Contribution of the UV component of natural sunlight to photoinhibition of photosynthesis in six species of subtidal brown and red seaweeds.

Field-collected specimens of three species of Laminaria and three species of subtidal red algae (Delesseria sanguinea, Plocamium cartilagineum and Phyllophora pseudoceranoides) were exposed to natural summer sunlight on Helgoland (southern North Sea) for up to 4 h at 15 °C. Dark-adapted variable fluorescence (Fv : Fm) was measured immediately after these treatments, and following 6, 24 and 48 h of recovery in moderate irradiances of white light. The response of plants to the full spectrum of natural sunlight was compared with that to PAR alone, UV-A + visible, UV-A + UV-B, or UV-A alone. The Fv : Fm values of all species were reduced to minimal values after 4 h in all of these treatments, but those of the more resistant species (Laminaria spp. and P. pseudoceranoides) were higher after shorter exposures to UV radiation alone than to PAR with or without UV. The recovery of Fv : Fm in all species was also more rapid in the two treatments that contained UV radiation alone than in those that included PAR. These results suggest that it is the high irradiances of PAR in natural sunlight which are responsible for the photoinhibition of photosynthesis of subtidal seaweeds and that the current ambient irradiances of UV radiation (either UV-B or UV-A) in northern temperate latitudes would not contribute significantly to this photoinhibition.

Seasonal variations in nitrate reductase activity and internal N pools in intertidal brown algae are correlated with ambient nitrate concentrations.

Nitrogen metabolism was examined in the intertidal seaweeds Fucus vesiculosus, Fucus serratus, Fucus spiralis and Laminaria digitata in a temperate Irish sea lough. Internal NO3- storage, total N content and nitrate reductase activity (NRA) were most affected by ambient NO3-, with highest values in winter, when ambient NO3- was maximum, and declined with NO3- during summer. In all species, NRA was six times higher in winter than in summer, and was markedly higher in Fucus species (e.g. 256 ± 33 nmol NO3- min1 g1 in F. vesiculosus versus 55 ± 17 nmol NO3- min1 g1 in L. digitata). Temperature and light were less important factors for N metabolism, but influenced in situ photosynthesis and respiration rates. NO3- assimilating capacity (calculated from NRA) exceeded N demand (calculated from net photosynthesis rates and C : N ratios) by a factor of 0.7–50.0, yet seaweeds stored significant NO3- (up to 40–86 µmol g1). C : N ratio also increased with height in the intertidal zone (lowest in L. digitata and highest in F. spiralis), indicating that tidal emersion also significantly constrained N metabolism. These results suggest that, in contrast to the tight relationship between N and C metabolism in many microalgae, N and C metabolism could be uncoupled in marine macroalgae, which might be an important adaptation to the intertidal environment.

Stress resistance and disease resistance in seaweeds: The role of reactive oxygen metabolism

It has become clear over the last 15-20 years that the immediate effect of a wide range of environmental stresses, and of infection, on vascular plants is to increase the formation of reactive oxygen species (ROS) and to impose oxidative stress on the cells. Since 1994, sufficient examples of similar responses in a broad range of marine macroalgae have been described to show that reactive oxygen metabolism also underlies the mechanisms by which seaweeds respond (and become resistant) to stress and infection. Desiccation, freezing, low temperatures, high light, ultraviolet radiation, and heavy metals all tend to result in a gradual and continued buildup of ROS because photosynthesis is inhibited and excess energy results in the formation of singlet oxygen. The response to other stresses (infection or oligosaccharides which signal that infection is occurring, mechanical stress, hyperosmotic shock) is quite different-a more rapid and intense, but short-lived production of ROS, described as an