2011, Gruber, Ansgar, Roleda, Michael Y., Bartsch, Inka, Hanelt, Dieter, Wiencke, Christian

To study the effect of different radiation conditions on sporogenesis of Laminaria digitata (Huds.) J. V. Lamour., excised disks were induced to form sporangia under PAR (P), PAR + ultraviolet-A (UVA) (PA), and PAR + UVA + ultraviolet-B (UVB) (PAB) conditions in the laboratory. Vitality of meiospores, released from sori induced under different radiation conditions in the laboratory and from sori of wild sporophytes acclimated to in situ solar radiation in the presence and absence of ultraviolet radiation (UVR), was measured in terms of their germination capacity. Sorus induction in disks of laboratory-grown sporophytes was not hampered under light supplemented with UVR, and sorus area was not significantly different among P, PA, and PAB. Vitality and germination rate of meiospores released from sori induced under different radiation treatments was comparable. Likewise, screening of UVR of the natural solar radiation did not promote higher germination rates of meiospores released from wild sporophytes. Germination rates were, however, higher in meiospores released from laboratory-induced sori compared to sori of wild sporophytes. Higher DNA damage (formation of cyclobutane pyrimidine dimers, CPDs) was observed in laboratory-grown nonsorus compared to sorus tissue, while CPDs were nondetectable in both sorus and nonsorus tissue of wild sporophytes. To explain the apparent protection of developing meiospores and the unexpected UV resistance of soral tissue, concurrent anatomical investigations of sporogenic tissue were performed. We observed the previously unreported existence of two types of sterile paraphysis cells. One type of paraphysis cells, the most frequent type, contained several red-fluorescing plastids. The other type, less frequently occurring, was completely filled with substances emitting blue fluorescence under violet excitation, presumably brown algal phenolic compounds (phlorotannins). Cells of this type were irregularly scattered within the sorus and did not contain red-fluorescing plastids. Meiospore-containing sporangia were positioned embedded between both types of paraphysis cells. In vegetative tissue, blue autofluorescence was observed only in injured parts of the blade. Results of our study suggest that the sorus structure with phlorotannins localized in the specialized paraphysis cells may be able to screen harmful UVR and protect UV-sensitive meiospores inside the sporangia.

2004, Gruber, Ansgar

Effects of ultraviolet radiation on the formation and the germination of zoospores have been investigated in Laminaria digitata. Excised phylloid discs were exposed to controlled ultraviolet radiation of different qualities and intensities in the laboratory. Both sides of the discs were irradiated alternately. Rate of sorus formation, sorus area, zoospore release, amount of cyclobutane-pyrimidine dimers in DNA and germination rate of the released zoospores were determined. In order to determine the relevance of ultraviolet radiation for L.digitata, field measurements of irradiance were collected. The absorption characteristics of zoospore suspensions of the three Helgoland species of Laminaria were investigated. The investigations showed that for phylloid discs irradiated on both sides, ultraviolet radiation has no effect on sorus formation and sorus area. The germination rate of the zoospores was not negatively influenced by ultraviolet radiation during the formation of the sorus. This suggests an effective protection of the sporogenic tissue from ultraviolet radiation. DNA damage was shown in UVB irradiated phylloid discs. The DNA damage was higher in the non-fertile regions of the phylloid discs than in the sorus tissue. Therefore, additional UVB protection mechanisms are suspected. In the field, UVB intensities capable of damaging zoospores were found. In L. digitata, both the zoospores and exudates contribute to the UV absorption of zoospore suspensions. The proportion of the total absorption caused by exudates is higher in L. saccharina and L. hyperborea. In these species, the zoospores themselves show hardly any absorption. Possibly, a significant proportion of exudation takes place in the sorus tissue. All results indicate that L. digitata has adaptive answers to ultraviolet radiation, which counteract the detrimental effects of ultraviolet radiation. This diploma thesis was carried out at the Biologische Anstalt Helgoland. 1. Referee (Advisor): Professor Dr. P. Kroth, University of Konstanz; 2. Referee: Professor Dr. C. Wiencke, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven.

