Per- and polyfluoroalkyl substances (PFASs) are synthetic organic chemicals with fully or partly fluorinated carbon chains connected to different functional end groups. Because of their excellent biochemical stability and high surface activity, PFASs have been manufactured and widely used in the electroplating, textile, and paper sectors for over 70 years. PFASs are highly persistent in the environment, which has resulted in their ubiquity in various abiotic and biotic matrices worldwide. Two conventional eight-carbon-chain (C8) PFASs, perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA), have been identified as persistent organic pollutants (POPs) and listed in the Stockholm Convention for global restriction and elimination since 2009 and 2019, respectively. With the ban of C8 PFASs, short-chain PFASs and PFASs with different structures have been manufactured for complementary applications. These emerging PFASs have similar physicochemical, bioaccumulative, and toxic properties as PFOS and PFOA but have not been studied as much.

The rapid industrialization and urbanization in the Pearl River Delta (PRD) region have resulted in substantial releases of PFASs. The northern South China Sea (SCS), adjacent to the Pearl River Estuary (PRE), is highly affected by PFASs discharged from the PRE, which pose a risk to the marine environment and organisms. In light of the above, in this work, field investigations were conducted to monitor legacy and emerging PFASs in biotic and abiotic samples from the PRE and the adjacent SCS, including seawater, invertebrates, teleost, and marine mammals. The predominant emerging PFASs in the field samples were selected, and their tissue-specific toxicokinetic and toxicodynamic characteristics were studied using the model organism, the marine medaka fish (Oryzias melastigma). The aims of this study are to obtain a preliminary PFAS inventory in the PRE and SCS, and to investigate the emerging PFASs’ environmental behavior, bioaccumulation characteristics, and toxicokinetic-toxicodynamic properties.

Considering PFASs’ bioaccumulation potential, marine mammals occupying the high tropic level are essentially biological sinks for PFASs in the marine environment. This study involved the target, nontarget, and suspect screening of PFASs in the liver of Indo-Pacific humpback dolphins (Sousa chinensis) and finless porpoises (Neophocaena phocaenoides), two resident marine mammals in the South China Sea, stranded between 2012 and 2018. In the marine mammal liver samples, PFOS and 6:2 chlorinated polyfluoroalkyl ether sulfonate (Cl-PFESA) predominated, accounting for 46% and 30% of the total PFASs, respectively. Significant increasing temporal trends (p < 0.05) were found in the concentrations of two emerging PFASs, perfluoroethylcyclohexane sulfonate (PFECHS) and hexafluoropropylene oxide-dimer acid (HFPO-DA) in the porpoises, indicating increasing pollution by these emerging PFASs in the PRE. Forty-four PFASs from 9 classes were additionally identified by nontarget and suspect screening, among which 15 compounds were reported for the first time in marine mammals. A primary risk assessment showed that 6:2 Cl-PFESA could have adverse effects in terms of reproductive injury potential on most of the investigated cetaceans.

High levels of emerging PFASs detected in cetaceans indicate PFASs’ bioaccumulation potential in these top predators. However, biomagnification characteristics of emerging PFASs in the marine food web are not well understood. Samples of the seawater and the marine food web, including fish, crustaceans, and marine mammals, from the coastal SCS were collected. One Cl-PFOS analysis interface compound with a predicted formula of C₁₄H₂₃Cl₆FO₄S⁻ was found in the biota samples. Significant trophic magnification was found for long-chain PFASs, and the trophic magnification factors (TMFs) increased with the carbon chain length. Linear PFOA and PFOS were found to have higher TMFs than their branched isomers. The compound trans-perfluoroethylcyclohexane sulfonate (PFECHS, 2.25) was found to have a higher TMF than cis-PFECHS (1.92), which was reported for the first time. The ratios between 6:2 H-PFESA from indirect and direct sources were estimated to be 3.86 and 0.729 for porpoises and dolphins, respectively. The estimated PFOA-equivalent uptake for all the examined marine organisms was higher than the guideline value, indicating a potential health risk from PFAS for humans via seafood consumption.

