Photoinhibition of photosynthesis was investigated in the Mediterranean green macroalga Acetabularia mediterranea using pulse amplitude modulated (PAM) chlorophyll fluorescence and oxygen evolution measurements in situ under solar radiation. Fluorescence parameters measured during the course of the day showed that moderate photoinhibition occured even in the natural environment of this alga when the sun was at high angles, but photosynthetic capacity was almost fully restored until sunset. A drastic decline in effective quantum yield was monitored when plants collected at 5 m depth were exposed to high irradiance close to the surface. Removal of the short wavelength band from solar radiation using cut-off filters revealed that UV has an overproportional inhibitory effect on photosynthesis. Especially the UV-B part of the spectrum contributes to photoinhibition whereas UV-A seems to be less effective in A. mediterranea. Total recovery of PAM chlorophyll fluorescence was obtained after 2 h of shading in all samples, indicating reversible photoinhibition rather than non-reversible photodamage. Oxygen evolution showed slower kinetics of inhibition.

Photoinhibition under solar radiation was measured in the green macroalgae Cladophora prolifera and C. pellucida, using pulse amplitude modulation fluorescence and oxygen production in situ. Photosynthetic oxygen production is inhibited within about 1 h and the quantum yield follows with similar time kinetics. Samples were collected from a habitat protected from exposure to direct solar radiation except for a few hours per day. Therefore, individuals of both species were adapted to low irradiance and readily inhibited by exposure to excessive radiation. Impairment occurred even at their natural habitat during the few hours of exposure to direct solar radiation of the day. Recovery from photoinhibition of both the photosynthetic quantum yield, defined as F-v'/F-m', and oxygen production took several hours and was not complete. Judging from both parameters indicated above, C. pellucida showed a higher degree of photoinhibition than C. prolifera. These results suggest that photoinhibition may be a significant factor in determining the spatial distribution of these benthic macroalgae.

ABSTRACTInhibition of photosynthesis after exposure to solar radiation was investigated in the marine green alga Dunaliella salina by monitoring photosynthetic optimal quantum yield Fv/Fm and efficiency of oxygen production. Samples were exposed to solar radiation in Ancient Korinth, Greece (37°58′ N, 23°0′ E) in August 1994. Within 30 min, Fv/Fm and efficiency of oxygen production decreased with similar kinetics with increasing exposure time. The inhibition, however, diminished when ultraviolet radiation was progressively excluded by means of colour filter glasses. Samples exposed for 3 h showed complete or partial recovery of photosynthesis, with almost the same rate under all irradition conditions. The fit of the experimental data with an analytical model describing inhibition of photosynthesis as a function of a linear combination of the photon fluence in the UV‐B, UV‐A and PAR allows one to estimate the relative mean effectiveness for inhibition by the three spectral ranges [about 2 × 10−4, 4 × 10−6 and 2 × 10−7 (μmol photons m−2)−1 for UV‐B, UV‐A and PAR, respectively].

Photoinhibition of photosynthesis was investigated in the Mediterranean green alga Caulerpa prolifera by using pulse amplitude modulation (PAM) chlorophyll fluorescence and oxygen evolution. After exposure to solar radiation at the water surface, oxygen production decreased drastically within 20 min; this inhibition of photosynthesis was only partially restored during the subsequent hours in the shade. Oxygen measurements at different depths showed optimal photosynthesis at a depth of 5 m. Exposure to solar radiation at the surface also caused a drastic decline in the effective photosynthetic quantum yield. Also these effects only partially recovered indicating that they are mainly caused by reversible photoinhibition and to a smaller extent to nonreversible photodamage. Photoinhibition occurs in this alga even in its natural habitat when the sun is at large angles. Despite the fact that UV-B accounts for a very small fraction of solar radiation, it has a considerable effect on photosynthesis while UV-A seems to contribute only a small amount to photoinhibition in C. prolifera.

Abstract— Photoinhibition of photosynthesis was investigated in the Mediterranean green alga Halimeda tuna measuring pulse amplitude modulation (PAM) chlorophyll fluorescence and oxygen evolution in situ under solar radiation. Exposure to solar radiation at the surface caused a drastic decline in the photosynthetic quantum yield accompanied by a decline in the photochemical quenching, while the nonphotochemical quenching dramatically increased. The algae recovered from these effects within a few hours indicating that these are mainly due to reversible photoinhibition and only to a smaller extent to nonreversible photodamage. Individuals harvested from deeper waters were more affected than those from shallower waters. Photoinhibition occurs in this alga even in its natural habitat when the sun is at high angles as shown by measuring the fluorescence parameters at hourly intervals during the course of the day. Photoinhibition was less pronounced when the short wavelength band was increasingly removed from solar radiation using cut‐off filters. After exposure of thalli to solar radiation at the water surface, oxygen production decreased drastically within 30 min; this inhibition was even more pronounced in algae harvested from deeper waters. Oxygen measurements at different depths showed optimal photosynthesis at a depth of 1 m. Also for photosynthetic oxygen production inhibited by high solar irradiance at least partial recovery could be observed within several hours. Despite the fact that UVB accounts for a very small fraction of solar radiation, it has a considerable effect on photosynthesis, whereas UVA seems to contribute only little to photoinhibition in H. tuna.

