1. IntroductionWhen photosynthetic organisms are exposed to higher irradiances which exceed their light energy requirement for photosyn-thetically operated metabolisms, a reduction of photosynthetic capa-city, called photo-inhi-bition, occurs (c.f. Powles, 1984; cf. Krause, 1988; Krause and Weis 1992). Under excessive light condi-tions Ohad et al.(1984) and Mattoo et al. (1984) found a de-grada-tion of the reaction center protein (D1) of photosystem II. Now, there is a controversial discus-sion whether this de-gradation is only a pho-to-damage or if it may protec-t the photosynthetic appa-ratus against continuous damaging ef-fects of in excess absorbed light energy (Osmond, 1994). Photo-da-mage occurs if the rate of the D1-protein damage exceeds the rate of its repair pro-cess, leading to a break-down of the D1-protein (Aro et al.,1993).Recently, a second process has been described which could be impor-tant for the regulation of quantum yield of photosynthesis, namely the xantho-phyll-cycle in the chloroplasts (Demmig-Adams and Adams, 1992). Quantum yield of photo-sy-stem II is diminished reversibly by increasing thermal energy dis-sipation. Caro-te-noids may control this harmless dissi-pation of excessively absor-bed light energy. Therefore, the pro-cess was called dynamic photo-in-hibition in order to distin-guish it clearly from chronic photoinhibi-tion due to the D1 turn-over(cf. Osmond, 1994). The short term ac-clima-tion of plants to high irra-di-ances and its re-lation to photosystem II pho-to-che-mistry and fluorescence emis-sion were reviewed in detail by Dau (1994a, b). Moreover, a gene-ral overview of photoinhi-bi-tion, its molecular aspects and its mecha-nisms in the field is given by seve-ral articles in the book edi-ted by Baker and Bowyer (1994). The effects of a changing irradiance environment especially on marine macrophyte physiology were also recently reviewed (Franklin 1997, Häder 1997).In the marine habitat macrophytes are exposed to con-si-derable diurnal changes of the impinging photon fluence rates due to the position of the sun, clouding and, especially, the tides. Therefore, at midday benthic marine algae can be exposed to extremely high irradiances during low tide. As a consequence, light energy is excessively absorbed by the photosynthetic apparatus and, hence, the extent of its photodamage increases. One of the damaging processes is the production of highly reactive oxy-gen species, which attack target molecules like the D1-protein, chlo-ro-phylls and unsaturated fatty acids (Asada and Takahashi, 1987). Thereby, it in-duces photodamage when scaven-ging by su-per-oxide dismu-tase, hydro-gen peroxi-dase or cata-lase is insuffi-cient. Under light stress condi-tions, the concentration of active oxygen is in-creased either by higher production rates or by in-suffi-cient capa-city of the oxygen scavenging systems. In general, the triplet sta-tes of excited chorophylls cause the generation of the extremely harmful singlet oxygen which can be quen-ched by caro-tenoids and ascor-bate.