Effect of high visible and UV irradiance on subtidal Chondrus crispus: stress, photoinhibition and protective mechanisms

Abstract

Stress and the mechanisms of protection under excessive photosynthetically active (PAR) and ultraviolet radiation (UVR) were examined in the red alga Chondrus crispus Stackh. collected from the subtidal zone (6 m depth) at Helgoland, North Sea, Germany. Three PAR (400–700 nm) treatments without UVR and three UVR-A/B (290–400 nm) treatments supplemented with 30 μmol m −2 s −1 PAR were given twice. All three stages of plant stress, irritation, damage and acclimation, were observed under high light conditions. In the earlier stages of photoinhibitory PAR and UVR treatment, an induction phase occurred. It was characterized by a significant increase in chlorophyll a and carotenoids. The capacity to cope with high light stress in C. crispus increased with increasing duration of the induction phase. Prolonged exposures to high irradiance induced a substantial decline in the potential quantum yield of photosynthesis ( F v/ F m) and progressive pigment destruction that may be responsible for the damage stage of stress. In plants exposed to elevated PAR (3600 μmol m −2 s −1) and UVR (57.3 and 3.6 W m −2 of UVRA and UVRB, respectively), photoinhibition of F v/ F m exceeded 95% of control. Even after 20 h in low irradiance it was similar to values measured immediately after stress, indicating severe photodamage. The 1800 μmol m −2 s −1 PAR treatment also decreased the pigment content and F v/ F m, although damage was not permanent and the tissue gradually recovered. In contrast, naturally high doses of UVR (37.7 and 2.3 W m −2 of UVRA and UVRB, respectively) resulted in less photoinhibition of photosynthesis with rapid recovery of F v/ F m and pigment content, indicating the dynamic nature of photosynthetic acclimation to growth irradiance. Thus, photoinhibitory PAR damage and its repair predominate in subtidal C. crispus during the process of acclimation to a full-sun environment. Comparison between the acclimation conditions suggests that fluorescence quenching is related to the concentration of carotenoids. The increase in concentration, not only in the first but also after repeated exposures to PAR and UVR, was attributed to a protection of the photosynthetic apparatus. Additionally, accumulation of mycosporine-like amino acids (MAAs), mainly during the repeated exposures to UVR, led to substantially less inhibition of photosynthesis than expected given the practically double UVR dose. This suggests incomplete protection of C. crispus by MAAs during UVR shock. Thus, these experiments indicate that a few basic physiological strategies seem to be active during acclimation to high solar irradiance: a prolonged induction phase, dynamic photoinhibition, increased concentration of carotenoids, and accumulation of MAAs, UV-screening substances.