Microalgae, which are the foundation of aquatic food webs, may be the indirect target of herbicides used for agricultural and urban applications. Microalgae also interact with other compounds from their environment, such as natural dissolved organic matter (DOM), which can itself interact with herbicides. This study aimed to evaluate the influence of natural DOM on the toxicity of three herbicides (diuron, irgarol and S-metolachlor), singly and in ternary mixtures, to two marine microalgae, Chaetoceros calcitrans and Tetraselmis suecica, in monospecific, non-axenic cultures. Effects on growth, photosynthetic efficiency (Ф’_M) and relative lipid content were evaluated. The chemical environment (herbicide and nutrient concentrations, dissolved organic carbon and DOM optical properties) was also monitored to assess any changes during the experiments.

The results show that, without DOM, the highest irgarol concentration (I0.5: 0.5 mg.L⁻¹) and the strongest mixture (M2: irgarol 0.5 μg.L⁻¹ + diuron 0.5 μg.L⁻¹ + S-metolachlor 5.0 μg.L⁻¹) significantly decreased all parameters for both species. Similar impacts were induced by I0.5 and M2 in C. calcitrans (around −56% for growth, −50% for relative lipid content and −28% for Ф’_M), but a significantly higher toxicity of M2 was observed in T. suecica (−56% and −62% with I0.5 and M2 for growth, respectively), suggesting a possible interaction between molecules.

With DOM added to the culture media, a significant inhibition of these three parameters was also observed with I0.5 and M2 for both species. Furthermore, DOM modulated herbicide toxicity, which was decreased for C. calcitrans (−51% growth at I0.5 and M2) and increased for T. suecica (-64% and −75% growth at I0.5 and M2, respectively).

In addition to the direct and/or indirect (via their associated bacteria) use of molecules present in natural DOM, the characterization of the chemical environment showed that the toxic effects observed on microalgae were accompanied by modifications of DOM composition and the quantity of dissolved organic carbon excreted and/or secreted by microorganisms. This toxicity modulation in presence of DOM could be explained by (i) the modification of herbicide bioavailability, (ii) a difference in cell wall composition between the two species, and/or (iii) a higher detoxification capacity of C. calcitrans by the use of molecules contained in DOM. This study therefore demonstrated, for the first time, the major modulating role of natural DOM on the toxicity of herbicides to marine microalgae.

微藻是水生食物网的基础，可能是用于农业和城市应用的除草剂的间接目标。微藻还与环境中的其他化合物发生相互作用，例如天然溶解的有机物（DOM），其本身可以与除草剂发生相互作用。这项研究旨在评估天然DOM对单种和非树种培养物中三种除草剂（地隆，依加洛尔和S-异丙甲草胺）单独和三元混合物对两种海洋微藻Chaetoceros calcitrans和Tetraselmis suecica毒性的影响。 。上生长的影响，光合效率（Ф”_中号）和相对脂质含量进行了评估。还监测了化学环境（除草剂和营养物的浓度，溶解的有机碳和DOM的光学性质），以评估实验过程中的任何变化。

结果表明，在没有DOM的情况下，irgarol的最高浓度（I0.5：0.5 mg.L ^-1）和最强的混合物（M2：irgarol 0.5μg.L ^-1  +敌草隆0.5μg.L ^{- 1}  + S-异丙甲草胺5.0μg.L ^-1）显着降低了两个物种的所有参数。类似的影响在诱导由I0.5和M2 C.钙质（约-56％的增长，-50％的相对脂质含量和Ф” -28％_中号），但在观察到M2的显著更高毒性Ť suecica（-56％和-62％分别具有I0.5和M2的生长），表明分子之间可能存在相互作用。

将DOM添加到培养基中后，两种物种的I0.5和M2均对这三个参数产生了显着抑制作用。此外，DOM调节了除草剂的毒性，C.calcitrans降低了（在I0.5和M2时-51％增长），而对T. suecica升高（在I0.5和M2时-64％和-75％增长）。 。

除了直接和/或间接（通过其相关细菌）使用天然DOM中存在的分子外，化学环境的表征还表明，对微藻观察到的毒性作用伴随着DOM组成和溶解有机物数量的变化。被微生物分泌和/或分泌的碳。在DOM的存在下进行该毒性调制可以通过进行说明（I）除草剂的生物利用度的修改，（ⅱ）在这两个物种之间细胞壁组成的差异，和/或（iii）的更高的解毒能力C.钙质由使用DOM中包含的分子。因此，这项研究首次证明了天然DOM对除草剂对海洋微藻毒性的主要调节作用。