Fertile Saccharina latissima sporophytes, collected in the Kongsfjorden, Ny-Ålesund, Spitsbergen, Norway (78°56.87′ N, 11°51.64′ E) were investigated in relation to its sensitivity to experimentally enhanced ultraviolet radiation : photosynthetically active radiation (UVR : PAR) ratios. Irradiance of UVR were 4.30 W m−2 of UV-A (320–400 nm) and 0.40 W m−2 of UV-B (280–320 nm), and PAR (400–700 nm) was ∼4.30 W m−2 (=20 µmol photons m−2 s−1). Excised soral (sporogenic) and non-soral (vegetative) tissues were separately irradiated for 16 h at 7°C. Transmission electron microscopy showed abundant occurrence of physodes, electron dense particles (∼300–600 nm) in the sorus. Paraphysis cells, with partly crystalline content, large mitochondria and abundant golgi bodies were towering over the sporangia. In soral tissue, cells were not visibly altered by the PAR + UVR irradiation. The chloroplasts, flagella and nucleus of unreleased meiospores inside the sporangial parent cells were visibly intact. Severe changes in the chloroplast structure of vegetative tissue occurred after PAR + UVR irradiation. These changes included wrinkling and dilatation of the thylakoid membranes, and appearance of electron translucent areas inside the chloroplasts. In vegetative cells exposed to PAR + UVR, the total amount of physodes, was slightly higher as in cells exposed to PAR only. Initial values of optimum quantum yield of photosystem II (Fv/Fm) were 0.743 ± 0.04 in non-soral and 0.633 ± 0.04 in soral tissue. Vegetative tissue was observed to be more sensitive to radiant exposure of PAR and PAR + UVR compared to reproductive tissue. Under PAR, a 20% reduction in Fv/Fm was observed in non-soral compared to no reduction in soral tissue, whereas under PAR + UVR, 60% and 33% reduction in Fv/Fm was observed in non-soral and soral tissues, respectively. This can be attributed to the corresponding three times higher antiradical power (ARP) capacity in soral compared to non-soral tissue.