity and abundance. After day 20, shifts in gut microbiota composition were mainly driven by host-selection processes. Therefore, colonization by environmental microbiota is key in the initial stages of establishing the host's gut microbiota which is subsequently shaped by host-selection processes. Our study provides a novel quantitative understanding of the host-environment interactions during gut microbial assemblage of oviparous reptiles.

Direct and indirect effects of geographic and environmental factors on ant beta diversity across Amazon basin.

Understanding the direct and indirect effects of niche and neutral processes in structuring species diversity is particularly challenging because environmental factors are often geographically structured. Here, we used Structural Equation Modeling to quantify direct and indirect effects of geographic distance, the Amazon River's opposite margins, and environmental differences in temperature, precipitation, and vegetation density (Normalized Difference Vegetation Index-NDVI) on ant beta diversity (Jaccard's dissimilarity) across Amazon basin. We used a comprehensive survey of ground-dwelling ant species from 126 plots distributed across eight sampling sites along a broad environmental gradient. We found that geographic distance and NDVI differences were the major direct predictors of ant composition dissimilarity. The major indirect effect was that of temperature through NDVI, whereas precipitation neither had direct or indirect detectable effects on beta diversity. Thus, ant compositional dissimilarity seems to be mainly driven by a combination of isolation by distance (through dispersal limitation) and selection imposed by vegetation density, and indirectly, by temperature. Our results suggest that neutral and niche processes have been similarly crucial in driving the current beta diversity patterns of Amazonian ground-dwelling ants.

Molecular features of Probopyrus sp. (Isopoda: Bopyridae) from Brazilian Amazonia and the parasitism of inland populations of Macrobrachium amazonicum (Decapoda: Palaemonidae).

Bopyrid isopods of the genus Probopyrus are well-known parasites of freshwater prawns of the genus Macrobrachium. The parasitism of coastal populations of Macrobrachium amazonicum by Probopyrus bithynis, for example, has been documented since the late 1980s. Despite this, molecular data on different populations are not available for any Probopyrus species. The present study is the first to describe Probopyrus populations from distinct regions of the Amazon basin based on sequences of two genes, the mitochondrial cytochrome oxidase C subunit I (COI) and the nuclear 18S ribosomal DNA (18S rDNA) gene. The analyses indicated the presence of two Probopyrus species, each parasitizing either the coastal or the inland populations of M. amazonicum. The results indicated the potential use of the COI barcode for the identification of Probopyrus species. We discuss the potential implications of the findings for the taxonomy of Probopyrus bithynis and other species of the genus Probopyrus.

Molecular and morphological evidence of a new species of Crassicutis Manter 1936 (Digenea), a parasite of cichlids in South America.

A new species of Crassicutis Manter, 1936 (Digenea: Megaperidae) is described from the intestine of Satanoperca jurupari (Cichlidae) in the Amazon River basin, Brazil. The genus Crassicutis currently contains eight species. Crassicutis manteri n. sp. is morphologically very similar to Crassicutis cichlasomae Manter, 1936, a parasite of cichlids reported from Mexico, the Antilles, and Central and South America. Molecular data revealed that C. cichlasomae represents a species complex in Middle American cichlids. The new species can be readily distinguished from C. cichlasomae sensu lato, and the other congeners, by a combination of morphological traits such as a narrow, elongate mouth opening (versus spherical in other species), the tandem position of testes (symmetrical or oblique in most congeners), narrow body widening towards its posterior end (versus widely oval, leaf-like in other species), and short intestinal caeca ending close to the posterior end of the posterior testis (versus reaching more posteriorly in other species). Six novel sequences of 28S rDNA, ITS1, and cox1 were generated for two isolates of the new species. Sequences of the 28S rRNA gene were used to corroborate that Crassicutis is sister taxa of Homalometron Stafford, 1904. Mitochondrial DNA corroborated the distinction of the new species with previously sequenced congeners in Middle American cichlids; the interspecific divergence between the new species and the genetic lineages of C. cichlasomae was very high, varying between 23.7 and 27.2%. Biogeographical implications of our findings are briefly discussed including questionable validity of records of C. cichlasomae from South America.

Genome-resolved metagenomics analysis provides insights into the ecological role of Thaumarchaeota in the Amazon River and its plume.

