The Effects of Natural Suspended Solids on Copper Toxicity to the Cardinal Tetra in Amazonian River Waters

The biotic ligand model as a promising tool to predict Cu toxicity in amazon blackwaters

The Rio Negro basin of Amazonia (Brazil) is a hotspot of fish biodiversity that is under threat from copper (Cu) pollution. The very ion-poor blackwaters have a high dissolved organic carbon (DOC) concentration. We investigated the Cu sensitivity of nine Amazonian fish species in their natural blackwaters (Rio Negro). The acute lethal concentration of Cu (96 h LC50) was determined at different dilutions of Rio Negro water (RNW) in ion-poor well water (IPW), ranging from 0 to 100%. The IPW was similar to RNW in pH and ionic composition but deficient in DOC, allowing this parameter to vary 20-fold from 0.4 to 8.3 mg/L in tests. The Biotic Ligand Model (BLM; Windward version 3.41.2.45) was used to model Cu speciation and toxicity over the range of tested water compositions, and to estimate lethal Cu accumulations on the gills (LA50). The modeling predicted a high relative abundance of Cu complexes with DOC in test waters. As these complexes became more abundant with increasing RNW content, a concomitant decrease in free Cu2+ was observed. In agreement with this modeling, acute Cu toxicity decreased (i.e. 96 h LC50 values increase) with increasing RNW content. The three most sensitive species (Hemigrammus rhodostomus, Carnegiella strigatta and Hyphessobrycon socolofi) were Characiformes, whereas Corydoras schwartzi (Siluriformes) and Apistogramma agassizii (Cichliformes) were the most tolerant. These sensitivity differences were reflected in the BLM-predicted lethal gill copper accumulation (LA50), which were generally lower in Characiformes than in Cichliformes. Using these newly estimated LA50 values in the BLM allowed for accurate prediction of acute Cu toxicity in the nine Amazonian fish. Our data emphasize that the BLM approach is a promising tool for assessing Cu risk to Amazonian fish species in blackwater conditions characterized by very low concentrations of major ions but high concentrations of DOC.

Acute toxicity of palm oil mill effluent on zebrafish (Danio rerio Hamilton-Buchanan, 1822): Growth performance, behavioral responses and histopathological lesions

Evaluating the toxicity of Palm Oil Mill Effluent (POME) is critical as part of the effort to develop waste management regulations for the palm oil industry. In this study, we investigated the acute toxicity of POME on growth performance, behavioral response, and histopathology of gill and liver tissues of zebrafish (Danio rerio). In total, 550 adult male zebrafish were used for the toxicity experiment including range finding test, acute toxicity test, growth performance and behaviour test. Static non-renewal acute toxicity bioassays were conducted by exposing fish to POME (1.584-9.968 mL/L) for 96 h. Growth performance, behavior response, and histopathological lesions in untreated and POME treated (96-h LC50: 5.156 mL/L) fish were measured at 24, 48, 72 and 96 h. Time-dependent significant decline in body length and body weight of POME-exposed zebrafish was observed. Furthermore, several behavioral changes were recorded, including hyperactivity, loss of balance, excessive mucus secretion, and depigmentation. Decreasing operculum movement and oxygen consumption rate as well as alterations in gill tissues (i.e. hyperplasia, hypertrophy, hemorrhage, and necrosis) of POME-exposed zebrafish were observed, suggesting a dysfunction in respiratory performance. On the other hand, liver tissue alterations (congestion, hemorrhage, hyperplasia, shrinkage of hepatocytes, hydrophilic degeneration, and necrosis) indicated a disruption in detoxification performance. We conclude that exposure to POME at acute concentration caused histopathological lesions both in gill and liver tissue along with changes in fish behaviors which disrupted respiratory and detoxification performance, resulting in mortality and reduced growth of zebrafish. These findings might provide valuable information for guiding POME management and regulation.

