Combined effects of the herbicide terbuthylazine and temperature on different flagellates from the Northern Adriatic Sea

The triazinic herbicide terbuthylazine (TBA) is becoming an emergent contaminant in Italian rivers and in coastal and groundwater. A preliminary analysis of the sensitivity of marine flagellates to TBA was performed by monitoring the photosynthetic efficiency of nine species (belonging to the Dinophyceae or Raphidophyceae class) isolated from the Adriatic Sea. Different sensitivity levels for each flagellate were observed and the most sensitive microalgae, based on PSII inhibition, were: Gonyaulax spinifera > Fibrocapsa japonica > Lingulodinium polyedrum while the most resistant were two species belonging to the Prorocentrum genus. Then the response of two microalgae to drivers, such as temperature and terbuthylazine, applied in combination was also investigated. Two potentially toxic flagellates, Prorocentrum minimum and G. spinifera, were exposed, under different temperature conditions (15, 20 and 25 °C), to TBA concentrations that did not completely affect PSII. For both flagellates, effects of TBA on algal growth, measured through cell density and carbon analysis, as well as on the photosynthetic activity are reported. All parameters analyzed showed a negative effect of TBA from the exponential phase. TBA effect on algal growth was significantly enhanced at the optimal temperature conditions (20 and 25 °C), while no difference between control and herbicide treatments were detected for G. spinifera grown at 15 °C, which represented a stress condition for this species. The maximum inhibition of photosynthetic efficiency was found at 20 °C for both organisms. Both flagellates increased cell carbon and nitrogen content in herbicide treatments compared to the control, except G. spinifera grown at 15 °C. Chlorophyll-a production was increased only in G. spinifera exposed to 5 μg L−1 of TBA and the effect was enhanced with the increase of temperature. Herbicide-induced variations in cellular components determined changes in cellular carbon:nitrogen (C:N) and chlorophyll:carbon (Chl:C) ratios. The C:N ratio decreased in both species, while only G. spinifera showed an increase in the Chl:C ratio at all temperature conditions. In response to TBA exposure G. spinifera increased extracellular polysaccharides release at 20 and 25 °C, while no difference was reported for P. minimum. Changes in nutrient uptake rates were also observed for P. minimum. Nitrate and phosphate uptake significantly increased in the presence of TBA and this response was enhanced at 25 °C, while nitrate uptake increased in G. spinifera only when grown at 25 °C. As for growth rates, the observed changes in intracellular component contents increased at optimal temperature conditions. In this work it is shown that temperature conditions can have an important role on the effect of terbuthylazine on algal growth and on the physiological responses of different species. Furthermore, the algal resistance and recovery can be dependent on nutrient availability.

► The effect of terbuthylazine was studied for the first time on several marine flagellates.
► Different sensitivity levels were observed with harmful species being both among the most sensitive and the most resistant.
► Algae were most affected at optimal temperature for growth.
► TBA-induced variations in cellular components determined changes in cellular C:N and Chl:C ratio.
► TBA treatments increased: extracellular polysaccharides and chlorophyll-a content (Gonyaulax spinifera), nutrient uptake rates (Prorocentrum minimum).