BACKGROUND: Thaumarchaeota are abundant in the Amazon River, where they are the only ammonia-oxidizing archaea. Despite the importance of Thaumarchaeota, little is known about their physiology, mainly because few isolates are available for study. Therefore, information about Thaumarchaeota was obtained primarily from genomic studies. The aim of this study was to investigate the ecological roles of Thaumarchaeota in the Amazon River and the Amazon River plume. RESULTS: The archaeal community of the shallow in Amazon River and its plume is dominated by Thaumarchaeota lineages from group 1.1a, which are mainly affiliated to Candidatus Nitrosotenuis uzonensis, members of order Nitrosopumilales, Candidatus Nitrosoarchaeum, and Candidatus Nitrosopelagicus sp. While Thaumarchaeota sequences have decreased their relative abundance in the plume, Candidatus Nitrosopelagicus has increased. One genome was recovered from metagenomic data of the Amazon River (ThauR71 [1.05 Mpb]), and two from metagenomic data of the Amazon River plume (ThauP25 [0.94 Mpb] and ThauP41 [1.26 Mpb]). Phylogenetic analysis placed all three Amazon genome bins in Thaumarchaeota Group 1.1a. The annotation revealed that most genes are assigned to the COG subcategory coenzyme transport and metabolism. All three genomes contain genes involved in the hydroxypropionate/hydroxybutyrate cycle, glycolysis, tricarboxylic acid cycle, oxidative phosphorylation. However, ammonia-monooxygenase genes were detected only in ThauP41 and ThauR71. Glycoside hydrolases and auxiliary activities genes were detected only in ThauP25. CONCLUSIONS: Our data indicate that Amazon River is a source of Thaumarchaeota, where these organisms are important for primary production, vitamin production, and nitrification.

PAH Profiles in Suspended Particulate Matter from an Urbanized River Within the Brazilian Amazon.

The distribution, seasonal variation and sources of 16 polycyclic aromatic hydrocarbons (PAH) from suspended particulate matter (SPM) of the Aurá River, a small amazon typical river located in Northern Brazil, were determined. Gas chromatography-mass spectrometry analysis of SPM revealed a mixture of PAH from different origins and a seasonal variation of PAH primary source for the studied area. Pyrene was the dominant PAH in both studied periods. Total PAH content (ΣPAH) ranged from lower than quantification limit (< LOQ) to 2498.2 ng g-1 dw during the dry season and < LOQ to 2865.8 ng g-1 dw during the wet season. Low molecular weight PAH (LMW) represented 51% of ΣPAH during the dry season and 29% during the wet season. It was noted, by comparing previous data, that the main source of these compounds was altered after the deactivation of an irregular landfill next to the river.

Histopathological alterations in gills of Amazonian shrimp Macrobrachium amazonicum parasitized by isopod Probopyrus bithynis (Bopyridae).

The present study describes, for the first time, histopathological alterations in the gills of Macrobrachium amazonicum caused by infestation of Probopyrus bithynis (Isopoda: Bopyridae). In every case (100%), the infestation by P. bithynis was by a single pair of parasites (male and female) and occurred in the right or left side of the branchial chamber; the gill structures were visibly compressed due to the presence of parasites. The gills of M. amazonicum parasitized by P. bithynis exhibited a chronic inflammatory response, with the presence of edema, greater quantities of hemocytes, necrosis, epithelial cell hyperplasia, rupture of the pillar cells at the ends of the gill lamellae, desquamation of the cuticle, lamellar fusion and rupture of the lamellar epithelium. Tissue lesions were found in the histological sections of the gills of the parasitized M. amazonicum. Structural alterations in the branchial chamber of the hosts caused by the presence of P. bithynis can lead to physiological changes that can impair host respiratory performance. Finally, histopathological alterations in the branchial chamber of hosts suggest that P. bithynis feed directly on the gill tissues of this shrimp.

Cryoprotectant agents and cooling effect on embryos of Macrobrachium amazonicum.

