Influence of UV radiation on the photosynthesis of arctic macroalgae in the field

Abstract

The photoinhibition in brown and red macroalgae from Spitsbergen (79°N, 12°E) was investigated to study the effects of UV under balanced UV /photosynthetically active radiation (PAR) conditions. Algae were collected from different depths and exposed to natural solar radiation. Various parts of the UV radiation spectrum were successively cut off by filters absorbing wavelengths of less than 295 nm, less than 320 nm and less than 400 nm. The samples were covered with these filters and exposed to daylight 10 cm below the water surface in outdoor tanks near the shore supplied with running seawater. PAR and UV radiation were continuously measured during the experiments. The degree of photoinhibition was observed during the course of the day by measuring the in vivo fluorescence of photosystem II ( F v F m ). In addition, the transmittance of the water body of the Kongsfjord (Spitsbergen, Norway) related to the UV and PAR wavelength ranges was determined. Even on a sunny day in August, only relatively low fluence rates of UVA (approximately 13 W m −2), UVB (approximately 0.14 W m −2) and PAR (approximately 1100 μmol m −2 s −1 ) were measured in the air at noon due to the low position of the sun at this high latitude. The UV transmittance of the water body in the fjord was also low. As a result, UV stress occurred only in seaweeds growing in the intertidal zone or in shallow water. The photoinhibition of photosynthesis was mainly induced by white light in shallow water. The inhibitory effects of UV radiation caused a delay in the recovery processes in the afternoon and evening, rather than an inhibitory effect on photosynthesis in the morning. Fucus distichus, growing in the upper intertidal zone, was most insensitive to UV radiation. Algae from the sublittoral zone had problems coping with the natural UV radiation in shallow water, whereas those from the intertidal zone were acclimatized to the unfavourable UV /PAR conditions. By cutting off sequentially the shorter wavelengths of the UV range, the investigated brown algae showed no significantly different effects. In contrast, the red alga Palmaria palmata showed a clear response to the different UV ranges, i.e. the UVB wavelength range was very effective in causing photoinhibition. In addition, the recovery phase was delayed in spite of the low fluence rates impinging on the alga during the course of the day.