Daphnia magna Gut-Specific Transcriptomic Responses to Feeding Inhibiting Chemicals and Food Limitation

Multi-generation effects of lead (Pb) on two Daphnia species

Two monophyletic Daphnia species (Daphnia magna and D. similis) were exposed to a sub-lethal concentration of Pb (50 µg/L) for nine generations under two food regimes (usual and restricted) and analyzed for acetylcholinesterase (AChE) activity, first reproduction delay, lifespan, and net reproductive rate (R0) at the subcellular, individual, and population levels, respectively. In the sixth generation, Pb-acclimated neonates were moved to clean media for three more generations to check for recovery. The net reproductive rate (R0) of D. magna was not affected by Pb. However, Pb stimulated reproduction, reduced lifespan, and decreased AChE activity. First reproduction delay and lifespan did not improve during the recovery process, suggesting a possible genetic adaptation. Food restriction reduced R0, lifespan, delayed hatching, and increased AChE activity; the opposite outcomes were observed for D. similis. The full recovery shown by R0 suggests the physiological acclimation of D. similis. Under food restriction, the animals exhibited a reduction of R0 and lifespan, delayed first reproduction, and increased AChE activity; however, there was no effect of Pb. The recovery process under food restriction showed that D. similis might not cope with Pb exposure, indicating a failed recovery. Such outcomes indicate that one model species' sensitivity may not represent another's sensitivity.

Feeding inhibition in daphnids - A sensitive and rapid toxicity endpoint for chemical stress?

The planktonic Crustacea Daphnia are among the most employed organisms in ecotoxicology, mainly in regulatory assays that follow OECD/ISO protocols. The most common endpoint for acute testing (24-48 h) without feeding of organisms is usually monitored as mortality or immobilization. A rapid and physiologically and environmentally more relevant toxicity endpoint could be the impaired feeding of daphnids. Decreased feeding of test organisms upon exposure to toxicants has been used to evaluate sub-lethal effects occurring already in minutes to hours. This endpoint, however, has not been used systematically and the respective data are inconsistent due to heterogeneity of experimental design. The aim of this review is to evaluate the scientific literature where impaired Daphnia feeding has been used in ecotoxicological research. The search made in WoS (June 5, 2024) using combination of keywords "Daphni* AND feed* yielded 152 articles. Out of these 152 papers 46 addressed feeding of d aphnids upon exposure to various toxicants (insecticides, heavy metals, pharmaceuticals, contaminated environmental samples and toxic cyanobacteria; in total 59 different chemicals/combinations). These 46 papers formed the basis of the critical analysis presented in the current review. For 18 chemicals it was possible to compare the sensitivity of the feeding and mortality endpoints. We conclude that although the feeding inhibition of Daphnia sp. did not prove systematically more sensitive than mortality/immobilization, it is a sub-lethal endpoint that allows rapid evaluation of toxic effects of chemicals to aquatic crustaceans - important and sensitive organisms in the aquatic food web.

Characterizing nutritional phenotypes using experimental nutrigenomics: Is there nutrient-specificity to different types of dietary stress?

The ability to directly measure and monitor poor nutrition in individual animals and ecological communities is hampered by methodological limitations. In this study, we use nutrigenomics to identify nutritional biomarkers in a freshwater zooplankter, Daphnia pulex, a ubiquitous primary consumer in lakes and a sentinel of environmental change. We grew animals in six ecologically relevant nutritional treatments: nutrient replete, low carbon (food), low phosphorus, low nitrogen, low calcium and high Cyanobacteria. We extracted RNA for transcriptome sequencing to identify genes that were nutrient responsive and capable of predicting nutritional status with a high degree of accuracy. We selected a list of 125 candidate genes, which were subsequently pruned to 13 predictive potential biomarkers. Using a nearest-neighbour classification algorithm, we demonstrate that these potential biomarkers are capable of classifying our samples into the correct nutritional group with 100% accuracy. The functional annotation of the selected biomarkers revealed some specific nutritional pathways and supported our hypothesis that animal responses to poor nutrition are nutrient specific and not simply different presentations of slow growth or energy limitation. This is a key step in uncovering the causes and consequences of nutritional limitation in animal consumers and their responses to small- and large-scale changes in biogeochemical cycles.

Persisting Effects in Daphnia magna Following an Acute Exposure to Flowback and Produced Waters from the Montney Formation

Hydraulic fracturing extracts oil and gas through the injection of water and proppants into subterranean formations. These injected fluids mix with the host rock formation and return to the surface as a complex wastewater containing salts, metals, and organic compounds, termed flowback and produced water (FPW). Previous research indicates that FPW is toxic to Daphnia magna (D. magna), impairing reproduction, molting, and maturation time; however, recovery from FPW has not been extensively studied. Species unable to recover have drastic impacts on populations on the ecological scale; thus, this study sought to understand if recovery from an acute 48 h FPW exposure was possible in the freshwater invertebrate, D. magna by using a combination of physiological and molecular analyses. FPW (0.75%) reduced reproduction by 30% and survivorship to 32% compared to controls. System-level quantitative proteomic analyses demonstrate extensive perturbation of metabolism and protein transport in both 0.25 and 0.75% FPW treatments after a 48 h FPW exposure. Collectively, our data indicate that D. magna are unable to recover from acute 48 h exposures to ≥0.25% FPW, as evidence of toxicity persists for at least 19 days post-exposure. This study highlights the importance of considering persisting effects following FPW remediation when modeling potential spill scenarios.