SummaryThere are few reports of cryopreservation and injuries in Macrobrachium amazonicum embryos. Thus, the aim of this study was to analyze the effects of cryoprotectants agents and cooling on stage VIII of this species. Fertilized eggs from ovigerous females were removed from the incubation chamber, then placed in 10 ml Falcon tubes with a cryoprotectant solution and saline-free calcium solution. Thus, the embryos underwent a cooling curve of 1°C per min until reaching 5°C, and then were stored for 2 h. The tubes containing the embryos were washed to remove the cryoprotectant, acclimated for 5 min and then transferred to 50 ml incubators. At the end of the 24-h period, living embryos from each tube were counted and tabulated. A pool of embryos was fixed with 4% formaldehyde and then subjected to histology using 3-mm thick sections and stained with haematoxylin/eosin. Another pool was used for biometric analysis in which length, width and volume were analyzed. The cryoprotectants agents used were: dimethylsulfoxide (DMSO), methyl alcohol, ethylene glycol at 1, 5 and 10% and sucrose (0.5 M). Variance analysis was performed followed by Tukey's honest significant difference (HSD) test at 5% significance level. DMSO cryoprotectant affected embryo survival the least with rates of 71.8, 36.2 and 0% for concentrations of 1, 5 and 10%, respectively. Ethylene glycol caused 100% mortality at all the concentrations used. It was not possible to observe the interference of cooling and cryoprotectants on embryonic structures in this study.

Role of the floodplain lakes in the methylmercury distribution and exchanges with the Amazon River, Brazil.

Seasonal variability of dissolved and particulate methylmercury (F-MeHg, P-MeHg) concentrations was studied in the waters of the Amazon River and its associated Curuai floodplain during hydrological year 2005-2006, to understand the MeHg exchanges between these aquatic systems. In the oxic white water lakes, with neutral pH, high F-MeHg and P-MeHg concentrations were measured during the rising water stage (0.70±0.37pmol/L, n=26) and flood peak (14.19±9.32pmol/g, n=7) respectively, when the Amazon River water discharge into the lakes was at its maximum. The lowest mean values were reported during the dry season (0.18±0.07pmol/L F-MeHg, n=10 and 1.35±1.24pmol/g P-MeHg, n=8), when water and suspended sediments were outflowing from the lakes into the River. In these lakes, the MeHg concentrations were associated to the aluminium and organic carbon/nitrogen changes. In the black water lakes, with acidic pH and reducing conditions, elevated MeHg concentrations were recorded (0.58±0.32pmol/L F-MeHg, n=16 and 19.82±15.13pmol/g P-MeHg, n=6), and correlated with the organic carbon and manganese concentrations. Elevated values of MeHg partition coefficient (4.87

Wide but not impermeable: Testing the riverine barrier hypothesis for an Amazonian plant species.

Wallace's riverine barrier hypothesis postulates that large rivers, such as the Amazon and its tributaries, reduce or prevent gene flow between populations on opposite banks, leading to allopatry and areas of species endemism occupying interfluvial regions. Several studies have shown that two major tributaries, Rio Branco and Rio Negro, are important barriers to gene flow for birds, amphibians and primates. No botanical studies have considered the potential role of the Rio Branco as a barrier, while a single botanical study has evaluated the Rio Negro as a barrier. We studied an Amazon shrub, Amphirrhox longifolia (A. St.-Hil.) Spreng (Violaceae), as a model to test the riverine barrier hypothesis. Twenty-six populations of A. longifolia were sampled on both banks of the Rio Branco and Rio Negro in the core Amazon Basin. Double-digest RADseq was used to identify 8,010 unlinked SNP markers from the nuclear genome of 156 individuals. Data relating to population structure support the hypothesis that the Rio Negro acted as a significant genetic barrier for A. longifolia. On the other hand, no genetic differentiation was detected among populations spanning the narrower Rio Branco, which is a tributary of the Rio Negro. This study shows that the strength of riverine barriers for Amazon plants is dependent on the width of the river separating populations and species-specific dispersal traits. Future studies of plants with contrasting life history traits will further improve our understanding of the landscape genetics and allopatric speciation history of Amazon plant diversity.

Effects of the exposure to atrazine on bone development of Podocnemis expansa (Testudines, Podocnemididae).

The use of pesticides is a widely spread practice in Brazilian agriculture, and dispersion of these substances is an important factor for the fauna and flora. Atrazine is an endocrine disruptor in the xenoestrogen class that is used worldwide in agricultural practices. In Brazil, its use is permitted in several crops. Podocnemis expansa is a representative of the Testudines order that is the largest freshwater reptile of South America. Its distribution enables it to get in contact with molecules that are commonly used as pesticides, which may cause deleterious effects in target populations. In order to evaluate the possible effects of the exposure to atrazine on bone ontogeny of this species, eggs were artificially incubated in sand moistened with water contaminated with atrazine at concentrations equal to 0, 2, 20 or 200 µg/L. Embryos were collected throughout incubation and submitted to diaphanization of soft tissues with potassium hydroxide (KOH); bones were stained with Alizarin red S and cartilages by Alcian blue. Embryos were evaluated for the presence of abnormalities during the different stages of pre-natal development of skeletal elements. No effect of atrazine was observed on bone development during the embryonic phase in P. expansa individuals, in the conditions of this study.

