Unraveling the combined toxicity and removal mechanisms of fluoxetine and sertraline co-contaminants by the freshwater microalga Chlorella pyrenoidosa

Deciphering the inhibitory mechanisms of didecyldimethylammonium chloride on microalgal removal of fluoxetine: Insights from the alterations in cell surface properties and the physio-biochemical and molecular toxicity

The COVID-19 pandemic has increased the co-occurrence of quaternary ammonium compounds (QACs) and antidepressants in aquatic environments. Microalgae are sustainable and cost-effective candidates for removing emerging pollutants. QACs have a robust ability to adsorb on cell surface and alter membrane permeability, but little is known about the influence of QACs on microalgal bioremediation of co-existing pollutants like antidepressants. In this study, the influence mechanisms of didecyldimethylammonium chloride (DDAC) on the removal of fluoxetine (FLX) by C. pyrenoidosa were explored. The results showed that C. pyrenoidosa exhibited high removal efficiency of single FLX (75.23 %-88.65 %) mainly through biodegradation (57.12 %-67.19 %). However, the coexisting medium and high concentrations of DDAC considerably decreased the biodegradation amount (10.50 %-33.30 %) and removal efficiency (29.47 %-52.89 %) of FLX by C. pyrenoidosa. In contrast, the presence of DDAC increased extracellular and intracellular FLX concentrations due to the enhanced extracellular polymeric substance content, cell surface hydrophobicity, and cell membrane permeability. Meanwhile, DDAC showed synergistic effects with FLX on microalgal growth through exacerbated oxidative damage and photosynthesis inhibition. Moreover, transcriptomics revealed that the dysregulations of key genes involved in the DNA replication and repair, ribosome biogenesis, photosynthesis-antenna proteins, peroxisomes, and glutathione metabolism pathways were important molecular mechanisms underlying the synergistic toxicity. Furthermore, the principal component analysis suggested that the enhancement of extracellular and intracellular FLX concentrations induced by the coexistence of DDAC increased the mixture's toxicity, resulting in the decreased biodegradation amount and ultimately a reduction in the removal efficiency of FLX. Our results highlight the significance of recognizing the influence of QACs on microalgal remediation and ecological risks of antidepressants.

Integrated physio-biochemical and transcriptomic analysis reveals the joint toxicity mechanisms of two typical antidepressants fluoxetine and sertraline on Microcystis aeruginosa

Selective serotonin reuptake inhibitor (SSRI) antidepressants are of increasing concern worldwide due to their ubiquitous occurrence and detrimental effects on aquatic organisms. However, little is known regarding their effects on the dominant bloom-forming cyanobacterium, Microcystis aeruginosa. Here, we investigated the individual and joint effects of two typical SSRIs fluoxetine (FLX) and sertraline (SER) on M. aeruginosa at physio-biochemical and molecular levels. Results showed that FLX and SER had strong growth inhibitory effects on M. aeruginosa with the 96-h median effect concentrations (EC50s) of 362 and 225 μg/L, respectively. Besides, the mixtures showed an additive effect on microalgal growth. Meanwhile, both individual SSRIs and their mixtures can inhibit photosynthetic pigment synthesis, cause oxidative damage, destroy cell membrane, and promote microcystin-leucine-arginine (MC-LR) synthesis and release. Moreover, the mixtures enhanced the damage to photosynthesis, antioxidant system, and cell membrane and facilitated MC-LR synthesis and release compared to individuals. Furthermore, transcriptomic analysis revealed that the dysregulation of the key genes related to transport, photosystem, protein synthesis, and non-ribosomal peptide structures was the fundamental molecular mechanism underlying the physio-biochemical responses of M. aeruginosa. These findings provide a better understanding of the toxicity mechanisms of SSRIs to microalgae and their risks to aquatic ecosystems.