Other photosynthesisrelated effects

There are a number of UVR effects that are closely related to the photosynthetic performance of aquatic primary producers. These effects are due to the couplings between radiation - especially UVR - and a number of morphological and biochemical factors within the cell [200]. Thus, for example, radiation-induced changes in nutrient uptake, synthesis and allocation of metabolic compounds, motility/orientation, and cell morphology will result in variations in photosynthetic rates. In the following paragraphs we will outline some of these effects -nutrient incorporation and enzyme activities related to carbon and nitrogen metabolism, accumulation or damage on pigments. Specific effects, such as DNA damage, which may induce a reduction in growth rates [195], and hence affect overall primary productivity, are addressed in Chapter 9.

11.8.1 Nutrient incorporation/assimilation and enzyme activities

Growth of aquatic autotrophic organisms is dependent not only on carbon assimilation, but also on the incorporation and assimilation of nitrogen, phosphate, sulfur and several micronutrients [201]. In general, it is considered that UVR - especially UV-B - is an inhibitor of uptake processes (especially nitrogenous), whereas UV-A stimulates or exerts no significant effects on the uptake of these ions [170,171]. In particular, studies carried out with phytoplanktonic organisms have demonstrated that nitrate and ammonium uptakes are affected by UVR [202-204]. Furthermore, Dohler [170,171,203,205], working with several Antarctic and North Sea phytoplanktonic species, has pointed out the diversity of responses among the organisms tested. Thus, samples dominated by the prymnesiophyte Pheaocystis pouchetii were very sensitive to UV-B doses (in terms of 15N-ammonium uptake), and so were those containing Ceratium sp., Coscinodiscus sp. and Noctiluca sp. 15N-nitrate uptake was not or only slightly affected by UV-B irradiances [203]. On the other hand, in experiments conducted with North Sea natural phytoplankton populations, Dohler and Hagmeier [206] found that UV-A radiation stimulated 15N-ammonium uptake. Fewer studies have addressed the effects of UVR on P-uptake of phytoplanktonic cells. Hessen et al. [176], working with the chlorophyte Chlamydomonas reinhardtii found a stimulation under low UV-B doses (< 3.6 kJ m~2 at 312 nm), but higher inhibition when UV-B doses were higher. In addition, studies on UV-B effects carried out in both sandy and muddy sediments have suggested that the nutrient availability may be an important factor for the susceptibility of MPB communities to UV-B exposure [47]. Wulff et al. [68] designed an experiment to test this hypothesis, and showed that the availability of nutrients indeed can act to mitigate the effects of UV-B on a microbenthic community on a sandy substratum.

Some studies have also addressed the UVR effects on nutrient incorporation in marine macroalgae [52,90]. In particular, these studies have focused on UVR effects upon carbonic anhydrase (CA) and nitrate reductase (NR) activities [32,192,207-210]. These are important enzymes involved in the incorporation of carbon and nitrogen within the cell [211], thus any stress factor that affects them will ultimately influence photosynthesis. Studies carried out with algae collected from southern Spain [32,192,208,210] found daily variations (i.e., circadian rhythms) in NR and CA activities. In Dasycladus vermicularis it was found that these variations were antagonistic during the onset of solar radiation, although these changes only partially matched those of photosynthesis [192], suggesting that these processes are affected differentially by UVR. In long-term studies, it has been shown that UV-A radiation stimulated NR activity, and UV-B decreased both nitrate uptake and NR activities [209,212]. On the other hand, UV-B radiation seems to stimulate CA activity in eulittoral algae but not in subtidal [13,209]. In addition, experiments were conducted to determine the effects of UVR on the activity of Calvin cycle enzymes, such as ribulose-1,5-biphosphate carboxylase/oxygenase (RUBISCO) and glyceraldehyde-3-phos-phate dehydrogenase (G3PDH), and in Arctic macroalgae it was found that the photosynthetic activity decreased due to the negative effects of UVR upon these enzymes [213].

11.8.2 Pigments

Several researchers have reported the decrease of photosynthetic pigments due to exposure to UVR [214-216]. This reduction can be due to a combination of factors, such as the inhibition of de novo synthesis and the natural turnover of pigments, or directly to photobleaching [216]. Bleaching can occur not only because of UVR, but also due to exposure to high PAR intensities [216]; it is species-specific and also depends on the spectral characteristics of the radiation treatments imposed to the cells. Helbling et al. [214], working with several marine phyto-plankton cultures, found that Nannochloris oculata (Eustig-matophyceae) had a decrease in chl-a content of 30, 60 and 80% under PAR only, PAR + UV-A, and PAR + UVR, respectively, after being exposed for 4.5 h to solar radiation. The prymnesiophyte Isochrysis galbana, on the other hand, did not experience significant changes in chl-a content (for the same radiation treatments) even after 7 h of exposure. Other experiments have also demonstrated the differential sensitivity to UVR of various pigments [217], with the phycobiliproteins being especially sensitive to these wavelengths [218],

Absolute amounts of photosynthetic pigments, commonly used as an estimator of growth in autotrophic organisms, seem to be also affected by UVR. During a long-term experiment, Helbling et al. [43] simulated ozone depletion events by moving Antarctic phytoplankton towards the Equator, so that the samples were exposed not only to increased levels of UVR, but also to natural changes in the relative proportions of UV-B and UV-A. They found a decrease in the growth rate of Antarctic phytoplankton exposed to UVR as compared to that exposed to the PAR-only treatment. However, growth rates were not significantly different when the samples were incubated under UVR levels similar to those found at their sampling site in the Antarctic. Data on long-term experiments conducted in both polar areas [137] showed that even though photosynthesis was initially affected by UVR (day 1), growth rates, evaluated either as carbon fixation or chl-a content, did not show any significant differences. In general, studies have demonstrated that different growth responses due to UVR exposure occur not only among taxa [198,219], but also within the same genus. For example, in the chlorophyte Dunaliella salina growth rate was not affected by UVR, whereas in D. tertiolecta it was significantly reduced after 3 days of exposure [198].