Toxicity of cryoprotectants agents in freshwater prawn embryos of Macrobrachium amazonicum.

The process of cooling and cryopreservation of prawn embryos is a viable alternative for a continuous supply of larvae for freshwater prawn farming ponds. However, studies involving the application of those techniques as well as on toxicity of cryoprotectants in freshwater prawn embryos are scarce. Thus, this study aims to test the toxicity of methylic alcohol (MET), dimethyl sulfoxide (DMSO) and ethylene glycol (EG) on Macrobrachium amazonicum embryos. For the present experiment, pools of embryos were taken from 15 M. amazonicum females and were divided into three groups and tested in duplicate at concentrations of 10, 5, 3; 1, 0.5 or 0.1%. Toxicity tests were conducted for 24 h in Falcon® pipes to obtain the lethal concentration for 50% of the larvae (LC50). After the set period for testing, random samples of embryos were removed for morphological analysis under stereoscopic microscopes. Results were analysed using analysis of variance (ANOVA) and Tukey's test at a 5% significance level and Trimmed Spearman-Karber Analysis to determine LC50-24 h. DMSO toxicity tests revealed that 5% and 10% concentrations showed the highest toxicity and differed from the control (P ≤ 0.05), 24h-LC50 was 437.4 ± 14.4 µL. MET was less toxic among the tested cryoprotectants and concentrations did not allow the determination of its LC50-24h. For tests with EG, concentrations of 3, 5 or 10% solutions resulted in a 100% mortality to tested embryos; EG was the tested cryoprotectant with the highest toxicity, with an LC50-24h average of 81.91 ± 35.3 µl.

A new genus and species of proteocephalidean tapeworm (Cestoda), first parasite found in the driftwood catfish Tocantinsia piresi (Siluriformes: Auchenipteridae) from Brazil.

Frezella gen. n. is proposed to accommodate Frezella vaucheri sp. n. from poorly known auchenipterid fish, Tocantinsia piresi (Miranda Ribeiro), from the Xingú River, one of the principal tributaries of the lower Amazon River in Brazil. The new genus belongs to the Proteocephalinae because of the medullary position of the testes, ovary (yet some follicles penetrate to the cortex on the dorsal side), vitelline follicles and uterus. It differs from other proteocephaline genera in the morphology of the scolex, which includes a metascolex composed of two distinct zones: anterior, strongly wrinkled part posterior to the suckers, and posterior, sparsely folded zone. Frezella can also be differentiated by having the internal longitudinal musculature hypertrophied laterally on both sides, the presence of some ovarian follicles in the cortex on the dorsal side and the presence of additional pair of tiny, thin-walled osmoregulatory canals situated slightly dorsomedian to ventral canals. Frezella vaucheri is the first helminth parasite reported from T. piresi, which occurs in the lower reaches of the Amazon and Tocantins River basins in Brazil.

The lowest diploid number in Testudines: Banding patterns, telomeric and 45S rDNA FISH in Peltocephalus dumerilianus, 2n = 26 and FN = 52 (Pleurodira, Podocnemididae).

The karyotype of the big-headed Amazon River turtle, Peltocephalus dumerilianus, is characterized based on a sample of seven juveniles from Reserva Biológica do Rio Trombetas, Pará State, Brazil (1°30' S, 56°34' W). Here we present the first results on GTG and CBG-banding patterns, Ag-NOR staining and FISH, with telomeric and 45S rDNA sequences as probes. A cytogenetic comparison with related Podocnemidae is also provided.

Faith healing: the threat of "Surucucu" and the local cure of Amazon floodplain dwellers.

