PFAS-herbicide diflufenican reduces the photosynthetic capacity in seagrass (Zostera marina L.)

Global decline of seagrasses is directly linked with anthropogenic activities. One of the increasing threats, is marine pollution with a wide range of chemically synthesized substances. Here, we investigated effects of the commonly used PFAS-herbicide, diflufenican (DFF), on Zostera marina L. photobiology, over a 3-week exposure period, followed by 2-weeks of recovery. Measurements were conducted on seagrass leaves exposed to three different DFF concentrations (i.e., untreated, 0.015, 0.03 and 0.3 mg DFF L-1). Photosynthesis and respiration rates, as a function of increasing photon scalar irradiance, were determined with the use of optical sensors measuring net gas exchange. Variable chlorophyll fluorescence imaging was used for determining the 2D photosynthetic capacity of the seagrass leaves. Photopigment concentrations in the seagrass leaves were quantified using HPLC-analysis. We show that DFF reduced the photosynthesis rates for plants exposed to 0.03 and 0.3 mg DFF L-1 already after 1 week of exposure. While the imaging of photosynthetic capacity only showed a reduction for the highest DFF concentration (0.3 mg L-1) after 3 weeks. Concentrations of chlorophyll a, chlorophyll b and β-carotene generally increased with exposure time for all treatments. Yet, no signs of recovery from exposure to high DFF concentrations were observed in the plants' photosynthetic capacity after 2 weeks. These findings indicate that exposure to high concentrations of DFF reduces the photosynthetic performance and capacity of seagrass (Z. marina L.), which over time and combined with other environmental stressors may cause synergetic negative effects that are detrimental to seagrasses.