• chlorophyll fluorescence;
  • Chondrus crispus;
  • growth;
  • MAA;
  • macroalgae;
  • mycosporine-like amino acids;
  • photosynthesis;
  • UVB radiation

The induction and protective role of the UV-absorbing compounds known as mycosporine-like amino acids (MAAs) were examined in sublittoral Chondrus crispus Stackh. transplanted for 2 weeks in the spring and summer to shallow water under three irradiance conditions: PAR (photosynthetically active radiation; 400–700 nm), PAR + UVA (PAR + 320– 400 nm), PAR + UVA + UVB (PAR + UVA + 280– 320 nm). Sublittoral thalli collected around Helgoland, North Sea, Germany, from 6 m below the mean low water of spring tides contained less than 0.1 mg·g−1 dry weight (DW) total MAAs, whereas eulittoral samples contained over 1 mg·g−1 DW. Transplantation to shallow water led to the immediate synthesis of three MAAs in the following temporal order: shinorine (λmax 334 nm), asterina (λmax 330 nm), and palythine (λmax 320 nm), with the shinorine content peaking and then declining after 2 days (exposure to 100 mol photons·m−2). Maximum total MAA content (2 mg·g−1 DW) also occurred after 2 days of induction, exceeding the content normally found in eulittoral samples. Furthermore, the relative proportion of the different MAAs at this time was different than that in eulittoral samples. After 2 days the total content declined to the eulittoral value, with palythine as the principal MAA. Similar data were obtained for all treatments, indicating that MAA synthesis in C. crispus was induced by PAR and not especially stimulated by UV radiation. The ability of photosystem II (PSII) to resist damage by UVB was tested periodically during the acclimation period by exposing samples to a defined UVB dose in the lab. Changes in chlorophyll fluorescence (Fv/Fm and effective quantum yield, φII) indicated that PSII function was inhibited during the initial stage of acclimation but gradually improved with time. No difference among screening treatments was detected except in spring for the samples acclimating to PAR + UVA + UVB. In this treatment Fv/Fm and φII were significantly lower than in the other treatments. During the first week of each experiment, growth rates were also significantly reduced by UVB. The reductions occurred despite maximum MAA content, indicating an incomplete protection of photosynthetic and growth-related processes.