BACKGROUND: Snakebites are considered a neglected tropical disease responsible for many accidents, some fatal, and are related to poverty. The Brazilian Amazon has the highest incidence of snakebites per inhabitant, with the state of Pará having the most reported cases. For those who have difficult access to hospitals and pharmacies, this issue is even more urgent. METHODS: In this research, we worked together with the population of five riverine communities in Aritapera (Santarém-PA), a fluvial island located in the Lower Amazon floodplain (várzeas), in order to identify the species of venomous snakes and create a record of snakebites in the region and treatments carried out. RESULTS: Dwellers reported a high frequency of encounters throughout the year and mentioned five venomous ethnospecies, although we identified only Bothrops atrox. Approximately 28.7% of the participants had already been bitten, and in 15.8% of the interviews, they mentioned deaths from snakebites. The treatments varied between hospitalization (42.8%), home treatments (23.8%), both together (25.4%) and healers (7.9%). There were cases where no treatment, or just religious treatment, was performed. In general, no serious sequelae were reported. Although home treatments were more common in the past, many people maintained the practice of using them before going to the hospital. Among the most used are Pau X and the fat of the Amazon River Dolphin. The latter appears to be a recent discovery by locals and is considered very efficient both for humans and animals. CONCLUSION: Difficult access to health centers, a lack of energy to store antivenom and a high rate of encounters with snake place Aritapera dwellers in a vulnerable situation regarding snakebite accidents. In this context, they discovered treatments that improved their well-being until hospitalization. As the Amazon River dolphin is an endangered species, the use of its fat requires attention. In this sense, the dissemination of this knowledge is important to encourage studies that investigate which properties of this fat act as counterpoisons. By discovering substitutes that can be incorporated in other rural and remote communities, an economic and ecologically viable option for the health of residents can be promoted, in addition to valuing traditional knowledge.

Assessing the ovarian development of Macrobrachium amazonicum (Heller, 1862) phenotypes by means of an integrative analysis.

Macrobrachium amazonicum is a species of economic interest with a wide distribution in the Americas and high morphological and reproductive variability. Three phenotypes can be observed in this species: i) large-size amphidromous, ii) large-size and iii) small-size hololimnetic prawns. In the present work, the morphological, histochemical and ultrastructural aspects of ovarian development in the three phenotypes were comparatively analyzed. In addition, the interaction between the ovary and the hepatopancreas was investigated in these phenotypes through the use of gonadosomatic (GSI) and hepatosomatic (HSI) indices. Despite the morphological differences and different reproductive strategies adopted by the females, the macroscopic, histochemical and ultrastructural patterns of ovarian development showed no differences between the phenotypes. The ovaries were macroscopically classified into five stages of development (I to V). In early stages (I and II), the ovaries are full of oogonia, previtellogenic oocytes and oocytes in primary or endogenous vitellogenesis. At these stages, the rough endoplasmic reticulum (RER) produces a granular electron-dense material and sends it to the Golgi apparatus, where it will be modified, compacted and transformed into immature yolk granules. From stage III, secondary or exogenous vitellogenesis begins (with no interruption of endogenous vitellogenesis), where follicular cells nourish the oocytes and extracellular material is absorbed by endocytic vesicles, which fuse with immature yolk granules (forming mature granules) or with existing mature yolk granules. In stages IV and V, secondary vitellogenesis continues and mature yolk granules progressively occupy the cytoplasm. In M. amazonicum, the patterns of increase in oocyte diameter are quite similar between phenotypes, being greater in the small-size phenotype. This is related to the formation of larger oocytes/eggs and the production of large lipid reserves for their larvae. Changes in GSI and HSI during ovarian development show strong similarity between phenotypes, supporting the results obtained by histology and ultrastructure. Females in stages III and IV mobilize hepatopancreas reserves for ovarian maturation, which justifies the higher HSI values recorded in these stages. On the other hand, females in stage V show higher GSI and lower HSI values, indicating a mobilization of resources for the end of ovarian development as the females are ready to spawn.

A systematic review on metal contamination due to mining activities in the Amazon basin and associated environmental hazards.

Metal contamination associated with mining activities has been considered one of the main environmental pollution problems in the Amazon region. Understanding the levels of metal contamination from mining activities requires a good understanding of background metal concentrations, which may vary notably according to the geology/lithology characteristics of the region, soil type, and predominant biogeochemical processes. This review assessed 50 papers and reports published between 1989 and 2020 describing environmental concentrations of different metals and metalloids (As, Hg, Mn, Fe, Cd, Cu, Cr, Pb, Ni, and Zn) in water and sediments of mining and non-mining areas in five geographic regions of the Amazon basin. Metal enrichment caused by mining activities was calculated and exposure concentrations were compared with sediment and water quality standards set for the protection of aquatic life. Significant enrichments of Cd, Cu, Cr, Fe, Hg, Mn, Ni and Zn were observed in mining areas in both sediment and water. Regarding background levels in the different geographic regions, the highest prevalence of metal enrichment (i.e., concentrations 10 to 100-fold higher than mean background values) in sediment samples was found for Fe (100% of samples), Ni (90%), and Mn (69%). For water, high prevalence of metal enrichment occurred for Zn, Mn, and Fe (100% of samples), and for Hg (86%). Hg, Fe, Pb, Cu, Cd, Ni and Zn exceeded water and/or sediment quality standards in a significant number of samples in the proximity of mining areas. This study indicates that mining activities significantly contribute to water and sediment contamination across the Amazon basin, posing hazards for freshwater ecosystems and potentially having human health implications.

