Relevance of tributary inflows for driving molecular composition of dissolved organic matter (DOM) in a regulated river system

River regulation by dams can alter flow regimes and organic matter dynamics, but less is known about how unregulated tributaries regulate organic matter composition and processing in the regulated river below the confluence. This study reports on water chemistry, especially dissolved organic matter (DOM) concentration and composition (dissolved organic carbon (DOC), organic nitrogen (DON), organic phosphorus (DOP) and combined amino acids (DCAA)) along the regulated Tumut and unregulated Goobarragandra (tributary) rivers under different flow conditions (base flow vs storm event) in south-east Australia. The tributary was significantly different from regulated and downstream sites during base flow conditions with higher temperature, pH, buffering capacity, DOC and nutrient concentrations (DON, DOP, DCAA). DOM characterisation by spectrometry and size exclusion chromatography revealed that the tributary contained a higher proportion of terrestrially derived humic-like and fulvic-like DOM. In contrast, regulated and downstream sites contained higher proportion of microbially derived DOM such as low molecular weight neutrals and protein-like components. Storm pulses of tributary flows into the regulated system, influenced both concentration and composition of DOM at the downstream site, which more strongly resembled the tributary site than the regulated site during the storm event. Additionally, we found that the tributary supplied fresh DOM, including small organic molecules to the regulated system during storm events. The presence of these different types of labile DOM can increase primary productivity and ecological functioning within regulated river reaches downstream of tributary junctions. This has important implications for the protection of unregulated tributary inflows within regulated river basins.

Two wild carnivores selectively forage for prey but not amino acids

In nutritional ecology the intake target is the diet that maximises consumer fitness. A key hypothesis of nutritional ecology is that natural selection has acted upon the behavioural and physiological traits of consumers to result in them Selectively Consuming prey to match the Intake Target (SCIT). SCIT has been documented in some herbivores and omnivores, which experience strong heterogeneity in the nutritional quality of available foods. Although carnivores experience a prey community with a much more homogeneous nutrient composition, SCIT by carnivores has nevertheless been deemed highly likely by some researchers. Here we test for SCIT for micronutrients (amino acids) in two freshwater carnivores: the river blackfish and the two-spined blackfish. Although both blackfishes exhibited non-random consumption of prey from the environment, this resulted in non-random consumption of amino acids in only one species, the river blackfish. Non-random consumption of amino acids by river blackfish was not SCIT, but instead an artefact of habitat-specific foraging. We present hypotheses to explain why wild populations of freshwater carnivores may not exhibit SCIT for amino acids. Our work highlights the need for careful, critical tests of the hypotheses and assumptions of nutritional ecology and its application to wild populations.

Biomolecular modifications in the sacfry of Mogurnda adspersa in response to copper stress

Copper (Cu) is one of the most harmful contaminants in fresh-water systems. Fish larvae such as sacfry are particularly vulnerable to metals such as copper (Cu) due to a less-developed excretory organ system and permeable skin that can absorb metals directly from the water. However, the sublethal effects of metals on this life stage are not well understood. This study assessed the sublethal toxicity of Cu on purple-spotted gudgeon sacfry (PSG, Mogurnda adspersa). For this purpose, 96 h Cu toxicity bioassays were performed and toxic effects of Cu on PSG were measured at different levels of biological organization, from the individual (loss of equilibrium, wet weight), to tissue (chemical changes in retinal tissue composition) and molecular responses (whole body amino acid (AA) profiles). The EC₁₀ and EC₅₀ (ECx: effect concentration that affected X% of test organisms) were found to be 12 (9 - 15) µg Cu L^-1 and 22 (19 - 24) µg Cu L^-1, respectively. Copper stress caused a decrease in total amino acid content and changed the AA profile of PSG compared to the controls. Proton-induced X-ray emission (PIXE) mapping techniques showed accumulation of Cu in the retinal tissues disturbing the distribution of other elements such as zinc, sulfur, phosphorus and potassium. Fourier-transform infrared (FTIR) microspectroscopy of control and Cu treated eye tissues revealed a change in protein secondary structure in retinal tissues in response to Cu accumulation, as well as decreased levels of the molecular retinal, consistent with the degradation of rhodopsin, a key protein in the visual sensory system. This is the first study to demonstrate the multi-level responses of PSG arising from exposure to environmentally realistic Cu concentrations and suggests that AA profiling can serve as a useful tool to assess the impacts of metals on fresh-water organisms.

