Light energy and oxygen drive photo-oxidation of dissolved organic carbon (DOC). The longer the wavelength the greater its depth of penetration into water, changing the spectral environment with depth. We asked how oxygen concentration and light spectral composition might affect photo-oxidation processes in DOC. Outdoor experiments compared responses of fluorescence and absorbance indices to photo-oxidation of filtered (0.45 µm) Rio Negro water (Amazon Basin) under near-anoxia, normoxia and hyperoxia exposed to natural sunlight or reduced sunlight (≥340, reduced-UVR). Near-anoxia decreased all absorbance and fluorescence indices. Absorbance changed across the spectrum (≥250 nm) even under reduced-UVR provided that oxygen was present. This phenomenon maintains broader photo-oxidation and the release of CO2 at depth. Slope350-400 was responsive to changes in the irradiance field but not to oxygen concentration, while Slope275-295 responded to both. Thus, larger molecules are broken down near the water\'s surface and medium to smaller molecules continue to be processed at depth. The production of fulvic acid-like fluorescence required both UVB and oxygen, restricting its production to surface waters. The relatively small increase in R254/365 compared with the loss of SUVA254 under near-anoxia indicated a slower breakdown of larger DOC molecules as oxygen becomes limiting. Breakdown of larger molecules which absorb in the 350-400 nm range, appears to involve two steps - one by radiant energy and another involving oxygen. The study results reflect the dynamic gradients in photo-oxidation with depth.

Rare earth elements (REEs) are generally defined as a homogenous group of elements with similar physical-chemical properties, encompassing Y and Sc and the lanthanides elements series. Natural REEs contents in soils depend on the parent material, the soil genesis processes and can be gradually added to the soil by anthropogenic activities. The REEs have been considered emerging pollutants in several countries, so the establishment of regulatory guidelines is necessary to avoid environmental contamination. In Brazil, REE soils data are restricted to some regions, and knowledge about them in the Amazon soils is scarce, although this biome covers more than 40% of the Brazilian territory. Thus, the objectives of this study were to determine the REE content in soils of two hydrographic basins (Solimões and Rio Negro) of the Amazon biome, establish their Quality Reference Values (QRV) and to investigate the existence of enrichment of REEs in urban soils. The ΣREE(Y + Sc) content of Solimões surface samples was 109.28 mg kg-1 and the ΣREE(Y + Sc) content in the subsurface samples was 94.11 mg kg-1. In soils of Rio Negro basin, the ΣREE(Y + Sc) was 43.95 15 mg kg-1 surface samples and 38.40 mg kg-1 in subsurface samples. The ΣREE(Y + Sc) in urban topsoils samples was 38.62 mg kg-1. The REEs contents pattern in three studied areas are influenced in different amplitude by natural soil properties. The REEs content in urban topsoils were slightly higher than the Rio Negro pristine soils, but the ecological risk was low. QRVs recommend for Solimões soils ranged from 0.01 (Lu) to 145.6 mg kg-1 (Ce) and for Rio Negro soils ranged from 0.05 (Lu) to 15.8 mg kg-1 (Ce).

The aim of the present study was to characterize the effects of particles on metal aquatic toxicity in a tropical system. To this end, we investigated the effects of natural suspended solids on copper (Cu) geochemistry and acute toxicity to the cardinal tetra (Paracheirodon axelrodi), in 2 Amazonian rivers with different total suspended solids (TSS) levels: the Rio Negro (low TSS ~ 8 mg L-1 ) and the Rio Solimões (high TSS ~ 70 mg L-1 ). The effect of particles on Cu aqueous geochemistry was assessed by measuring total, dissolved, and free ionic Cu concentrations in filtered (<0.45 μm) and unfiltered waters. Furthermore, acute Cu toxicity to fish was assessed in both filtered and unfiltered waters, by measuring physiological net fluxes of Na+ , Cl- , K+ , and total ammonia (in both river waters) and 96-h fish mortality (in Rio Solimões only). The particles in the Rio Negro were not abundant enough to play a significant role in our study. On the other hand, the Rio Solimões particles bound approximately 70% of total aqueous Cu in our tests. In agreement with bioavailability-based models, this decrease in dissolved (and free ionic) Cu concentration decreased Cu lethality in the 96-h toxicity tests. In the physiological measurements, the best evidence of particle protection was the total alleviation of Cu-induced Cl- losses. These flux tests also suggested that the particles themselves may negatively affect Na+ balance in the fish. Overall, the present study supports the use of bioavailability concepts to account for the role of natural suspended solids on metal biological effects in the Amazon River basin. Environ Toxicol Chem 2019;38:2708-2718. © 2019 SETAC.