Higher Risks of Copper Toxicity in Turbid Waters: Quantifying the Bioavailability of Particle-Bound Metals to Set Site-Specific Water Quality Criteria

In coastal waters, particulate metals constitute a substantial fraction of the total metals; however, the prevalent water quality criteria are primarily based on dissolved metals, seemingly neglecting the contribution of particulate metals. Here we developed a method to quantify the toxicity risk of particulate metals, and proposed a way to calculate modifying factors (MFs) for setting site-specific criteria in turbid waters. Specifically, we used a side-by-side experimental design to study copper (Cu) bioaccumulation and toxicity in an estuarine clam, Potamocorbula laevis, under the exposure to "dissolved only" and "dissolved + particulate" ⁶⁵Cu. A toxicokinetic-toxicodynamic model (TK-TD) was used to quantify the processes of Cu uptake, ingestion, assimilation, egestion, and elimination, and to relate mortality risk to tissue Cu. We find that particulate Cu contributes 40-67% of the Cu bioaccumulation when the suspended particulate matter (SPM) ranges from 12 to 229 mg L^-1. The Cu-bearing SPM also increases the sensitivity of organisms to internalized Cu by decreasing the internal threshold concentration (C_IT) from 141 to 76.8 μg g^-1. MFs were derived based on the TK-TD model to consider the contribution of particulate Cu (in the studied SPM range) for increasing Cu bioaccumulation (MF = 1.3-2.2) and toxicity (MF = 2.3-3.9). Water quality criteria derived from dissolved metal exposure need to be lowered by dividing by an MF to provide adequate protection. Overall, the method we developed provides a scientifically sound framework to manage the risks of metals in turbid waters.

Investigating the mechanisms of dissolved organic matter protection against copper toxicity in fish of Amazon's black waters

We investigated how natural dissolved organic matter (DOM) of the Rio Negro (Amazon) affects acute copper (Cu) toxicity to local fish: the cardinal tetra (Paracheirodon axelrodi) and the dwarf cichlid (Apistogramma agassizii). It is established that Cu²⁺ complexation with DOM decreases Cu bioavailability (and thus toxicity) to aquatic organisms, as conceptualized by the Biotic Ligand Model (BLM). However, we also know that Rio Negro's DOM can interact with fish gills and have a beneficial effect on Na⁺ homeostasis, the main target of acute Cu toxicity in freshwater animals. We aimed to tease apart these potential protective effects of DOM against Cu-induced Na⁺ imbalances in fish. In the laboratory, we acclimated fish to Rio Negro water (10 mg L^-1 DOC) and to a low-DOM water (1.4 mg L^-1 DOC) with similar ion composition and pH (5.9). We measured 3-h Cu uptake in gills and unidirectional and net Na⁺ physiological fluxes across a range of Cu concentrations in both waters. Various DOM pre-acclimation times (0, 1 and 5 days) were evaluated in experiments with P. axelrodi. Copper exposure led to similar levels of net Na⁺ loss in the two fish, but with distinct effects on Na⁺ influx and efflux rates reflecting their different ionoregulation strategies. Rio Negro DOM protected against Cu uptake and toxicity in the two fish species. Both Cu uptake in fish gills and Na⁺ regulation disturbances were relatively well predicted by the modelled aqueous free Cu²⁺ ion concentration. These findings suggest that protection by DOM occurs mainly from Cu complexation under the tested conditions. The prevalence of this geochemical-type protection over a physiological-type protection agrees with the BLM conceptual framework, supporting the use of the BLM to assess the risk of Cu in these Amazonian waters.

Global change and physiological challenges for fish of the Amazon today and in the near future

Amazonia is home to 15% (>2700, in 18 orders) of all the freshwater fish species of the world, many endemic to the region, has 65 million years of evolutionary history and accounts for 20% of all freshwater discharge to the oceans. These characteristics make Amazonia a unique region in the world. We review the geological history of the environment, its current biogeochemistry and the evolutionary forces that led to the present endemic fish species that are distributed amongst three very different water types: black waters [acidic, ion-poor, rich in dissolved organic carbon (DOC)], white waters (circumneutral, particle-rich) and clear waters (circumneutral, ion-poor, DOC-poor). The annual flood pulse is the major ecological driver for fish, providing feeding, breeding and migration opportunities, and profoundly affecting O2, CO2 and DOC regimes. Owing to climate change and other anthropogenic pressures such as deforestation, pollution and governmental mismanagement, Amazonia is now in crisis. The environment is becoming hotter and drier, and more intense and frequent flood pulses are now occurring, with greater variation between high and low water levels. Current projections are that Amazon waters of the near future will be even hotter, more acidic, darker (i.e. more DOC, more suspended particles), higher in ions, higher in CO2 and lower in O2, with many synergistic effects. We review current physiological information on Amazon fish, focusing on temperature tolerance and ionoregulatory strategies for dealing with acidic and ion-poor environments. We also discuss the influences of DOC and particles on gill function, the effects of high dissolved CO2 and low dissolved O2, with emphasis on water- versus air-breathing mechanisms, and strategies for pH compensation. We conclude that future elevations in water temperature will be the most critical factor, eliminating many species. Climate change will likely favour predominantly water-breathing species with low routine metabolic rates, low temperature sensitivity of routine metabolic rates, high anaerobic capacity, high hypoxia tolerance and high thermal tolerance.