Changes to the amino acid profile and proteome of the tropical freshwater microalga Chlorella sp. in response to copper stress

Contamination of freshwaters is increasing globally, with microalgae considered one of the most sensitive taxa to metal pollution. Here, we used 72 h bioassays to explore the biochemical effects of copper (Cu) on the amino acid (AA) profile and proteome of Chlorella sp. and advance our understanding of the molecular changes that occur in algal cells during exposure to environmentally realistic Cu concentrations. The Cu concentrations required to inhibit algal growth rate by 10% (EC₁₀) and 50% (EC₅₀) were 1.0 (0.7-1.2) µg L^-1 and 2.0 (1.9-2.4) µg L^-1, respectively. The AA profile of Chlorella sp. showed increases in glycine and decreases in isoleucine, leucine, valine, and arginine, with increasing Cu. Proteomic analysis revealed the modulation of several proteins involved in energy production pathways, including: photosynthesis, carbon fixation, glycolysis, and oxidative phosphorylation, which likely assists in meeting increased energy demands under Cu-stressed conditions. Copper exposure also caused up-regulation of cellular processes and signalling proteins, and the down-regulation of proteins related to ribosomal structure and protein translation. These changes in biomolecular pathways have direct effects on the AA profile and total protein content and provide an explanation for the observed changes in amino acid profile, cell growth and morphology. This study shows the complex mode of action of Cu on Chlorella under environmentally realistic Cu concentrations and highlights several potential biomarkers for future investigations.

Spatial variation in the amino acid profile of four macroinvertebrate taxa along a highly polluted river

Acid mine drainage (AMD) is one of the major environmental problems impacting aquatic ecosystems globally. We studied changes in the community composition of macroinvertebrates and amino acid (AA) profiles of dominant taxa along an AMD contamination gradient within the Dee River, Queensland, Australia to understand how AMD can affect the biomolecular composition of macroinvertebrates. Taxa richness and community composition of macroinvertebrates changed widely along the AMD gradient with significantly lower taxa richness recorded at the polluted sites compared to upstream and downstream sites. The Dipteran families: Chironomidae and Ceratopogonidae, the Odonata family Gomphidae, and the Coleoptera family Dytiscidae were the only families found at all sampling sites and were used here for AA analysis. There were significant variations in the AA profiles among the studied taxa. The AA profile of each taxon also varied among upstream, polluted and downstream sites suggesting that contamination of a river system with acid mine drainage not only alters the overall macroinvertebrate community composition but also significantly influences the AA profile of organisms that are tolerant to AMD. This study highlights the potential of using AA profiling to study the response of aquatic organisms to contamination gradients such as those associated with AMD.

Dissolved organic matter and metabolic dynamics in dryland lowland rivers

Dissolved organic matter (DOM) within freshwaters is essential for broad ecosystem function. The concentration and type of DOM within rivers depends on the relative contributions of allochthonous sources and the production and consumption of DOM by microbes. In this work we have examined the temporal patterns in DOM quality and productivity in three lowland rivers in dryland Australia using fluorescence excitation emission scans. We assessed the production and consumption of DOM within light and dark bottle assays to quantify the relative contribution of bacteria and algae to the DOM pool and simultaneously assessed whether the systems were autotrophic or heterotrophic. DOM varied temporally within the three river systems over the course of the study period. Characterisation of DOM within light and dark bottles following a 6-hour incubation revealed microbial consumption of a humic-like component and production of protein-like components similar in nature to the amino acids tryptophan and tyrosine. The lack of a significant difference in DOM quality between the light and dark bottles indicated that the protein-like DOM is likely derived from bacterial activity. Respiration was shown to be higher than gross primary production in both whole river and bottle assays, yielding negative net production values and demonstrating that these rivers were predominately heterotrophic. Our work suggests that bacterial metabolism of DOM may be a significant contributor to the production of protein-like components within heterotrophic freshwater systems.

Novel chitosan goethite bionanocomposite beads for arsenic remediation

We report on the synthesis and As adsorption properties of a novel chitosan - iron (oxyhydr)oxide composite material for the remediation of arsenic-contaminated water supplies. FE-SEM, Mössbauer spectroscopy, ICP-OES and synchrotron (Bulk XAS, μXRF) techniques were applied to determine the composition of the new material and investigate the As uptake efficiency and mechanism. The iron (oxyhydr)oxide phase has been identified as a nano-sized goethite, well dispersed in the chitosan matrix, leading to the name 'chitosan goethite bionanocomposite' (CGB). The CGB material is prepared in the form of beads of high density and excellent compression strength; the embedding of the goethite nanoparticles in the chitosan matrix allows for the high adsorption capacity of nanoparticles to be realized. CGB beads remove both As(III) and As(V) efficiently from water, over the pH range 5-9, negating the need for pre-oxidation of As(III). Kinetic studies and μXRF analysis of CGB bead sections show that diffusion-adsorption of As(V) into CGB beads is faster than for As(III). Using CGB beads, synthetic high-arsenic water (0.5 mg-As/L) could be purified to world drinking standard level (<0.01 mg-As/L) using only 1.4 g/L CGB. When considered in combination with the advantages of the low-cost of raw materials required, and facile (green) synthesis route, CGB is a promising material for arsenic remediation, particularly in developing countries, which suffer a diversity of socio-economical-traditional constraints for water purification and sanitation.