: Photoinhibition of photosynthesis was measured in two Mediterranean Corallinaceae, Jania rubens and Corallina mediterranea, using pulse-amplitude modulation (PAM) fluorescence and oxygen production on site. Both algae were found to be adapted to low irradiances of solar radiation and easily inhibited by exposure to excessive radiation. Both algae were impaired even in their natural habitat under overhanging rocks which protected them from direct solar radiation, except for a few hours in the early morning. Recovery from photoinhibition of both the photosynthetic quantum yield, defined as Fv′/Fm′, and oxygen production took several hours and was not complete. Judging from both parameters indicated above, Jania seems to be even more sensitive than Corallina, even though the former alga was found in more exposed habitats.

The effects of solar radiation on photosynthetic oxygen production and pulse amplitude modulated (PAM) fluorescence were measured in the marine brown macroalga Padina pavonia harvested from different depths from the Greek coast near Korinth. In fluence rate-response curves the light compensation point for photosynthetic oxygen production increased and the saturation level decreased with increasing exposure time to solar radiation. Cutting off the UV-B wavelength range (280–315 nm) from solar radiation reduced the inhibition of photosynthesis, and the organisms were less affected when all of the UV radiation was filtered out. Algae collected from 7 m depth were much more prone to photoinhibition than those harvested from rock pools exposed to unfiltered solar radiation. During continuous exposure to solar radiation, rock pool algae showed photoinhibition after longer periods of time than specimens from 7 m or from dark adapted habitats. When subjected to unfiltered solar radiation the ratio of the variable fluorescence to the maximal fluorescence FvFm (Fv = Fm − Fo) rapidly declined with increasing exposure time. However, again algae from 7 m depth were more prone to photoinhibition than rock pool algae. The differences between the two ecological strains were less obvious when UV-B or total UV was removed from solar radiation. Only in the latter case a complete recovery was observed after 2 h while, when exposed to unifiltered sunlight, only the rock pool algae recovered completely within that time.

Photoinhibition of photosynthesis was investigated in the marine macroalga Ulva laetevirens by measuring chlorophyll fluorescence (PAM) and oxygen evolution. After exposure to unfiltered solar radiation the ratio of the variable fluorescence to the maximal fluorescence F-v/F-m (F-v = F-m-F-O) as well as the oxygen production declined rapidly with increasing duration of the exposure. Fluence rate-response curves for the oxygen production showed a decrease in the slope with exposure time and a shift of the compensation point to higher fluence rates as well as a decrease in the photosynthetic oxygen production. Solar radiation deprived of most of the W-B (long pass above 320 nm) induced a similar degree of photoinhibition of the ratio F-v/F-m as unfiltered solar radiation, whereas the oxygen production was less affected. The photoinhibitory effect of solar radiation was diminished when the solar radiation was filtered by a filter which passes wavelengths above 400 nm. Even though UV-B accounts for a very small fraction of solar radiation, it has a considerable effect on photosynthesis. In addition, UV-A and visible radiation contribute to photoinhibition in Ulva.

Fast chloroplast orientation in Mesotaenium from profile position to face position cannot be induced by either red (R) or blue (BL) light (in contrast to Mougeotia). Rather interaction of light signals mediated by phytochrome and blue-light photoreceptor(s) is essential for the response. If both light treatments are separated in time, the irradiation sequence R-BL is much more effective than BL-R, i.e. a gradient of the far-red (FR)-absorbing form of phytochrome (Pfr) renders following BL highly effective, but BL cannot increase the responsiveness to following Pfr. The memory of a R irradiation before BL holds only for some minutes, indicating that the physiological activity of Mesotaenium-Pfr and its photoproducts is very short-lived. This transient signal mediated by Pfr can be transformed to a more stable internal signal by interaction with BL. The interaction process does not occur at the level of photoperception. Rather, early products of the phytochrome-initiated signal transduction chain interact with excited cryptochrome or an early product of it; Pfr can be removed by FR before the onset of BL. The internal signal stores the directional information of the Pfr-gradient, so that BL is now fully effective and induces chloroplast movement.