**3. Results**

For the three ciliates tested, we identified species-specific responses regarding their photoprotection strategies as described in detail below (Figure 1).

## *3.1. Stokesia Vernalis*

No significant mortality occurred in any of the three treatments, neither over 4 h (i.e., natural daily dose) nor over 7.5 h in exp 1 (*p* > 0.05). Moreover, no abnormal swimming behavior, changes in shape, or symbiont disarrangement were detected (B.S. personal observation). After 48 h (exp 3), about 20% of survivors were left over after the UV treatment (*p* < 0.05) in contrast to PAR and the control (*p* > 0.05; Figure 2). After exposure (exp 1), the concentration of MAAs was highest in the UV treatment, followed by PAR and then the control. Shinorine was the dominant MAA (99% of the total

amount of MAAs) after all three experimental treatments, and asterina-330, palythene, and usujirene were only found in traces (Table 1).

**Figure 1.** Living individuals of *Vorticella chlorellata* (**<sup>a</sup>**–**<sup>c</sup>**), *Stokesia vernalis* (**d**), and *Pelagodileptus trachelioides* (**<sup>e</sup>**,**f**): (**a**) Overall view of an intact stalked individual after photosynthetically active radiation (PAR) (experiment (exp) 1); (**b**) unusual globular appearance of the cell 48 h after the UV treatment (exp 3), with the distorted myoneme (stalk protein) visible (arrow); (**c**) *V. chlorellata* attached to colonies of *Botryococcus braunii* (**d**), showing the right lateral view, where the arrows point out to the unique "packages" of algal symbionts; (**e**) total view, showing the prominent proboscis and trunk; and (**f**) left lateral view of an individual without proboscis after 6.5 h under ultraviolet radiation (UVR) (exp 1), where the arrow points to the stump of the proboscis left over, the symbionts were accumulated in the posterior cell portion. A, algal symbionts; *B.b.*, colonies of *Botryococcus braunii*; M, myoneme of the stalk; P, proboscis; T, trunk, *V.c*., individual of *V. chlorellata*. Scale bars: 50 μm (**<sup>a</sup>**–**<sup>e</sup>**) and 10 μm (**f**).



1 Mainly shinorine with proportions of porphyra-334 (peaks in chromatogram not distinctly separated). 2 Peaks in chromatogram not distinctly separated in either palythine or asterina-330.

**Figure 2.** Surviving individuals (% ± std) of *Stokesia vernalis* after 48 h after exp 3 (follow-up on exp 2): *n* = 4 replicate wells containing five individuals each; \* indicates a significant difference between the UV treatment and the control (*p* < 0.05).

## *3.2. Vorticella Chlorellata*

In the PAR treatment and in the control, no changes in the ciliates' appearance and behavior were recorded (exp 1; Figure 1a). During UV exposure, some individuals had broken myonemes (i.e., stalk proteins) from 6 h onward (Figure 1b). After 48 h, their stalks did not contract any more, the adoral membranelles did not filter anymore, and finally, the ciliates died.

## *3.3. Pelagodileptus trachelioides*

All individuals tested survived 7.5 h of exposure in any treatment as well as in the control (exp 1; *p* > 0.05). After the dark repair period (exp 3), significant mortalities were observed in individuals from the UV treatment and at 280, 295, and 305 nm (*p* ≤ 0.001) in contrast to 320, 335, 345, and 360 nm, PAR, and the control (*p* > 0.05; Figure 3a). Individuals already moved slower than usual after 2.5 h in the UV treatment and 280 nm, after 3.5 h in 295 and 305 nm, and after 5.5 h in 320 nm but not in the other treatments (B.S. personal observation). Morphological alterations, such as a spherical appearance with a reduced trunk and a reduction in proboscis length (Figure 1f), were observed in the UV treatment and at 280, 295, and 305 nm after 7.5 h in approximately 10% of the individuals but not in all other treatments (i.e., exp 1 and exp 2). A symbiont accumulation in the posterior cell portion of *P. trachelioides* was observed after 4.5 h and later only in the UV-B treatments (i.e., exp 1 and exp 2).

In the PER exp 3, all ciliates survived the exposure over 4 h (UVR\_4h), the PER period (5.5 h), and the dark period (12.5 h; *p* > 0.05; Figure 3b and Figure S1). Approximately 15–20% of the individuals survived three weeks. In the UVR\_6h treatment, survival significantly decreased after the dark period (12.5 h; *p* ≤ 0.001; Figure 3b), accompanied by decreased swimming activity and dislocated symbionts, seen after 9.5 h in 24% of the survivors (B.S. personal observation). No individual from the UVR\_6h treatment was sustained 48 h after exposure.

Among the MAAs detected, shinorine was the dominant one (approximately 90%) in all three treatments, followed by palythine and asterina-330, and then traces of palythene and usujirene (Table 1). The concentration of MAAs decreased from the UV treatment to the dark control.

**Figure 3.** (**a**) Mortality (% ± std) of *Pelagodileptus trachelioides* 24 h after T0 (exp 1 and 2): The experimental treatments were UVR, 280, 295, 305, 320, 335, 345, and 360 nm, PAR, and control (Cont). Data of three individual experiments were pooled (*n* = 60 for all treatments, except for 280 and 295 nm with *n* = 40). \* indicates significant mortality against the control (*p* ≤ 0.001). (**b**) Means of the surviving individuals (% ± std) of *P. trachelioides* after experimental exposure to UVR\_4h (without UVR after 4 h) and UVR\_6h (without UVR after 6 h) and in the control (exp 3). X-axis: duration (h) of the UVR + PAR period, the photoenzymatic repair (PER) period (without UVR), and the dark period. a, b, and c indicate significant mortality (*p* < 0.05) between treatments.
