Comparative Study of the toxicity of Perfluorooctane sulfonic acid and Perfluorohexane sulfonic acid using algal and vertebrate models
Per- and polyfluoroalkyl substances (PFAS) are a class of fluorinated chemicals that are ubiquitous in the environment because of their widespread use for consumer and industrial goods. They have unique properties such as hydrophobicity, hydrophilicity, low surface energy, low friction properties, resistance to heat and chemical agents owing to their carbon-fluorine covalent bond. Perfluoroalkane sulfonic acids (PFSAs) are a family of PFASs that consist of a fluorinated carbon chain as the main structure capped with sulfonate functional groups. They are nonbiodegradable and exceedingly persistent in the environment. The most commonly researched PFSA compounds are Perfluorooctane sulfonic acid (PFOS) Perfluorohexane sulfonic acid (PFHxS) and Perfluorobutanesulfonic acid (PFBS). PFOS has been shown to have toxic effects on plants and animals and has been banned from use in production, but it is still being found in the environment because it is a degradation product of fluorotelomers-related substances. Shorter chain-length perfluoroalkyl acids were thought to be less dangerous because of shorter half-life, however there is limited data to support this assertion. PFHxS is a shorter chain alternative to PFOS, and some studies have reported adverse effects of PFHxS in human as well as some aquatic organisms. However, there is limited data on the effect of PFHxS on algae and zebrafish. Therefore, this study aimed to compare the effect of PFOS and PFHxS on two algae species- Prymnesium parvum and Chlorella sorokiniana under varying nutrient levels and the developmental effects of PFOS and PFHxS on zebrafish larvae. In this dissertation, an introduction on Per- and polyfluorinated compounds and their effects on aquatic organisms will be discussed. This is followed by research input from nutrient-dependent effects of PFOS and PFHxS on P. parvum, nutrient-dependent effects of PFHxS on C. sorokiniana and developmental toxicity in zebrafish larvae exposed to perfluorooctane sulfonic acid and perfluorohexane sulfonic acid. The endpoints evaluated in algae were growth, morphology, and chlorophyll content. The effect of PFOS and PFHxS on growth of P. parvum and C. sorokiniana was estimated using growth rate and highest density on the last day of the incubation period, The effect of PFOS and PFHxS on morphology was estimated using cell size and volume and chlorophyll content was estimated by measuring in vivo chlorophyll. The endpoints evaluated in zebrafish larvae were body length, behavior, reactive oxygen species, and gene expression of some Glucose Transporters and neurodevelopmental genes. Zebrafish larvae behavior was estimated using white light startle response, reactive oxygen species induction was estimated using 2’-7’dichlorofluorescin diacetate assay and gene expression was carried out using quantitative polymerase chain reaction. The results revealed that PFOS was more toxic to P. parvum than PFHxS at both nutrient levels tested. In the nutrient sufficient medium, when exposed to 100 mg/l, the P. parvum cells exhibited a negative growth rate and at the lower nutrient level (f/8 medium) the toxicity of PFOS to P. parvum at 100 mg/l was reduced. PFHxS exposure did not significantly affect the growth rate of C. sorokiniana, irrespective of nutrient availability. However, a reduction in in vivo chlorophyll content was observed under nutrient-limited conditions at 100 mg/L. PFOS affected the behavior of embryos at 1 and 10 µg/L. Similar results were observed at at 1 µg/L of PFHxS. PFOS and PFHxS induced significant levels of reactive oxygen species at 10 and 100 µg/L. Exposure to PFHxS led to upregulation of GLUTs 1a, 2, 10, and 12 at 100 µg/L at 3-dpf and PFOS led to upregulation of GLUTa 3 and 10 at 5-dpf. This research will contribute to the body of knowledge of the ecological implications of PFOS and PFHxS contamination in aquatic environments and highlight the importance of continued research in this area.