2006, Wiencke, Christian, Roleda, Michael Y., Gruber, Ansgar, Clayton, Margaret N., Bischof, Kai

1. The UV susceptibility of zoospores of the brown seaweeds Saccorhiza dermato- dea, Alaria esculenta and Laminaria digitata (Laminariales) was determined in field experiments in June 2004 on Spitsbergen (78° 55' N, 11° 56' E).
2. Freshly released zoospores were exposed for one or two days at various water depths to ambient solar radiation, ambient solar radiation depleted of UVB radiation (UVBR) and ambient solar radiation depleted of both UVBR and UVAR. Subsequently, germination rates were determined after exposure to favourable light and temperature conditions in the laboratory.
3. The radiation regime was monitored at the water surface and in the water adjacent to the exposure platforms for the duration of the field exposure.
4. Under ambient solar radiation the tolerance of zoospores to UVR was highest in the shallow water species S. dermatodea, intermediate in the upper to mid sublittoral A. esculenta and lowest in the upper to mid sublittoral L. digitata. There was, however, no difference in the susceptibility of the zoospores to ambient solar radiation or to solar radiation depleted of UVBR.
5. The water body was relatively UV transparent especially in the upper water layers. The 1 % UVB depth mostly varied between 5.35 and 6.87 m. On one stormy day the 1 % UVB depth was only 3.57 m indicating resuspension of sediments.
6. We propose that as the zoospores are the developmental stages most susceptible to UVR their UVR tolerance is a major if not the most important factor for the determination of the upper depth distribution limit of these species on the shore. The results are discussed with respect to enhanced UVBR due to stratospheric ozone depletion.

2005, Roleda, Michael Y., Wiencke, Christian, Hanelt, Dieter, van de Poll, Willem H., Gruber, Ansgar

Depth distribution of kelp species in Helgoland (North Sea) is characterized by occurrence of Laminaria digitata in the upper sublittoral, whereas L. saccharina and L. hyperborea dominate the mid and lower sublittoral region. Laminaria digitata is fertile in summer whereas both other species are fertile in autumn/winter. To determine the light sensitivity of the propagules, zoospores of L. digitata, L. saccharina and L. hyperborea were exposed in the laboratory to different exposure times of photosynthetically active radiation (PAR; 400 700 nm), PAR + UVA radiation (UVAR; 320 400 nm) and PAR + UVAR + UVB radiation (UVBR; 280 320 nm). Optimum quantum yield of PSII and DNA damage were measured after exposure. Subsequently, recovery of photosynthetic efficiency and DNA damage repair, as well as germination rate were measured after 2 and 3 d cultivation in dim white light. Photosynthetic efficiency of all species was photoinhibited already at 20 µmol photons m−2 s−1 PAR, whereas UV radiation (UVR) had a significant additional effect on photoinhibition. Recovery of the PSII function was observed in all species but not in spores exposed to irradiation longer than 4 h of PAR + UVA + UVB and 8 h of PAR + UVA. The amount of UVB-induced DNA damage measured as cyclobutane pyrimidine dimers (CPDs) increased with exposure time and highest damage was detected in the spores of lower subtidal L. hyperborea relative to the other two species. Significant removal of CPDs indicating repair of DNA damage was observed in all species after 2 d in low white light especially in the spores of upper subtidal L. digitata. Therefore, efficient DNA damage repair and recovery of PSII damage contributed to the germination success but not in spores exposed to 16 h of UVBR. UV absorption of zoospore suspension in L. digitata is based both on the absorption by the zoospores itself as well as by exudates in the medium. In contrast, the absorption of the zoospore suspension in L. saccharina and L. hyperborea is based predominantly on the absorption by the exudates in the medium. This study indicates that UVR sensitivity of zoospores is related to the seasonal zoospore production as well as the vertical distribution pattern of the large sporophytes.