The differential sensitivity of pigments to UVR has also been studied for MPB organisms. Phycobilins of cyanobacterial mats appear to be more sensitive than chlorophylls and carotenoids, the latter often increasing at UV-B exposure [56,57,217]. However, in experiments conducted on intact sediment communities dominated by diatoms, no changes in pigment composition (expressed as ratios to chl-a) were observed [29,46,47,64,68], with one exception: higher caro-tenoid content was observed at enhanced UV-B levels in a Gyrosigma mat, probably reflecting a UV-B-protecting strategy [65].

In marine macroalgae, various responses were also found when addressing the effects of UVR upon various pigments. Exposure of Porphyra umbilicalis to artificial UVR levels decreased chl-a and phycocyanin concentrations by 65 and 67%, respectively, whereas carotenoids and phycoerythrin decreased by as much as 75 and 82%, respectively [220]. Furthermore, and under ambient levels, UVR not only decreased the concentration of chl-a and biliproteins in the red alga Porphyra leucosticta, but the pattern of daily variation was also affected [30]. The damage of photosynthetic pigments by UVR in P. leucosticta was suggested to be the cause of a decrease in photosynthetic rates. However, in Macrocystis pyrifera, it was found that the main light-harvesting complex of this alga, the fucoxanthin-chlorophyll protein, was the specific site for UV damage [158]. Finally, in Ulva rigida [221] and Dasycladus vermicularis [191] the content of chlorophyll and carotenoids was significantly higher in the presence of UV-B

than that in the control (PAR only), suggesting the presence of an efficient protective-pigment mechanism.

11.8.3 Cell morphology and size

When evaluating the photosynthetic responses to UVR of diverse organisms, some studies have revealed the importance of cell size [43,81,82,112,177]. In phytoplanktonic organisms, it was found that, although there is certainly variability in responses, small cells - with a relatively high surface to volume ratio -are more resistant to photosynthesis inhibition but more vulnerable to DNA damage [81,82,195]. On the other hand, and provided that microplanktonic cells (20-200 fim) do not have high concentrations of UV-absorbing compounds, they are more vulnerable to UVR (in terms of photosynthesis). This has been demonstrated in a comparative study carried out in the Andean lakes [82], where it was found that larger phytoplanktonic cells had a higher kinetics of inhibition and hence were more affected by UVR than smaller cells. For MPB organisms, on the other hand, there are contradictory findings as to whether UV-B-related changes in species composition are related to cell size or are due to taxon-specific sensitivity [82,119,222]. As seen for planktonic algae, increasing size may occur both on an individual species level, as cell division is hampered [177,197], and on community level, as species with larger cell-size could be favoured [116,222]. For MPB there is some indication for the latter, but not for the former. Bothwell et al. [58] found that large, stalked diatom species increased their dominance at UV-B exposure during periphyton succession, and Vinebrook and Leavitt [121] found that the growth of the small-sized diatom Achanthes minutissima was suppressed under UV-B exposure.

Besides size, the morphology also seems to influence the response of algae to solar radiation. This has been shown particularly for macroalgae. A comparison between the red algae Porphyra leucosticta and Rissoella verruculosa, which have comparable zonation patterns at intertidal sites, shows the different photo-protective strategies of these algae [30,32]. This is probably related to different absorption properties because of the thallus thickness and pigment composition. P. leucosticta has a thin thallus consisting of one cell layer in which light transmits rapidly and homogenously towards the harvesting complexes, whereas in R. verruculosa, which has a more complex structure, some scattering of photons through the multilayered thallus (self-shading) may take place. This was evident when the algae were exposed to full solar radiation, and in Porphyra UVR accounted for about 30% of the total photoinhibition, whereas no effects were observed in Rissoella. In addition, some studies hint about the importance different life stages, which are closely related with size. Although studies have focused on the macrothallus or adult stages, it is expected that UVR stress would be more evident in the microscopic life stages (single- and few-celled), mainly due to their structural simplicity. These studies, in addition, bring about important consequences for algal zonation. For example, depth distribution patterns of large kelps have been frequently thought to reflect the light requirements of establishment stages (spores, embryos, etc.). Paradoxically, most studies performed to address the relationship between the physiological performance under different light environments have been done with the large sporophytes, whereas spore adaptation has been largely overlooked [223]. The question of whether early developmental stages of macroalgae, particularly spores, are more susceptible to UVR than larger life history phases has been less addressed [73], If this is so, it is reasonable to think that the physiological adaptation of spore stages (such as the ability to acclimate to different light climates) will have consequences for the whole population dynamics [224].

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