Exposure to ultraviolet radiation delays photosynthetic recovery in Arctic kelp zoospores

Abstract

Seasonal reproduction in some Arctic Laminariales coincides with increased UV-B radiation due to stratospheric ozone depletion and relatively high water temperatures during polar spring. To find out the capacity to cope with different spectral irradiance, the kinetics of photosynthetic recovery was investigated in zoospores of four Arctic species of the order Laminariales, the kelps Saccorhiza dermatodea, Alaria esculenta, Laminaria digitata, and Laminaria saccharina. The physiology of light harvesting, changes in photosynthetic efficiency and kinetics of photosynthetic recovery were measured by in vivo fluorescence changes of Photosystem II (PSII). Saturation irradiance of freshly released spores showed minimal I k values (photon fluence rate where initial slope intersects horizontal asymptote of the curve) values ranging from 13 to 18 lmol photons m )2 s )1 among species collected at different depths, confirming that spores are low-light adapted. Exposure to different radiation spectra consisting of photosynthetically active radiation (PAR; 400-700 nm), PAR+UV-A radiation (UV-A; 320-400 nm), and PAR+ UV-A+UV-B radiation (UV-B; 280-320 nm) showed that the cumulative effects of increasing PAR fluence and the additional effect of UV-A and UV-B radiations on photoinhibition of photosynthesis are species specific. After long exposures, Laminaria saccharina was more sensitive to the different light treatments than the other three species investigated. Kinetics of recovery in zoospores showed a fast phase in S. dermatodea, which indicates a reduction of the photoprotective process while a slow phase in L. saccharina indicates recovery from severe photodamage. This first attempt to study photoinhibition and kinetics of recovery in zoospores showed that zoospores are the stage in the life history of seaweeds most susceptible to light stress and that ultraviolet radiation (UVR) effectively delays photosynthetic recovery. The viability of spores is important on the recruitment of the gametophytic and sporophytic life stages. The impact of UVR on the zoospores is related to the vertical depth distribution of the large sporophytes in the field.

Photosynthetic response of Arctic kelp zoospores exposed to radiation and thermal stress

“…22, 64 Kelp zoospores have a transitory planktonic phase. Considering their small size and the viscosity of water, 65 they may be able to vertically swim at a speed of 120 mm s -1 covering a distance of 1 m in 2 h. 22 Therefore, zoospores released from a 2-4 m tall kelp sporophyte should be able to touch-down and settle into a low-light environment under algal canopies within 8 h. Propagules are also released as a plume of spore cloud. Zoospores with phlorotannin-containing physodes could effectively buffer each other acting as a UV-biofilter.…”

“…22 Immediately after adjustment of spore density, the suspension was filled into the 5 ml quartz cuvettes and the optimum quantum yield (F v /F m ) was measured inside the Emitter-Detector Unit at time zero (n = 5). After 3 min of dark incubation, F 0 was measured with a red measuring light pulse (~0.3 mmol photons m -2 s -1 , 650 nm), and F m was determined with a 800 ms completely saturating red light pulse (~2750 mmol photons m -2 s -1 , 650 nm).…”

“…20 UV contributes to further photodamage by inactivation of the oxygenevolving complex and photochemical reaction centers of the PSII 21 which delays photosynthetic recovery in Arctic kelp zoospores. 22 The photophosphorylation and electron transport enzymes as well as plastoquinone diffusion enzyme are temperature dependent. Variation in temperature can, therefore, affect the light harvesting efficiency (a, the slope of the initial light-limited region of the photosynthesis-irradiance curve) of phototrophs.…”

Physiological, biochemical, and ultrastructural responses of the green macroalga Urospora penicilliformis from Arctic Spitsbergen to UV radiation

“…Curve fit was calculated with the Solver Module of MS-Excel using the least squares method comparing differences between measured and calculated data (Roleda et al 2006). The saturation irradiance for electron transport (E k ) was calculated as the light intensity at which the initial slope of the curve (α) intercepts the horizontal asymptote (rETR max ).…”

Ultraviolet radiation shapes seaweed communities

“…The first attempt to study kinetics of photoinhibition and recovery in zoospores of Arctic Laminariales showed that zoospores of the lower sublittoral L. saccharina were more sensitive to PAR-and UV-induced photoinhibition than upper-to midsublittoral S. dermatodea, A. esculenta and L. digitata. Kinetics of recovery in zoospores showed a fast phase in S. dermatodea which indicates a reduction of the photoprotective process while a slow phase in L. saccharina indicates recovery from severe photodamage (Roleda et al 2006d). These experiments were focussed on short-term effects, thus the preadaptive setting of species could be revealed, but not the respective potential of acclimation, also determined by the genetic features of species.…”