Molecular mechanisms by which polyethylene terephthalate (PET) microplastic and PET leachate promote the growth of benthic cyanobacteria

Toxic blooms of benthic cyanobacteria greatly threaten freshwater ecological health and drinking water safety. Meanwhile, microplastic pollution is becoming increasingly severe and microplastics accumulate in large quantities at the bottom of lakes and rivers, widely coexisting with algae. However, impacts of microplastics on benthic cyanobacteria are still unknown. This study investigated effects of microplastic polyethylene terephthalate (PET) - which is commonly found at the bottom of lakes and rivers - and its leachate at environmentally relevant concentration (0.3 mg/L) and high exposure concentration (3.0 mg/L) on typical benthic cyanobacteria (Oscillatoria sp. and Pseudanabaena sp.), and clarified the related molecular mechanisms through transcriptomic analysis. Results show that PET or PET leachate (PET-L) can promote benthic cyanobacterial growth and promotive effect of PET-L is more obvious than that of PET system. Promotion effect of PET or PET-L is more significant at environmentally relevant concentration (39-63 % increase compared with the control) compared with high exposure concentration (21-58 % increase compared with the control). In the presence of PET or PET-L, due to an increase in the number of cyanobacterial cells, concentrations of harmful metabolites (cylindrospermopsin, geosmin, and 2-methylisoborneol) in water also increased. Although PET particles may not be conducive to benthic cyanobacterial growth due to shading effect and mechanical damage, photosynthetic efficiency of algae was improved and dysregulated genes related to photosynthesis and extracellular transport of glycolipid were upregulated according to transcriptome analysis. Moreover, PET decomposition components, such as terephthalic acid and ethylene glycol, may be able to serve as carbon sources for cyanobacterial growth. Upregulation of genes associated with glycolysis, oxidative phosphorylation, and translation revealed that PET can promote the growth of benthic cyanobacteria. This study has important value in evaluating the impact of benthic cyanobacteria on aquatic ecological health and drinking water safety with the coexistence of microplastics.