99
(Markich et al. 2000). The complexation with the DOM (Hogan et al. 2005), carbonate (Nakajima et al. 1979), and phosphate (Fortin et al. 2002) was also demonstrated to decrease the U toxicity due a reduction on the UO 2
2+
activity.
The UO 2
2+
and UO 2 OH
+
are considered the most bioavailable U species and represent the major threat for aquatic organisms (Markich et al. 2000). However, in
some situations, other U species (e.g., UO 2 (CO 3 ) 2
2) might represent the bioavailable fraction of U (Croteau et al. 2016). A diversity of studies using analytical biological approaches including toxicity tests, bioaccumulation evaluation, chemical
speciation techniques, and computational methods were already performed to
understand the mechanisms involved on the bioavailability of the U species and/or
their toxicity to aquatic organisms.
Due to the heterogeneity of natural aquatic systems, the presence of diverse compounds, and the complexity of biological systems, the representation of the whole
mechanism involved in the bioavailability and toxicity of U in the environment is a
challenging task for any technique (Eismann et al. 2018). Furthermore, each organism has different uptake mechanisms, internalizing different fractions of contaminants. In this sense, it is observed the organism-specific concept of the bioavailability.
The understanding of each mechanism which is occurring in each exposure scenario
is important. Integrative strategies employing more than one approach are a powerful tool to improve the understanding about bioavailability and toxicity trends of the
U in natural aquatic ecosystems (Ferrari et al. 2017).
Analytical methods are useful to determine different fractions/species of U
potentially bioavailable to aquatic organisms. However, no single method can provide unequivocal information about the U behavior in the natural ecosystem
(Markich 2002). Physical separation techniques are employed to separate the U
species based on size (e.g., ultrafiltration (Guo et al. 2007) and size exclusion chromatography (Trenfield et al. 2011)). Physical separation techniques are also
employed to separate U species based on charge (e.g., electrophoresis (Pacheco and
Havel 2001)).
Electrochemical methods and the diffusive gradients in thin films technique
(DGT) (Drozdzak et al. 2016) are also employed to determine labile (weakly complexed) U species. Computational methods rely on the inputted information of the
researcher and use mathematical equations to generate a profile about the chemical
speciation of a given medium based on a speciation model. While there are several
limitations to their use, they provide useful information when applied properly.
Also, computational methods represent an advantageous tool to be used complementarily with field experiments performed in the natural systems (Markich 2002).
When studies are performed using biological responses or bioaccumulation, biological factors such as feeding habits, depuration, and movement on the environment should be considered to interpret and compare the data obtained through the
chemical and biological approaches (Eismann et al. 2018).
Previous studies have indicated that the U uptake by aquatic organisms is directly
related to the concentration of the bioavailable species in the water, suggesting the
direct exposure to the water as a main source of U (Bunn 2007). For some U species,
the food ingestion represents the main source of this element (Bergmann et al.
Biogeochemistry of Uranium in Tropical Environments
(Markich et al. 2000). The complexation with the DOM (Hogan et al. 2005), carbonate (Nakajima et al. 1979), and phosphate (Fortin et al. 2002) was also demonstrated to decrease the U toxicity due a reduction on the UO 2
2+
activity.
The UO 2
2+
and UO 2 OH
+
are considered the most bioavailable U species and represent the major threat for aquatic organisms (Markich et al. 2000). However, in
some situations, other U species (e.g., UO 2 (CO 3 ) 2
2) might represent the bioavailable fraction of U (Croteau et al. 2016). A diversity of studies using analytical biological approaches including toxicity tests, bioaccumulation evaluation, chemical
speciation techniques, and computational methods were already performed to
understand the mechanisms involved on the bioavailability of the U species and/or
their toxicity to aquatic organisms.
Due to the heterogeneity of natural aquatic systems, the presence of diverse compounds, and the complexity of biological systems, the representation of the whole
mechanism involved in the bioavailability and toxicity of U in the environment is a
challenging task for any technique (Eismann et al. 2018). Furthermore, each organism has different uptake mechanisms, internalizing different fractions of contaminants. In this sense, it is observed the organism-specific concept of the bioavailability.
The understanding of each mechanism which is occurring in each exposure scenario
is important. Integrative strategies employing more than one approach are a powerful tool to improve the understanding about bioavailability and toxicity trends of the
U in natural aquatic ecosystems (Ferrari et al. 2017).
Analytical methods are useful to determine different fractions/species of U
potentially bioavailable to aquatic organisms. However, no single method can provide unequivocal information about the U behavior in the natural ecosystem
(Markich 2002). Physical separation techniques are employed to separate the U
species based on size (e.g., ultrafiltration (Guo et al. 2007) and size exclusion chromatography (Trenfield et al. 2011)). Physical separation techniques are also
employed to separate U species based on charge (e.g., electrophoresis (Pacheco and
Havel 2001)).
Electrochemical methods and the diffusive gradients in thin films technique
(DGT) (Drozdzak et al. 2016) are also employed to determine labile (weakly complexed) U species. Computational methods rely on the inputted information of the
researcher and use mathematical equations to generate a profile about the chemical
speciation of a given medium based on a speciation model. While there are several
limitations to their use, they provide useful information when applied properly.
Also, computational methods represent an advantageous tool to be used complementarily with field experiments performed in the natural systems (Markich 2002).
When studies are performed using biological responses or bioaccumulation, biological factors such as feeding habits, depuration, and movement on the environment should be considered to interpret and compare the data obtained through the
chemical and biological approaches (Eismann et al. 2018).
Previous studies have indicated that the U uptake by aquatic organisms is directly
related to the concentration of the bioavailable species in the water, suggesting the
direct exposure to the water as a main source of U (Bunn 2007). For some U species,
the food ingestion represents the main source of this element (Bergmann et al.
Biogeochemistry of Uranium in Tropical Environments