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

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 He lgoland, North Sea, Germany. Three PAR (400-700 nm) treatments without UVR and three UVR-AB (290-400 nn) treatments supplemented with 30 mu mol m(-2) s(-1) PAR were given twice. All three stages of plant stress, irritation, d amage and acclimation, were observed under high light conditions. In the ea rlier stages of photoinhibitory PAR and UVR treatment, an induction phase o ccurred. It was characterized by a significant increase in chlorophyll a an d carotenoids. The capacity to cope with high light stress in C. crispus in creased 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 pigmen t destruction that may be responsible for the damage stage of stress. In pl ants exposed to elevated PAR (3600 eta mol 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 exce eded 95% of control. Even after 20h in low irradiance it was similar to val ues measured immediately after stress, indicating severe photodamage. The 1 800 mu mol 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 recove red. 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, photoinhi bitory 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 qu enching is related to the concentration of carotenoids. The increase in con centration, not only in the first but also after repeated exposures to PAR and UVR, was attributed to a protection of the photosynthetic apparatus. Ad ditionally, accumulation of mycosporine-like amino acids (MAAs), mainly dur ing the repeated exposures to UVR, led to substantially less inhibition of photosynthesis than expected given the practically double UVR dose. This su ggests incomplete protection of C. crispus by MAAs during UVR shock. Thus, these experiments indicate that a few basic physiological strategies seem t o be active during acclimation to high solar irradiance: a prolonged induct ion phase, dynamic photoinhibition, increased concentration of carotenoids, and accumulation of MAAs, UV-screening substances. (C) 2001 Elsevier Scien ce B.V. All rights reserved.