Skeletonema marinoi (Bacillariophyceae) sensitivity to herbicides andeffects of temperature increase on cellular responses toterbuthylazine exposure

tThe North East area of Italy is an intensively farmed area, where the use of herbicides has increaseddramatically during the last years. Some of the most detected herbicides are triazine compounds, such as:simazine (SIM), terbuthylazine (TBA), its degradation product desethyl-terbuthylazine (D-TBA) and otherherbicides, such as metolachlor (MET). In this paper, the sensitivity of the diatom Skeletonema marinoito the most detected herbicides (TBA, D-TBA, SIM and MET) was preliminarily studied. All the pollutantstested significantly inhibited the diatom growth and photosynthetic efficiency (from the concentrationof 15 g L−1) with the exception of TBA which had the strongest effects on S. marinoi starting from theconcentration of 5 g L−1. Consequently, cellular physiological responses to TBA exposure (1, 5, 10, 20and 30 g L−1) were further studied at increasing temperature conditions (15, 20 and 25◦C). Inhibitionof growth rate and photosynthetic efficiency was observed earlier and determined by lower TBA levelsthan those affecting cell growth. These responses were significantly enhanced at increasing temperatureconditions when growth rates were higher than those measured at 15◦C. Carbon cell content increased inthe cultures exposed to high concentrations of TBA (from 20 g L−1) compared to the controls, especiallyat high temperatures. Cell chlorophyll significantly increased from the added concentration of 10 g L−1of TBA at all the temperatures and, as a consequence, also the Chl:C ratio significantly increased. TheC:N ratio followed the pattern of nitrate uptake and was characterized, at all the temperatures, by lowvalues during the lag phase in cultures with 20 and 30 g L−1of TBA; in these conditions, in fact, thenutrient in the medium was exhausted later then in the controls. Only cultures exposed to 30 g L−1of TBA at 25◦C, which stopped to take up nutrients earlier and could not increase chlorophyll levels,did not display any growth capacity. This study shows that S. marinoi is affected by TBA concentrationslower than those affecting some harmful flagellate species frequently observed in the Adriatic Sea. Thus,it raises the question of the combined effects of herbicides pollution and high temperature pressures onphytoplankton composition.