*Article* **Toxicity Effects of Combined Mixtures of BDE-47 and Nickel on the Microalgae** *Phaeodactylum tricornutum* **(Bacillariophyceae)**

**Xiaolai Shi, Ruoyu Guo, Douding Lu, Pengbin Wang \* and Xinfeng Dai \***

Key Laboratory of Marine Ecosystem Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, 36 Baochubei Road, Hangzhou 310012, China; shxl7509@sio.org.cn (X.S.); dinoflagellate@sio.org.cn (R.G.); doudinglu@sio.org.cn (D.L.)

**\*** Correspondence: algae@sio.org.cn (P.W.); microalgae@sio.org.cn (X.D.);

Tel.: +86-182-6886-1647 (P.W.); +86-137-3546-6556 (X.D.)

**Abstract:** Nickel and 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) are two environmental pollutants commonly and simultaneously present in aquatic systems. Nickel and BDE-47 are individually toxic to various aquatic organisms. However, their toxicity mechanisms are species-dependent, and the toxic effects of combined mixtures of BDE-47 and nickel have not yet been investigated. The present study investigated the toxic effects of combined mixtures of BDE-47 and nickel in the diatom *Phaeodactylum tricornutum*. BDE-47 and nickel mixtures significantly decreased cell abundance and photosynthetic efficiency, while these cells' reactive oxygen species (ROS) production significantly increased. The EC50-72 h for BDE-47 and mixtures of BDE-47 and nickel were 16.46 ± 0.93 and 1.35 ± 0.06 mg/L, respectively. Thus, combined mixtures of the two pollutants enhance their toxic effects. Interactions between BDE-47 and nickel were evaluated, revealing synergistic interactions that contributed to toxicity in *P. tricornutum*. Moreover, transcriptomic analyses revealed photosynthesis, nitrogen metabolism, the biosynthesis of amino acids, the biosynthesis of secondary metabolites, oxoacid metabolism, organic acid metabolism, carboxylic acid metabolism, and oxidation-reduction processes were considerably affected by the mixtures. This study provides evidence for the mechanisms of toxicity from combined BDE-47 and nickel exposure while also improving our understanding of the ecological risks of toxic chemicals on microalgae.

**Keywords:** toxicity; physiological; transcriptome; the combination of BDE-47 and nickel; *Phaeodactylum tricornutum*