The severity of sediment desiccation affects the adsorption characteristics and speciation of phosphorus

Phosphorus is an important nutrient for plants and algae, and can be the limiting nutrient in aquatic ecosystems. However, oversupply can lead to significant water quality issues. The largest source and sink of P in most aquatic systems is the sediment. As a consequence of drought, in many places sediments that normally would have remained inundated are now being desiccated. Based on previous studies, it is often difficult to predict what impact drying will have on the cycling of P. This is because most of these studies have looked at drying across a chronosequence in the field, where there may be differences in sediment composition or microbial community structure. In this paper we present the results of a study where sediment was exposed to progressively more severe drying in the laboratory - starting with wet sediment, followed by air drying and then sequential oven drying at 30, 50 and 85 °C. Drying resulted in a shift in P speciation, notably with an increase in NaHCO3-extractable reactive P and a decline in NaHCO3-extractable unreactive P, likely indicating an increase in bioavailable, easily exchangeable P. Drying also resulted in a decline in the microbial-P fraction. Drying significantly affected the P adsorption characteristics of the sediment. The total amount of P adsorbed by the sediment and the linear adsorption co-efficient both declined, while the amount of native P adsorbed to the sediment and the equilibrium P concentration both increased. Drying also affected iron speciation with a shift from more reactive oxalate-extractable Fe to more recalcitrant citrate-dithionate-bicarbonate-extractable Fe, suggesting an increase in iron crystallinity and hence decrease in P adsorption capacity. The increase in crystallinity is consistent with Fe EXAFS results, which showed that drying resulted in an increase in edge-sharing neighbours. We hypothesise that the shifts in P speciation, the decline in P adsorption capacity, the increase in the equilibrium P concentration, as well as the death of micro-organisms (as evidenced by a decline in microbial P) on drying all contribute to the Birch effect - the initial pulse of P and/or N upon inundation of dried soils or sediments.

Mediated electron transfer between Fe(II) adsorbed onto hydrous ferric oxide and a working electrode

The redox properties of Fe(II) adsorbed onto mineral surfaces have been highly studied over recent years due to the wide range of environmental contaminants that react with this species via abiotic processes. In this work the reactivity of Fe(II) adsorbed onto hydrous ferric oxide (HFO) has been studied using ferrocene (bis-cyclopentadienyl iron(II); Fc) derivatives as electron shuttles in cyclic voltammetry (CV) experiments. The observed amplification of the ferrocene oxidation peak in CV is attributed to reaction between the electrochemically generated ferrocenium (Fc(+)) ion and adsorbed Fe(II) species in a catalytic process (EC' mechanism). pH dependence studies show that the reaction rate increases with Fe(II) adsorption and is maintained in the absence of aqueous Fe(2+), providing strong evidence that the electron transfer process involves the adsorbed species. The rate of reaction between Fc(+) and adsorbed Fe(II) increases with the redox potential of the ferrocene derivative, as expected, with bimolecular rate constants in the range 10(3)-10(5) M(-1) s(-1). The ferrocene-mediated electrochemical method described has considerable promise in the development of a technique for measuring electron-transfer rates in geochemical and environmental systems.

Proton and iron binding by the cyanobacterial toxin microcystin-LR

Microcystins (MCs) are a group of hepatotoxins produced by cyanobacteria that have not had their functional role or the environmental factors that trigger production clearly determined. One suggestion is that microcystins are siderophores (i.e., ligands with an extremely high affinity with iron, typically with stability constants substantially greater than 10(25)). In this work, we explore proton and iron binding with microcystin-LR (MC-LR). Using UV-visible spectroscopy and a HPLC peak retention time-based method, the two acid dissociation constants associated with the carboxylic groups of MC-LR were determined to be: pKa₁ = 2.17 and pKa₂ = 3.96. Cyclic voltammetry provides evidence for the formation of at least two Fe(III)-MC-LR complexes, with the Fe(III) reduction peak significantly shifted to more reducing potentials in the presence of MC-LR. These complexes have been interpreted as a rapidly formed initial complex (Complex 1) and a more stable, and slower forming, Complex 2. The stability constant for Fe(III)-MC-LR (Complex 2) was estimated to be approximately 10(13) in 60% v/v MeOH/water at 0.1 M ionic strength. The electrochemical experiments provide no evidence for the formation of a complex between Fe(2+) and MC-LR. Given that most MC-LR is released only upon cell lysis, and coupled with the moderate strength of the stability constant with Fe(III) determined in this study, it appears unlikely that that MC-LR is an extracellular siderophore. If MC-LR is involved in iron regulation in cyanobacteria, it is more likely as a shuttle for iron across the cell membrane or in intracellular processes.

Acidification and buffering mechanisms in acid sulfate soil wetlands of the Murray-Darling Basin, Australia

The acid generation mechanisms and neutralizing capacities of sulfidic sediments from two inland wetlands have been studied in order to understand the response of these types of systems to drying events. The two systems show vastly different responses to oxidation, with one (Bottle Bend (BB) lagoon) having virtually no acid neutralizing capacity (ANC) and the other (Psyche Bend (PB) lagoon) an ANC that is an order of magnitude greater than the acid generation potential. While BB strongly acidifies during oxidation the free acid generation is less than that expected from the measured proton production and consumption processes, with additional proton consumption attributed to the formation of an acid-anion (chloride) FeIII (oxyhydr)oxide product, similar to akaganéite (Fe(OH)2.7Cl0.3). While such products can partially attenuate the acidification of these systems, resilience to acidification is primarily imparted by sediment ANC.