Hotspots of geogenic arsenic and manganese contamination in groundwater of the floodplains in lowland Amazonia (South America).

Arsenic enrichment in groundwater resources in deltas and floodplains of large sediment-rich rivers is a worldwide natural hazard to human health. High spatial variability of arsenic concentrations in affected river basins limits cost-effective mitigation strategies. Linking the chemical composition of groundwater with the topography and fluvial geomorphology is a promising approach for predicting arsenic pollution on a regional scale. Here we correlate the distribution of arsenic contaminated wells with the fluvial dynamics in the Amazon basin. Groundwater was sampled from tube wells along the Amazon River and its main tributaries in three distinct regions in Peru and Brazil. For each sample, the major and trace element concentrations were analyzed, and the position of the well within the sedimentary structure was determined. The results show that aquifers in poorly weathered sediments deposited by sediment-rich rivers are prone to mobilization and accumulation of aqueous arsenic and manganese, both in sub-Andean foreland basins, and in floodplains downstream. Two zones at risk are distinguished: aquifers in the channel-dominated part of the floodplain (CDF) and aquifers in the overbank deposits on the less-dynamic part of the floodplain (LDF). Some 70 % of the wells located on the CDF and 20 % on the LDF tap groundwater at concentrations exceeding the WHO guideline of 10 µg/L arsenic (max. 430 µg/L), and 70 % (CDF) and 50 % (LDF) exceeded 0.4 mg/L manganese (max. 6.6 mg/L). None of the water samples located outside the actual floodplain of sediment-rich rivers, or on riverbanks of sediment-poor rivers exceed 5 µg/L As, and only 4 % exceeded 0.4 mg/L Mn. The areas of highest risk can be delineated using satellite imagery. We observe similar patterns as in affected river basins in South and Southeast Asia indicating a key role of sedimentation processes and fluvial geomorphology in priming arsenic and manganese contamination in aquifers.

Large-scale monitoring and risk assessment of microplastics in the Amazon River.

Microplastics (MPs) are one of the most widespread contaminants worldwide, yet their risks for freshwater ecosystems have seldom been investigated. In this study, we performed a large monitoring campaign to assess the presence and risks of MPs in Amazonian freshwater ecosystems. We investigated MP pollution in 40 samples collected along 1500 km in the Brazilian Amazon, including the Amazon River, three major tributaries, and several streams next to the most important urban areas. MPs in the 55-5000 µm size range were characterized (size, shape, color) by microscopy and identified (polymer composition) by infrared spectroscopy. Ecotoxicological risks were assessed using chronic Species Sensitivity Distributions for effects triggered by food dilution and tissue translocation using data alignment methods that correct for polydispersity of environmental MPs and bioaccessibility. This study shows that MPs are ubiquitous contaminants in Amazonian freshwater ecosystems, with measured concentrations (55-5000 µm) ranging between 5 and 152 MPs/m3 in the Amazon River and its main tributaries, and between 23 and 74,550 MPs/m3 in urban streams. The calculated Hazardous Concentration for the 5% of species (HC5) derived from the SSDs for the entire MP range (1-5000 µm) were 1.6 × 107 MPs/m3 (95% CI: 1.2 × 106 - 4.0 × 108) for food dilution, and 1.8 × 107 MPs/m3 (95% CI: 1.5 × 106 - 4.3 × 108) for translocation. Rescaled exposure concentrations (1-5000 µm) in the Amazon River and tributaries ranged between 6.0 × 103 and 1.8 × 105 MPs/m3, and were significantly lower than the calculated HC5 values. Rescaled concentrations in urban streams ranged between 1.7 × 105 and 5.7 × 108 MPs/m3, and exceeded both calculated HC5 values in 20% of the locations. This study shows that ecological impacts by MP contamination are not likely to happen in the Amazon River and its major tributaries. However, risks for freshwater organisms may be expected in near densely populated areas, such as the cities of Manaus or Belem, which have limited wastewater treatment facilities.