To better understand the bioaccumulation mechanism of PFASs in the filter-feeders, samples of oysters and their ambient water were collected in the littoral zone of the PRE. Total PFAS concentrations ranged from 13.8 to 58.8 ng/L, 3.60 to 11.2 ng/g dry weight (dw), and 0.969 to 1.98 ng/g dw in the dissolved phase, suspended particulate matter (SPM), and oysters, respectively. Most short-chain PFASs occurred in the dissolved phase (>95%), and long-chain PFASs were generally more deposited in the SPM than short-chain ones. The field-based bioconcentration factor (BCF) of long-chain perfluoroalkyl carboxylic acids (PFCAs) increased linearly (r² = 0.95, p < 0.01) with increasing carbon-chain length. Considering the widespread occurrence of perfluorohexanoic acid (PFHxA) and perfluoroheptanoic acid (PFHpA) precursors, the precursor transformation’ contribution should be a significant source of PFHxA and PFHpA in oysters as biomonitors.

The field-based investigation results indicate that PFECHS and Cl-PFESAs were the predominant emerging PFASs in the biotic and abiotic samples from the coastal SCS. Additionally, a significant increasing temporal trend was found for PFECHS in marine mammals stranded between 2012 and 2018, indicating the increasing use of this emerging PFAS in the PRD region. However, the toxicokinetic data on these compounds were still limited. The tissue-specific uptake and depuration kinetics of PFECHS and 6:2 and 8:2 Cl-PFESAs were therefore studied in marine medaka. These substances were investigated upon 28 days of exposure (0.2 μg/L), followed by a 14-day clearance period. The depuration constant (k_d) of PFECHS (0.196±0.007 day⁻¹) was reported for the first time. Among six studied tissues, higher concentrations of 6:2 Cl-PFESA, 8:2 Cl-PFESA, and PFECHS were found in the liver than the other tissues at day 28; these PFASs had the longest residence time in the eyes. No significant positive correlation was found between the bioconcentration factors of the studied PFASs and the phospholipid or protein contents in various tissues of the studied fish. Potential metabolites of Cl-PFESAs, i.e., their hydrogen-substituted analogs (H-PFESAs), were identified by time-of-flight mass spectrometry. However, the biotransformation rates were low (<0.19%), indicating the low metabolism capacity of Cl- to H-PFESAs in marine medaka.

To understand the possible impairment due to the studied emerging PFASs, the marine medaka were exposed to environmentally realistic concentrations of PFECHS and 6:2 Cl-PFESA (0.1, 0.3, and 1 μg/L) from eggs until sexual maturity (90 days). All the PFAS treatment groups showed insignificant hepatotoxicity. The gut microbiota composition of the PFECHS-treatment groups was significantly different from that of the control group, and activities of the digestive enzymes were significantly lower in the PFECHS exposed group (1 μg/L) than in the control group. These results indicated that PFECHS could cause dysbiosis of the intestinal microbiome and enzyme. Exposure to PFECHS (0.1-1 μg/L) and 6:2 Cl-PFESA (1 μg/L) could significantly disrupt medaka larvae’s swimming activity, and significant increases in acetylcholine activity and decreased γ-aminobutyric acid concentrations were found in fish brains for the PFECHS-treated group than in the control group, indicating effects on their nervous system. This was the first report on PFECHS’s toxic effects in teleost.

In summary, this PhD study investigated the PFAS pollution status in the PRE and the adjacent SCS and studied emerging PFASs’ environmental behavior and bioaccumulation characteristics in the marine environment. This work provided an in-depth understanding of the emerging PFASs’ toxicokinetic-toxicodynamic properties at environmentally realistic concentrations and demonstrated their significant bioaccumulation and toxic effects in marine medaka. In future field-based investigations on PFAS, tissue-specific studies on marine mammals should be conducted, and investigations on cation and amphipathic PFASs should be carried out. Furthermore, more toxicokinetic-toxicodynamic studies on PFCA alternatives are needed, and molecular mechanisms of the observed visual impairment caused by PFAS should be elucidated.