*3.1. P. tricornutum Cell Abundance in Response to Toxicity Tests*

No significant differences were observed in cell abundance in the BDE-47 toxicity tests at up to 8 mg/L of BDE-47 exposure over 72 h (*p* > 0.05) (Figure 1A). The cell abundance of BDE-47-treated cultures at concentrations of 16-65 mg/L was significantly lower than controls with the same exposure time (*p* < 0.001). After exposure to 65 mg/L BDE-47, cell abundance decreased at 24 h. Overall, the EC50-72 h of BDE-47 for *P. tricornutum* was 16.46 ± 0.93 mg/L.

*P. tricornutum* growth was inhibited when exposed to BDE-47 and nickel. Further, the inhibitory effect on cell abundances gradually increased with increasing exposure concentration and time (Figure 1B). Significant inhibition of cell abundance was detected at 48, 72, and 96 h when using mixtures at a concentration > 0.6 mg/L (*p* < 0.01). The EC50-48 h and EC50-72 h of the BDE-47 and nickel mixtures for *P. tricornutum* were 3.68 ± 0.11 and 1.35 ± 0.06 mg/L, respectively.

#### *3.2. Photosynthetic Efficiency Response*

The photosynthetic efficiency of *P. tricornutum* under different toxin treatments is shown in Figure 2A. BDE-47 exposure caused slight decreases in *P. tricornutum* photosynthetic efficiency. However, significant inhibitory effects were only detected at very high

concentrations of BDE-47 (>8 mg/L). Photosynthetic efficiency was still relatively high in the 65 mg/L BDE-47 treatment.

**Figure 2.** Photosynthetic efficiency of *P. tricornutum* after individual BDE-47 treatment (**A**) and treatment with mixtures of BDE-47/nickel (**B**). Error bars indicate ± SD. *p*-values are shown in Table S4.

Significant differences were not observed after exposure to mixtures of BDE-47 and nickel over 24 h. However, exposure to the mixtures strongly inhibited photosynthesis after 48 h of exposure. Photosynthetic efficiency gradually decreased with increasing exposure time and mixture concentrations (Figure 2B).

#### *3.3. Reactive Oxidative Species (ROS) Production*

ROS production levels over 72 h are shown in Figure 3. ROS production was induced by combinations of BDE-47 and nickel, although increases in ROS levels were not detected in the BDE-47-treated samples. The individual nickel treatments also demonstrated significantly increased ROS production [16]. Exposure to the BDE-47 and nickel mixtures significantly increased ROS production over 24 h at an exposure level of 12 mg/L (Figure 3). The highest ROS production was observed with 12 mg/L of mixtures after 72 h of exposure (Figure 3).
