Evaluation of phenol-induced ecotoxicity in two model ciliate species: Population growth dynamics and antioxidant enzyme activity

Adaptive Strategies and Underlying Response Mechanisms of Ciliates to Salinity Change with Note on Fluctuation Properties

The adaptability of marine organisms to changes in salinity has been a significant research area under global climate change. However, the underlying mechanisms of this adaptability remain a debated subject. We hypothesize that neglecting salinity fluctuation properties is a key contributing factor to the controversy. The ciliate Euplotes vannus was used as the model organism, with two salinity fluctuation period sets: acute (24 h) and chronic (336 h). We examined its population growth dynamics and energy metabolism parameters following exposure to salinity levels from 15‱ to 50‱. The carrying capacity (K) decreased with increasing salinity under both acute and chronic stresses. The intrinsic growth rate (r) decreased with increasing salinity under acute stress. Under chronic stress, the r initially increased with stress intensity before decreasing when salinity exceeded 40‱. Overall, glycogen and lipid content decreased with stress increasing and were significantly higher in the acute stress set compared to the chronic one. Both hypotonic and hypertonic stresses enhanced the activities of metabolic enzymes. A trade-off between survival and reproduction was observed, prioritizing survival under acute stress. Under chronic stress, the weight on reproduction increased in significance. In conclusion, the tested ciliates adopted an r-strategy in response to salinity stress. The trade-off between reproduction and survival is a significant biological response mechanism varying with salinity fluctuation properties.

How antibiotic exposure affect predator-prey interactions by population dynamics in ciliates?

Biodiversity loss resulting from environmental pollution is a global concern. While interspecific interactions are central to ecology, the impact of environmental pollution on predator-prey interactions and its ecological consequences, such as extinction and biodiversity loss, remain unclear. To investigate the effects of antibiotic exposure on predation strength and the resulting ecological consequence, the Didinium-Paramecium was utilized as a predator-prey model and exposed to nitrofurazone or erythromycin, two common pollutants, respectively. Initially, we determined prey population growth dynamics, body size, and predator numerical-functional responses. Subsequently, these above parameters were integrated into a mathematical model of predator-prey predation. Then both the long time-series data and phase portraits obtained through model simulation were used to estimate interaction strength and to predict the outcome of predator-prey coexistence. Our results revealed that exposure to either antibiotic significantly reduced prey population growth parameters (e.g., μmax and K) while increasing individual body size. The combined effects of antibiotic exposure and predation pressure on population growth inhibition or body size promotion were variable, mostly additive, with a few cases of synergy and extremely rare antagonism, depending on antibiotic exposure concentration. As antibiotic exposure concentration increased, the predator rmax generally declined, while functional responses varied depending on specific parameters, implying a decrease in predator-prey interaction strength. Analyses of phase portrait features showed that the population oscillation amplitude decreased with increasing antibiotic exposure concentrations, the cycle length of adjacent peaks increased, and prey extinction occurred earlier. In conclusion, antibiotic exposure reduced both predator and prey fitness, underlying the reason antibiotics reduces the strength of predator-prey interaction. Despite the indirect benefits of prey gain from this, the presence of predators can expedite the process of prey extinction caused by antibiotic exposure.

Predation risk affects the ecotoxicity evaluation of antibiotics: Population growth and antioxidase activity in the ciliate Paramecium jenningsi

Although predation risk exists under natural conditions, its role is usually ignored when evaluating the ecotoxicity of environmental contaminants, and the interaction between predation risk and antibiotic ecotoxicity is not yet clear. To investigate the nonconsumptive effects (NCEs) of predation on the ecotoxicity evaluation of antibiotics, the median lethal concentration (LC₅₀), relative population growth rate (RGR), and activities of three antioxidases were measured in the ciliate Paramecium jenningsi exposed to graded concentrations of the antibiotics nitrofurazone (NFZ) or erythromycin (ERY) in the presence or absence of a predator, i.e., the ciliate Didinium nasutum. The results showed that (1) NCEs significantly reduced the LC₅₀ of NFZ but had no effect on that of ERY; (2) predation pressure alone had no significant effect on the inhibitory rate of the P. jenningsi population, but the interaction with NFZ was synergistic, while that with CRY was additive; (3) the concentrationresponse (i.e., mortality) model for each antibiotic exposure with and without predation pressure differed significantly in the parameter slope; (4) RGRs were significantly reduced by antibiotic exposure or NCEs; only in NFZ-exposed groups did the RGRs decrease linearly with increasing exposure concentration; and (5) the activities of all three antioxidases significantly increased due to NCEs or following exposure to antibiotics. In brief, NCEs were detected in P. jenningsi, and these had additive or synergistic effects on antibiotic ecotoxicity, but their magnitude depended on the properties and exposure concentrations of the antibiotics. Our findings suggest that it is necessary to consider the roles of NCEs in the ecotoxicity evaluation of environmental contaminants.

