Effects of UVR on marine macrophyte photosynthesis

Studies on comparative primary productivity of marine macrophytes under different scenarios of UV climate are rather scarce; moreover, there is a diffuse picture of their photoadaptive strategies. Taking into account the distinct origin and the morpho-functional divergences of macroalgal species, a common adaptive strategy is unlikely. Thus, a number of responses can be determined among species. However, a general pattern is observed: under natural radiation levels they show daily photoinhibition - a decrease in the photosynthetic rates/yield [48,131], at least at high zenith angles [48-51]. In most cases, high PAR irradiances at noon cause a decrease in photosynthetic rates [88,90], but UVR also contributes largely to this process [30,72,77,157,158].

In intertidal algae in particular, the highest photoinhibition values (mainly due to PAR) are found when low tide coincides with local noon [88,97]. Even algae harvested from rock pools, where they are normally exposed to extreme solar irradiances, show signs of photoinhibition after prolonged periods of exposure [30,90]. Under these conditions, increases in temperature and partial desiccation of algal thallus also contribute to the observed photoinhibition [38,87]. Deep-water algae and those adapted to shaded environments are inhibited even faster when exposed to direct solar radiation [90].

Recovery of photosynthesis - measured as an increase in fluorescence quantum yield - starts when irradiance begins to decrease, but remains still at saturating levels. Recovery is species-specific and occurs faster in sun-adapted algae than in algae growing at deep or shaded locations and then transferred to the surface. In the eulittoral red algae Porphyra leucosticta [30], Asparagopsis armata and Felmanophycus rayssae [97] from southern Spain, recovery of photosynthesis occurs immediately after a decrease of only 10-20% of solar radiation. However, the brown alga Padina boryana recovers with a 30% irradiance decline, whereas in Sargassum polycystum a reduction of 70% in the incident radiation is required [96]. In their review on red macroalgae, Figueroa and Gómez [38] reported photoinhibition of ~ 30-80% at noon, but most of the species showed full recovery in the afternoon. In contrast, only partial recovery was observed in red algae from the North Sea [72] or from Patagonian waters (Helbling et al., unpublished). The recovery of macroalgae after UVR exposure (as compared to the PAR control) is highly variable, with little recovery found in Macrocystis pyrifera [159] and in Gelidium sesquipedale [31], and high recovery with beneficial effects of UV-B in the brown alga Dictyota dichotoma [35] and in the marine angiosperm Posidonia oceanica [42]. These specific responses provide important information, as the recovery kinetics gives insights into the photo-adaptive strategies of macroalgae and their light-stress tolerance capacity. Thus, those algae capable of dynamic (reversible) photoinhibition under high solar radiation levels and with a rapid recovery capacity will have competitive advantages as compared to those without any efficient photoprotection mechanism.

The ability for dynamic photoinhibition during exposure to high radiation, as well as the general degree of photosynthetic adaptation of individual species to different light regimes influences the upper depth distribution of algal zonation [86,88,94,98]. In fact, several taxa and life history stages of inter- and subtidal polar algae show a strong correlation between their depth distribution and their capacity to cope with high radiation stress [33,34,77,94]. Thus, species growing in the upper subtidal zone show in general more tolerance to high solar radiation levels, especially to UVR, than algae from deeper waters

Relatively few studies have been conducted on a long-term basis to determine the effects of UVR on photosynthesis and growth of macroalgae [54,151], In experiments carried out with Ulva sp., UV-B caused a decrease of both growth rates and photosynthesis during the first week of exposure to solar radiation, but UV-A stimulated growth as compared to the PAR treatment [54]. However, after two weeks of exposure, no differences were observed between treatments, a fact that hints to the action of acclimation mechanisms, which protect algae against UV stress (see below and Chapter 10).

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