Recurrent blooms of the toxic dinoflagellate Ostreopsis cf. ovata are frequently reported in the Northwestern Mediterranean Sea. The impact of these proliferations on other microalgal species inhabiting the same habitats is of interest from an ecological prospective. In vitro experiments were carried out to investigate the influence of O. cf. ovata on the growth of the co-occurring benthic diatoms Licmophora paradoxa, Navicula arenaria and the benthic dinoflagellates Prorocentrum lima and Coolia monotis. Overall, O. cf. ovata exhibited weak allelopathic effects towards these microalgal species, with a reduction in the cell abundance for L. paradoxa and P. lima only. Interestingly, dead cells of L. paradoxa and N. arenaria were observed embedded in the thick mucus surrounding O. cf. ovata cells, suggesting that the mucous layer could act as a toxic phycosphere, especially for non-motile cells. All competitors were further exposed for 24 h to ovatoxins, the major toxins produced by O. cf. ovata, and the maximum quantum yield efficiency of L. paradoxa, N. arenaria and P. lima was affected at a minimum concentration of 10 µg mL⁻¹. We then hypothesized that the diffusion of solubilized ovatoxins in the culture medium affects only moderately the competitors’ growth, whereas their accumulation in the mucus would yield deleterious effects. More precisely, the competitors’ sensitivity to ovatoxins was enhanced in their stationary phase of growth and resulted from a rapid inhibition of an uncharacterized photosynthetic step downstream photosystem II. Altogether, these results emphasize the predominant role of the O. cf. ovata’s mucus in driving ecological interactions and suggest that it can affect the growth of several benthic microalgae by accumulating the potent ovatoxins.

有毒甲藻Ostreopsis的反复开花cf。ovata经常在地中海西北部报告。从生态学的角度来看，这些增殖对居住在同一栖息地的其他微藻物种的影响是令人感兴趣的。进行了体外实验以研究O. cf.的影响。ovata对同时发生的底栖硅藻Licmophora paradoxa、Navicula arearia和底栖甲藻Prorocentrum lima和Coolia monotis的生长。总体而言，O. cf. 卵形对这些微藻物种表现出弱化感作用，仅L. paradoxa和P. lima的细胞丰度降低。有趣的是，观察到L. paradoxa和N. arearia的死细胞嵌入在O周围的厚粘液中。参看。卵形细胞，这表明粘液层可以充当有毒藻类，特别是对于非运动细胞。所有参赛者进一步暴露于卵毒素 24 小时，卵毒素是O产生的主要毒素。参看。卵形, L. paradoxa , N. arearia和P. lima的最大量子产率^{在 10 µg mL -1}的最小浓度下受到影响。然后我们假设溶解的卵毒素在培养基中的扩散仅对竞争者的生长产生适度影响，而它们在粘液中的积累会产生有害影响。更准确地说，竞争者对卵毒素的敏感性在其生长的稳定阶段增强，这是由于下游光系统 II 的未表征的光合作用步骤的快速抑制所致。总之，这些结果强调了O. cf.的主要作用。ovata的粘液在驱动生态相互作用中的作用，并表明它可以通过积累有效的卵毒素来影响几种底栖微藻的生长。