Co-metabolic biodegradation of 4-bromophenol in a mixture of pollutants system by Arthrobacter chlorophenolicus A6

Brominated phenols are listed as priority pollutants together with nitrophenol and chlorophenol are the key components of paper pulp wastewater. However, the biodegradation of bromophenol in a mixed substrate system is very scanty. In the present investigation, simultaneous biodegradation kinetics of three substituted phenols 4-bromophenol (4-BP), 4-nitrophenol (4-NP), and 4-chlorophenol (4-CP) were investigated using Arthrobacter chlorophenolicus A6. A 2³ full factorial design was applied with varying 4-BP and 4-CP from 75-125 mg/L and 4-NP from 50-100 mg/L. Almost complete degradation of this mixture of substituted phenols was achieved at initial concentration combinations of 125, 125, and 100 mg/L of 4-CP, 4-BP, and 4-NP, respectively, in 68 h. Statistical analysis of the results revealed that, among the three variables, 4-NP had the most prominent influence on the degradation of both 4-CP and 4-BP, while the concentration of 4-CP had a strong negative interaction effect on the biodegradation of 4-NP. Irrespective of the concentration levels of these three substrates, 4-NP was preferentially biodegraded over 4-CP and 4-BP. Furthermore, 4-BP biodegradation rates were found to be higher than those of 4-CP, followed by 4-NP. Besides, the variation of the biomass yield coefficient of the culture was investigated at different initial concentration combinations of these substituted phenols. Although the actinomycetes consumed 4-NP at a faster rate, the biomass yield was very poor. This revealed that the microbial cells were more stressed when grown on 4-NP compared to 4-BP and 4-CP. Overall, this study revealed the potential of A. chlorophenolicus A6 for the degradation of 4-BP in mixed substrate systems.

Toxicity of silver nanoparticles (AgNPs) in the model ciliate Paramecium multimicronucleatum: Molecular mechanisms of activation are dose- and particle size-dependent

Current understanding of toxicity mechanisms of nanoparticles is still far from comprehensive, partly because of the neglect of control factors such as the dependence of mechanism activation on the exposure dosage and particle size. To reveal molecular mechanisms of silver nanoparticle (AgNP) toxicity, the model ciliate Paramecium multimicronucleatum was exposed for 12 h to different concentrations of AgNPs with particle size of 20 nm (0.08, 0.12, and 0.30 mg/l) and 40 nm (0.08 and 0.30 mg/l). Transcriptomes of the tested ciliates were then analyzed based on dendrograms of gene expression, Gene Ontology (GO) terms, KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways, and up- and down-regulated genes. Results showed that: (1) toxicity mechanisms of AgNP revealed by analyses of GO and KEEG were significantly involved in the metabolic pathways of nutrients and the biosynthesis of macromolecules; (2) the top five up-regulated genes were mainly related to biological oxidation, biosynthesis, and oxidative stress, while top five down-regulated genes were mainly related to glycolysis; (3) activated mechanisms varied both in quantity and in type with dosages and particle sizes of AgNPs; (4) AgNP-treatments with different exposure dosages and particle sizes can produce the same toxicity in terms of 12 h-EC₅₀, but the underlying molecular mechanisms differed significantly. In brief, this study provides insights into the molecular mechanisms of AgNP toxicity through transcriptome analyses and confirmed their dependence of activation on the exposure dosage and particle size of AgNPs.

Combined effects of food resources and exposure to ammonium nitrogen on population growth performance in the bacterivorous ciliate Paramecium caudatum

Ciliated protozoa (ciliates) play vital roles in biological wastewater-treatment processes, however, combined effects of abiotic and biotic factors as well as the importance of species-specificity of bacterial food organisms on population growth dynamics remain poorly understood, which are hampering the management and optimization of biological wastewater treatment processes. This study investigated the effects of food resources and ammonium nitrogen (NH₄⁺) exposure, both independently and in combination, on the population growth of the bacterivorous ciliate Paramecium caudatum. Results showed that, when fed with two different bacterial food organisms, population growth performance of P. caudatum differed significantly and increased with the addition of protozoa pellet medium. When exposed to NH₄⁺ population growth declined and metabolic enzyme activities were altered. The negative effects of NH₄⁺ on population growth could be weakened by supplementing the food resource with protozoa pellet media. In brief, it was confirmed that the existence of interactive effect of food resources and ammonium nitrogen, as well as the importance of species-specificity of bacterial food organisms on the population growth performance of ciliates. These findings might lead to the development of a valuable strategy for improving the performance of biological wastewater-treatment processes.