Effects of Temperature and Photorepair Radiation on a Marine Ciliate Exposed to UVB Radiation

Abstract

The influences of intensity and repeated exposure to ultraviolet-B radiation (UVB), photoreactivating repair radiation (PRR), and temperature on the scuticociliate Parauronema acutum were explored under laboratory conditions. Population growth was negatively affected after exposure to the equivalent of one sunny summer day of ambient UVB, especially in the absence of PRR. Repeated daily exposure to UVB severely compromised ciliate survival. UVB-exposed treatments without PRR recovered slower and reached lower final abundances than treatments receiving PRR. Reducing the daily UVB exposure approximately 25% improved ciliate recovery after exposure. In the single exposure treatments, temperature effects were not consistent, except that growth was slowest for control and treatments at the lowest temperature (15 °C). These data suggest that dark repair and/or photoprotection are present in P. acutum, but photoenzymatic repair was the more effective mechanism in reversing UVB damage. Repeated exposure treatments without PRR had zero or declining growth at all temperatures (15, 20 and 25 °C), as did those with PRR at 15 °C. Significant temperature/dose differences were identified in the repeated exposure treatments; ciliates subjected to the higher UVB intensity with PRR survived only at 25 °C, while ciliate populations under reduced UVB increased at 20 and 25 °C.

Effectiveness of Photoprotective Strategies in Three Mixotrophic Planktonic Ciliate Species

“…Our results add to previous reports of the existence of PER in the ciliates Glaucoma sp. and P. acutum as a major strategy in UV-B tolerance [22,23].…”

“…Alternatively, organisms may have a nucleotide excision repair mechanism ("dark repair"), where the damaged part of the DNA is removed and resynthesized [17,21]. PER and dark repair appear to be widespread among taxonomically diverse organisms and have been also identified in protists [17,[22][23][24]. When all of these mechanisms are inefficient, diverse negative effects are observed.…”

“…In protists, damage by UVR is known to lead to reduced motility and retarded division as well as a reduction in growth rates [25][26][27][28][29]. However, from the few studies available, for example, on ciliates, it is known that both damage by UVR and the presence of photoprotective mechanisms are species-specific [10,[14][15][16]22,23,[30][31][32][33][34][35]. For example, in laboratory experiments, Glaucoma sp.…”

“…For example, in laboratory experiments, Glaucoma sp. and Parauronema acutum recovered under photoreactive radiation or in the dark, whereas a Cyclidium species did not [22,23]. In some mixotrophic ciliates, two photoprotective mechanisms provided by the algal symbionts have been identified, namely, the synthesis of sunscreen compounds (i.e., mycosporine-like amino acids (MAAs)) and the self-shading effect given by the formation of dense algal layers inside the cell that prevent UVR from reaching the sensitive nuclear material [14][15][16].…”