Points of departure for an algal species (Raphidocelis subcapitata) exposed to 22 per- and polyfluoroalkyl substances

Due to their environmental prevalence, persistence, and potential toxicity, per- and polyfluoroalkyl substances (PFAS) are contaminants of concern. However, except for a few well studied PFAS, hazard data required to evaluate their risk to aquatic ecosystems are limited. The present study used a 24-hour, high throughput assay to screen 22 PFAS with varying chain lengths and functional groups for effects on the transcriptome of a green algal species, Raphidocelis subcapitata. The tested PFAS included perfluoroalkyl carboxylic acids, perfluoroalkyl carboxylic acid ethers, alcohols, sulfonic acids, and sulfonamides. Algae were exposed in a 96-well microplate format to 8 concentrations (nominally, 100 to 0.03 uM) of each PFAS, in a 1/2 log dilution series. Analytical measurements of PFAS concentration were made at the initiation of exposure and after 24 hours. In addition to traditional endpoints (i.e., biomass, viability, and chlorophyll), RNA was extracted for sequencing of the transcriptome. Concentration response curves were fit to the resulting transcriptomic data to calculate gene-specific benchmark concentrations. The distribution of benchmark concentrations was subsequently used to calculate a transcriptomic point of departure (tPOD) which represents a concentration below which no concerted molecular change is detected in response to PFAS exposure. The resulting tPODs were compared with traditional endpoints and environmentally detected concentrations of specific PFAS. RNA-seq was performed using mRNA extracted from Raphidocelis subcapitata that were exposed for 24 h to a dilution series of 22 different PFAS. The nominal exposure concentrations ranged from 100-0.03 µM. All assays were conducted in four replicate, 96-well plates. Control wells contained the same concentration as PFAS wells (0.33%) of either DMSO or methanol (depending on which solvent the test chemical was solubilized in).