Mechanisms to reduce the effects of UVR on photosynthesis

Adaptation to UVR assumes the existence of mechanisms that protect the organism or reduce the deleterious effects. According to Roy [140] four basic mechanisms allow an organism to cope with a stressful situation, i.e., UVR exposure, (1) avoidance, (2) reducing the stress by a physiological behavioral mechanism - e.g., through the synthesis of UV-absorbing compounds, (3) repairing the damage produced and, (4) acclimating to stress when allowed enough time. More details on these mechanisms can be found in other chapters of this book.

Avoidance mechanisms seem to be a common strategy against exposure to high levels of UVR. For microalgae living in soft substrata, such as motile cyanobacteria and diatoms [15,56,65], this involves downward vertical migration. Bebout and Garcia-Pichel [15] showed that, by migrating down to 300 fim depth, cyanobacteria could reduce their UV exposure to 10% of that at the sediment surface. For benthic diatoms, the observed downward movement of Gyrosigma balticum at high light levels was first suggested to be related to PAR rather than UV-B [46,64]. However, a subsequent fluorescence study indicated that the migration could in fact be a direct response to UV-B [65]. Avoidance can also be achieved by means of circadian rhythms that allow an organism to swim down at noon to depths where radiation intensities are low, as occur in some dinoflagellates [175]. However, it should be considered that UVR can alter the motility and phototaxis of some autotrophic organisms, as found in several microalgal species [5]. Moreover, in other organisms, loss of flagella has also been reported [176]. Thus, in some sensitive organisms, avoidance mechanisms can be severely altered by UVR exposure.

Another strategy to minimize the effects of UVR is through the presence of UV-screening compounds. The most studied compounds are those collectively named mycosporine-like amino acids (MAAs), which are found in many marine and freshwater autotrophic and heterotrophic organisms [177,178] (see also Chapter 10). Evidence of their protective role upon physiological mechanisms remains still unclear, and in some cases it seems that they just provide partial protection, as in some cyanobacteria [179]. In other cases, though, MAAs have been proved to be an effective protection mechanism [107,180] so that photosynthesis in phytoplanktonic cells with higher amounts of MAAs was less inhibited. In benthic diatoms, however, the production of such protective substances does not appear to be a major strategy. Although MAAs have been detected in MPB of shallow-water subtidal sediments, the concentrations are low and show no significant increase at UV exposure (ambient or increased) [29,46,47,64], which agrees with the findings of Peletier et al. [119]. Jeffrey et al. [181] tested 152 algal species and found that diatoms generally had low concentrations of UV-screening compounds as compared with other algal groups. Moreover, Helbling et al. [107] found that pennate diatoms (which usually dominate benthic diatom communities) contained less MAAs than centric diatoms. Other compounds may also have a protective role, functioning as UV-screening agents (see Chapter 10). For example, scytonemin is a UV-absorbing extracellular substance found in the sheath of cyanobacterial filaments [182]. In addition, high concentrations of carotenoids as a result of UVR exposure have been observed in diatom mats [65], and some cyanobacteria as well as chloro-phytes [162], suggesting an UV-protecting function of these pigments.

MAAs have also been reported in green, red and brown macroalgae from tropical, temperate and polar regions [55,125,158,177,183,184]. The concentration of MAAs in macroalgae has been found to be related to depth zonation and UV exposure [177]. Their accumulation seems to be higher under high than under low daily irradiance values (i.e. different latitudes), and moreover, generally higher in intertidal than in subtidal algae [184-186], In addition, this accumulation seems to be a wavelength-dependent process [125,185,187,188], and an UV-B-mediated increment of these compounds has been shown in a variety of algae [155,189]. In Chondrus crispus, both UV-A and UV-B stimulated a strong accumulation of shinorine, whereas the content of palythinol and palythine was mainly stimulated by PAR, indicating a MAA-specific induction triggered by these wavelengths [125]. In Palmaria palmata, on the other hand, and when exposed only to PAR, a 6-fold increase in the porphyra-334 concentration was observed; the treatment receiving PAR -I- UV-A gave similar results plus an accumulation of shinorine; under full solar radiation, accumulation of porphyra-334, shinorine and palythine was observed [185]. In addition, in Chondrus crispus, pre-exposure to blue light followed by growth under natural UV-A led to a 7-fold increase in the synthesis of shinorine as compared with growth without the blue light pre-treatment [188]. So, it has been hypothesized that there are two photoreceptors for MAAs synthesis in C. crispus, one for blue light and one for UV-A, which act synergistically [188]. In macroalgae, other types of potentially protective compounds are also found, such as phlorotanins in brown algae [190] and coumarins in the green alga Dasycladus [191,192].

In addition, and while UVR-mediated DNA damage occurs in aquatic autotrophic organisms [168,193-196], repair mechanisms of the DNA molecule (see Chapter 9) are also present [193]. However, the presence of one or other mechanism (i.e., photoreactivation, nucleotide excision repair or recombination repair) is clearly dependant on the species under study and the radiation conditions at which the cells are exposed (see Chapter 9).

Finally, acclimation mechanisms to cope with high UVR intensities are important in several aquatic organisms. These usually occur on a long-term basis, when organisms have been exposed for enough time to the stress factor (UVR). One of these acclimation mechanisms is the previously mentioned synthesis of MAAs, as found in some natural populations and cultures of phytoplankton

[107,110,141]. However, the synthesis of UV-absorbing compounds is not a general response, and several species do not show an increase of MAAs content even after several weeks of exposure to UVR [197,198], Acclimation can also occur through a change in the community composition [110], so that those species more adapted to a particular light regime will dominate. For example, a natural Antarctic phytoplankton population dominated by flagellates (80% in terms of carbon biomass) changed to a diatom dominated population when receiving UVR + PAR, whereas in those samples receiving only PAR, small flagellates still dominated. However, diverse responses are observed at different sites. For example, Mousseau et al. [199] in their study conducted with an estuarine community also observed changes in diversity when samples were exposed to different radiation treatments. A shift from a diatom-dominated community to small flagellates occurred more rapidly in the treatment receiving enhanced UV-B as compared to those receiving natural UV-B levels. Clearly, responses are strongly species-specific and depend on radiation levels and quality to which organisms are exposed.

0 0

Post a comment