The toxicity of four herbicides in mixture (alachlor, diuron, des-isopropyl-atrazine and 31 simazine) on the growth and the photosynthesis parameters of two marine diatoms Pseudo-32 niszchia mannii and Chaetoceros decipiens have been investigated for 9 days in monoculture 33 and co-culture tests. The catalase (CAT) and guaiacol peroxidase (GPX) were also monitored 34 to assess the oxidative stress response. In single-species assays, while both species displayed 35 no affected instantaneous growth rate by herbicides, their physiological responses were 36 different. Chl a content of P. mannii significantly decreased upon herbicide exposure, due 37 probably to pigment destruction or inhibition of their synthesis. This decrease was associated 38 with a reduction in the chlorophyll fluorescence parameters (ABS0/RC, TR0/RC, ET0/RC and 39 DI0/RC). In contrast, C. decipiens maintained an effective photosynthetic performance under 40 herbicide exposure, as Chl a per cell content and the specific energy fluxes per reaction center 41 remained unchanged relative to control values. GPX activity was significantly higher in 42 contaminated P. mannii and C. decipiens monocultures than in controls at early herbicide 43 exposure (1 day), whereas a significant induction of CAT activity occurred later (from day 3 44 for C. decipiens and at day 9 for P. mannii) in response to herbicides. In control co-culture, P.45 mannii was eliminated by C. decipiens. As observed in the monoculture, the herbicides did not 46 affect the photosynthetic performance of C. decipiens in co-culture, but significantly reduced 47 its instantaneous growth rate. The oxidative stress response in co-culture has similar trends to 48 that of C. decipiens in monoculture, but the interspecies competition likely resulted in higher 49 CAT activity under herbicide exposure. Results of this study suggest that herbicide toxicity for 50 marine diatoms might be amplified by interspecies interactions in natural communities, which 51 might lead to different physiological and growth responses.