Impact of natural ultraviolet radiation on rates of photosynthesis and on specific marine phytoplankton species

Abstract

Natural phytoplankton populations from both Antarctic and tropical waters were exposed to solar radiation to determine the effects of ultraviolet radiation (UVR) on rates of photosynthesis. Radiation in the UV-A region (320 to 400 nm) was responsible for over 50 % of the total inhibition due to UVR, with less than 50 % due to UV-B (280 to 320 nm). Wavelengths c305 nm, which is the spectral region most enhanced under conditions of low ozone concentrations in the atmosphere, accounted for only 15 to 20% of the total inhibition due to UV-B radiation. Under high-light conditions on sunny days, photosynthetic rates were increased 200 to 300 % by screening off all UVR below 378 nm. When the average UVR (295 to 385 nm) during the incubation period was below a threshold value of 5 to 10 W m-', there was no significant depression of photosynthetic rates. Microscopic examinations of a phytoplankton population which was allowed to grow for 5 d under high solar irradiance indicated that UVR is more inhibitory to microplankton than to nanoplankton, and that it induced the formation of resting spores in diatom species of the genus Chaetoceros. Phytoplankton from tropical waters showed marked resistance to UVR as compared to Antarctic phytoplankton. Under the highest light conditions an increase in photosynthetic rates of 10 to 20 % was recorded in some experiments by screening off all UVR c 3 7 8 nm, but when data from all experiments with samples from surface waters in the tropics were analyzed, there was no significant difference between samples with or without natural solar UVR. Phytoplankton samples from below the pycnocline in tropical waters, however, were very sensitive to solar radiation, suggesting that the resistance shown by phytoplankton from surface waters reflects a photoadaptive process.

Effects of solar ultraviolet radiation on photosynthesis of the marine red tide alga Heterosigma akashiwo (Raphidophyceae)

“…Previous studies [7,18,[40][41][42][43] have shown that solar UVR can significantly inhibit photosynthesis of aquatic organisms in short time scales (i.e., less than a day). Nevertheless, cells can have different mechanisms to cope with the excessive solar radiation energy in the short-term scale; for example, photoprotection has been shown to be very effective in many algae (e.g.…”

Sensitivity of intertidal and subtidal red algae to UVA and UVB radiation, as monitored by Chlorophyll fluorescence measurements: Influence of collection depth and season, and length of irradiation

“…However, most action spectra of the inhibitory effects of UV radiation on photosynthesis indicate that UVA wavelengths are effective (e.g. Jones & Kok, 1966;Cullen eta]., 1992; Helbling et aI., 1992) and, when allowance is made for the higher irradiances at UVA than at UVB wavelengths, account for observations that more than 50% of the inhibition of photosynthesis in phytoplankton caused by solar radiation is due to UVA (Helbling et al, 1992). A recent study of the effects of sunlight on photosynthesis and chlorophyll fluorescence in the green macroalga Ulva (Herrrnann et al, I995) also indicated that the removal of UVB wavelengths had a relatively small effect on the degree of photoinhibition, so that the results reported here are not atypical.…”

“…A recent study of the effects of sunlight on photosynthesis and chlorophyll fluorescence in the green macroalga Ulva (Herrrnann et al, I995) also indicated that the removal of UVB wavelengths had a relatively small effect on the degree of photoinhibition, so that the results reported here are not atypical. The ecological significance of the relatively high effectiveness of UVA wavelengths is that thinning of the ozone layer will have little effect on these responses because the irradiance of wavelengths longer than 330 nm is almost independent of ozone concentration (Helbling et al, 1992).…”

“…Between two and four of these lamps were supplemented by one to three white fluorescent lamps (Osram-L 65W/25S) to provide maximal irradiances of I-4 Wm -z of UVB (280-320nm), 15 W m -2 of UVA (320-400 nm) and 20-25/,mol m ~ s -* of photosynthetically active radiation (PAR) at a distance of about 20 cm below the lamps (precise values for each experiment are given in the figure legends). Lower irradiances in each waveband were obtained by reducing ~he number of lamps and increasing the distance between the lamps and the samples, and UVB was removed completely in some treatments by covering the culture dishes with Mylar film (0"1 mm thick; absorption spectrum given by Helbling et al, 1992). UV irradiance was measured with UVA and UVB cosine sensors (Dr Gr6bel UV-Elektronik, Ettlingen, Germany), which were calibrated in each experimental light field against a spectroradiometer (LI-I800UW; LiCor Instruments, Lincoln, NE).…”

“…The detailed work on the effects of UV radiation on marine organisms and ecosystems, which was stimulated by this discovery, has concentrated on the Southern Ocean (e.g. Smith et aI., I992;Helbling et al, 1992), but recent reports that last winter's (i995-6) hole in the ozone layer over the Arctic was the 'deepest ever seen in the Northern Hemisphere' (Pearce, 1996) have underlined the need for comparable studies in northern seas and oceans. A review of UVB (280-320 nrn) effects on Nordic marine ecosystems (W~ingberg ef at., 1996) identified critical differences between the environmental conditions for planktonic organisms in the northern and southern cool-temperate zones that could affect their response to increased UV radiation in the field, and concluded that more basic work on northern ecosystems was needed.…”