To assess how the quality and properties of the natural dissolved organic carbon (DOC) could drive different effects on gill physiology, we analysed the ionoregulatory responses of a native Amazonian fish species, the tambaqui Colossoma macropomum, to the presence of dissolved organic carbon (DOC; 10 mg l-1 ) at both pH 7.0 and pH 4.0 in ion-poor water. The DOC was isolated from black water from São Gabriel da Cachoeira (SGC) in the upper Rio Negro of the Amazon (Brazil) that earlier been shown to protect a non-native species, zebrafish Danio rerio against low pH under similar conditions. Transepithelial potential (TEP), net flux rates of Na+ , Cl- and ammonia and their concentrations in plasma and Na+ , K+ ATPase; v-type H+ ATPase and carbonic anhydrase activities in gills were measured. The presence of DOC had negligible effects at pH 7.0 apart from lowering the TEP, but it prevented the depolarization of TEP that occurred at pH 4.0 in the absence of DOC. However, contrary to our initial hypothesis, SGC DOC was not protective against the effects of low pH. Colossoma macropomum exposed to SGC DOC at pH 4.0 experienced greater net Na+ and Cl- losses, decreases of Na+ and Cl- concentrations in plasma and elevated plasma ammonia levels and excretion rates, relative to those exposed in the absence of DOC. Species-specific differences and changes in DOC properties during storage are discussed as possible factors influencing the effectiveness of SGC DOC in ameliorating the effects of the acid exposure.

We examined ionoregulatory characteristics of two species of characiform fish and two species of cichlids that are not native to the Rio Negro, an extremely ion-poor, acidic tributary of the Amazon River, in order to gain insight into the origin of the specializations possessed by Rio Negro fish. The two characiform species examined, Congo tetras (Phenacogrammus interruptus) and black neon tetras (Hyphessobrycon herbertaxelrodi), had high-affinity/high-capacity transporters that produce high rates of uptake in dilute water. Na+ uptake for both was pH insensitive (down to pH 3.5). Exposure to 100 μmol L-1 phenamil had no effect on Na+ uptake in either species, while exposure to 1 mmol L-1 NH4Cl- (high external ammonia [HEA]) slightly stimulated Na+ uptake in Congo tetras. Exposure to \"Na+-free\" water significantly inhibited Na+ uptake (by 65%-85%) but had no effect on net ammonia flux. The two cichlid species examined, convict cichlids (Amatitlania siquia) and red point cichlids (Archocentrus sp.), had high-affinity but low-capacity transporters that yield low rates of uptake in dilute media. Sodium transport was pH sensitive and completely inhibited at pH 3.5. Phenamil exposure inhibited Na+ uptake by 60% in convict cichlids but had no effect on red point cichlids, and exposure to HEA reduced Na+ uptake in both species by 70%-85%. Exposure to \"Na+-free\" water reduced Na+ uptake by 80%-85%, and in convict cichlids it also reduced net ammonia flux by about 50%. The ionoregulatory characteristics described for both groups are strikingly similar to those for Rio Negro species, and we suggest that they may be ancestral physiological traits for these two groups. Further, if this is the case, it seems likely that these traits existed before the Rio Negro and may explain the great success of these species-rich groups in colonizing the river despite its challenging chemistry.

Copper at high concentrations is an ionoregulatory toxicant in fish and its toxicity is known to be strongly modulated by the water chemistry. The toxicity of Cu to the tropical fish cardinal tetra (Paracheirodon axelrodi) was investigated in waters from two major rivers of the Amazon watershed: the Rio Negro (filtered <0.45μm, pH 5.6, DOC=8.4 mgL-1, Na=33μM, Ca=8μM) and the Rio Solimões (filtered <0.45μm, pH 6.7, DOC=2.8 mgL-1, Na=185μM, Ca=340μM), as well as in a natural \"reference water\" (groundwater) which was almost DOC-free (pH 6.0, DOC=0.34 mgL-1, Na=53μM, Ca=5μM). Acute 96-h mortality, Cu bioaccumulation and net flux rates of Na+, Cl-, K+ and total ammonia were determined in P. axelrodi exposed in each water. Copper speciation in each water was determined by two thermodynamic models and by potentiometry, and its toxicity was predicted based on the biotic ligand model (BLM) framework. Our results indicate that high Na+ loss is the main mode of toxic action of Cu in P. axelrodi, in accordance with general theory. Cardinal tetra showed a particularly high ability to tolerate Cu and to maintain Na+ balance, similar to the ability of this and other endemic Rio Negro species to tolerate low pH and ion-poor conditions. Cu toxicity was lower in Rio Negro than in the other two waters tested, and the free [Cu2+] at the LC50, as determined by any of the three speciation methods tested, was approximately 10-fold higher. This variation could not be captured by a realistic set of BLM parameters. At least in part, this observation may be due to gill physiological alterations induced by the abundant dissolved organic matter of the Rio Negro. The implication of this observation is that, for metals risk assessment in tropical waters, similar to the Rio Negro, care must be used in applying BLM models developed using temperate DOC and temperate species.