The physiology of fish in acidic waters rich in dissolved organic carbon, with specific reference to the Amazon basin: Ionoregulation, acid-base regulation, ammonia excretion, and metal toxicity

Although blackwaters, named for their rich content of dissolved organic carbon (DOC), are often very poor in ions and very acidic, they support great fish biodiversity. Indeed, about 8% of all freshwater fish species live in the blackwaters of the Rio Negro watershed in the Amazon basin. We review how native fish survive these harsh conditions that would kill most freshwater fish, with a particular focus on the role of DOC, a water quality parameter that has been relatively understudied. DOC, which is functionally defined by its ability to pass through a 0.45-µm filter, comprises a diverse range of compounds formed by the breakdown of organic matter and is quantified by its carbon component that is approximately 50% by mass. Adaptations of fish to acidic blackwaters include minimal acid-base disturbances associated with a unique, largely unknown, high-affinity Na⁺ uptake system that is resistant to inhibition by low pH in members of the Characiformes, and very tight regulation of Na⁺ efflux at low pH in the Cichliformes. Allochthonous (terrigenous) DOC, which predominates in blackwaters, consists of larger, more highly colored, reactive molecules than autochthonous DOC. The dissociation of protons from allochthonous components such as humic and fulvic acids is largely responsible for the acidity of these blackwaters, yet at the same time, these components may help protect organisms against the damaging effects of low water pH. DOC lowers the transepithelial potential (TEP), mitigates the inhibition of Na⁺ uptake and ammonia excretion, and protects against the elevation of diffusive Na⁺ loss in fish exposed to acidic waters. It also reduces the gill binding and toxicity of metals. At least in part, these actions reflect direct biological effects of DOC on the gills that are beneficial to ionoregulation. After chronic exposure to DOC, some of these protective effects persist even in the absence of DOC. Two characteristics of allochthonous DOC, the specific absorbance coefficient at 340 nm (determined optically) and the PBI (determined by titration), are indicative of both the biological effectiveness of DOC and the ability to protect against metal toxicity. Future research needs are highlighted, including a greater mechanistic understanding of the actions of DOCs on gill ionoregulatory function, morphology, TEP, and metal toxicity. These should be investigated in a wider range of native fish Orders that inhabit one of the world's greatest biodiversity hotspots for freshwater fishes.

Acute exposure to environmentally relevant concentrations of copper affects branchial and hepatic phosphoryl transfer network of Cichlasoma amazonarum: Impacts on bioenergetics homeostasis

The toxic effects of copper (Cu) are linked to dysfunction of metabolism and depletion of adenosine triphosphate (ATP). Nevertheless, the effects related to phosphoryl transfer network, a network of enzymes to precise coupling of the ATP-production and ATP-consuming process for maintenance of bioenergetic, remain unknown. Therefore, the aim of this study was to determine whether the phosphoryl transfer network could be one pathway involved in the bioenergetic imbalance of Cichlasoma amazonarum exposed for 96 h to environmentally relevant concentrations of Cu found in Amazonia water around mines. Branchial mitochondrial creatine kinase (CK) activity was significantly lower in fish exposed to 1500 μg/L Cu than in the control group, while branchial cytosolic CK activity was significantly greater. Branchial (exposed to 750 and 1500 μg/L Cu) and hepatic (exposed to 1500 μg/L Cu) pyruvate kinase (PK) activity was significantly lower in fish exposed to Cu than in the control group. Branchial and hepatic ATP levels were significantly lower in fish exposed to 1500 μg/L than in the control group. Branchial reactive oxygen species (ROS) and lipid peroxidation (LPO) levels were significantly higher in fish exposed to 750 and 1500 μg/L Cu compared to control. Hepatic ROS and LPO levels were significantly higher in fish exposed to 1500 μg/L than in the control group. Branchial and hepatic Cu levels were significantly higher in fish exposed to 1500 μg/L compared to other groups. Exposure to 750 and 1500 μg/L Cu impairs bioenergetics homeostasis, which appears to be mediated by ROS overproduction and lipid peroxidation.