Synthesis of mycosporine-like amino acids in Chondrus crispus (Florideophyceae) and the consequences for sensitivity to ultraviolet B radiation

The induction and protective role of the W-absorbing compounds known as myc osporine-like amino acids (MAAs) were examined in sublittoral Chondrus cris pus Stackh. transplanted for 2 weeks in the spring and summer to shallow wa ter under three irradiance conditions: PAR (photosynthetically active radia tion; 400-700 nm), PAR + UVA (PAR + 320-400 nm), PAR + UVA + UVB (PAR + UVA + 280-320 mm). Sublittoral thalli collected around Helgo-land, North Sea, Germany, from 6 m below the mean low water of spring tides contained less t han 0.1 mg.g(-1) dry weight (DW) total MAAs, whereas eulittoral samples con tained over 1 mg.g(-1) DW. Transplantation to shallow water led to the imme diate synthesis of three MAAs in the following temporal order: shinorine (l ambda(max) 334 nm), asterina (lambda(max) 330 nm), and palythine (lambda(ma x) 320 mn), with the shinorine content peaking and then declining after 2 d ays (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 norm ally found in eulittoral samples. Furthermore, the relative proportion of t he different MAAs at this: time was different than that in eulittoral sampl es. After 2 days the total content declined to the eulittoral value, with p alythine as the principal MAA. Similar data were obtained for all treatment s, indicating that MAA synthesis in C. crispus was induced by PAR and not e specially stimulated by UV radiation. The ability of photosystem II (PSII) to resist damage by UVB was tested periodically during the acclimation peri od by exposing samples to a defined UVB dose in the lab. Changes in chlorop hyll fluorescence (F-v/F-m and effective quantum yield, phi(II)) indicated that PSII function was inhibited during the initial stage of acclimation bu t gradually improved with time. No difference among screening treatments wa s detected except in spring for the samples acclimating to PAR + UVA + UVB. In this treatment F-v/F-m and phi(II) were significantly lower than in the other treatments. During the first week of each experiment, growth rates w ere also significantly reduced by WE. The reductions occurred despite maxim um MAA content, indicating an incomplete protection of photosynthetic and g rowth-related processes.