2.3.2. Release of K<sup>+</sup> by Algal Cells

As K<sup>+</sup> is absorbed into the vacuole of algae cells and is mainly stored as an enzyme activator, the algal release of K<sup>+</sup> can be manifested for cell membrane damage [37,38]. During the incubation process, 10-mL supernatant samples of three species were regularly taken and immediately filtered through 0.2-µm mixed cellulose ester filters (Whatman, Little Chalfont, Buckinghamshire, UK) after daily irradiation. Then, the solution was acidified to pH = 2 with HNO<sup>3</sup> and K<sup>+</sup> content was determined by the inductively coupled plasma mass spectrometry (IC-PMS) (XII series, Thermo, Waltham, MA, USA). Afterwards, the release rate of K<sup>+</sup> by algal cells was calculated as a percentage and ultrasonic disrupted samples were adopted to make a comparison.

#### 2.3.3. Characterization of Extracellular Polymeric Substance (EPS)

The extraction of EPS was conducted according to the methods described by Gao et al. [39] and Yang et al. [40]. Firstly, samples of the algal cultures were sonicated with 100 W ultrasound treatment for 5 min to obtain a uniform distribution, then filtered through 0.45-µm filters (Whatman, Little Chalfont, Buckinghamshire, UK) in order to separate soluble EPS (SEPS) from cell pellets [41]. The supernatant was collected and stored at 4 ◦C in the dark. Then, the harvested cells were washed in ultra-pure water, re-suspended in 0.05% NaCl solution, and centrifuged at 16,000× *g* for 20 min. The resulting supernatant was collected as bounded EPS (BEPS) and also stored at 4 ◦C in the dark.

On Day 1, the filtered SEPS and BEPS fractions were taken without dilution and the excitation emission matrix (EEM) spectra were determined by using a fluorescence spectrometer (F-7000, Hitachi, Tokyo, Japan). The excitation wavelengths were increased from 200 to 400 nm in 5-nm steps and the emission spectra were recorded from 250 to 500 nm in 1-nm increments. The increments were all set at 5 nm, and a scan speed of 2400 nm min−<sup>1</sup> was applied. The blank scans were performed using modified BG<sup>11</sup> medium, in which no fluorescence substance was present.

In addition, the contents of SEPS and BEPS were quantified spectrophotometrically (UV-2700, Shimadzu, Kyoto, Japan) during the whole incubation process by the anthrone sulfuric acid method and the values were normalized to cell density [42].

#### 2.3.4. Reactive Oxygen Species (ROS) in Cells and Superoxide Dismutase (SOD) Activity

Before and during the incubation process in the mono-cultures, subsamples were regularly taken for the determination of ROS in algal cells of three species and activity of SOD. The details can be seen in the Supplementary Materials (SM).

#### 2.3.5. Cell Adsorption Spectra and Contents of Photosynthetic Pigments

At the beginning of incubation, a scanning spectrophotometer (Beckman Coulter, Fullerton, CA, USA) was used to measure the whole-cell absorption spectra of three species between 400 and 750 nm. Cell cultures with the optical density value at 680 nm (OD680) of 0.10 were used for measurement and the absorption peaks could indicate the existence of photosynthetic pigments in algal cells [43].

At different stages of the incubation (Day 1 and 8) in the mono-cultures, a subsample of algal cultures was collected and filtrated through 0.2-mm mixed cellulose ester filters (Whatman) to determine the contents of photosynthetic pigments (pg cell−<sup>1</sup> ) in single cells, including chlorophyll a (Chl-a), carotenoid (CAR), and phycocyanin (PC). The details can be seen in SM.

#### *2.4. Statistical Analysis*

All experiments were conducted in triplicate and means ± standard deviations of three replicates were calculated. The parametric three-way repeated-measures analysis of variance (RM-ANOVA) was used to determine the effects of irradiation treatments (PAR and UV-B), species (*C. pyrenoidosa*, non-toxic and toxic *M. aeruginosa*), and sampling time on the cell density, growth rates, EPS contents, and other parameters. Data were tested for normality and the variance assumptions of parametric ANOVA, and no data transformation was needed. If the interaction factor was significant at *p* < 0.05, a one-way ANOVA followed by Tukey's test was adopted to determine where differences lie. Meanwhile, the student's *t*-test was adopted to test the differences in the algal cell density of different species on a specific day in the co-cultures. All statistical analyses were performed using SPSS 22.0 (Chicago, IL, USA).

#### **3. Results**
