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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% (EC 10 ) and 50% (EC 50 ) 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., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2019 Elsevier B.V. All rights reserved.)