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Macroalgae and Seagrasses

While phytoplankton are motile in the water column (Häder et al., 1995), most macroalgae are sessile and therefore restricted to their growth site (Lüning, 1990). Macroalgae show a distinct and fixed pattern of vertical distribution in their habitat. Some of these plants inhabit the supralittoral (coast above high water mark) exposed only to the spray from the surf, whereas others populate the eulittoral (intertidal zone), which is characterized by the regular temporal change in the tides (Häder, 1997d). Still others are never exposed to air since they are restricted to the sublittoral zone. The range in exposure can be substantial, from over 1000 Wm-2 (total solar radiation) at the surface to less than 0.01% of that which reaches the understory of a kelp habitat (Markager and Sand-Jensen, 1994). Macroalgae have developed mechanisms to regulate their photosynthetic activity to adapt to the changing light regime and protect themselves from excessive radiation (Foster and Franklin, 1997). They use the same mechanism of photoinhibition as higher plants to decrease the photosynthetic electron transport during periods of excessive radiation. This phenomenon facilitates thermal dissipation of excessive excitation. Different algal species occupy different depth niches and are adapted to different solar exposure (Häder and Figueroa, 1997). They also differ in their ability to cope with enhanced UV radiation (Dring et al., 1996). A broad survey was carried out to understand photosynthesis in aquatic ecosystems and the different adaptation strategies to solar radiation of ecologically important species of green, red and brown algae from the North Sea, Baltic Sea, Mediterranean, Atlantic, polar and tropical oceans (Markager and Sand-Jensen, 1994; Wiencke et al., 1994; Figueroa et al., 1996; Beach and Smith, 1996a,b; Kirst and Wiencke, 1996; Häder and Figueroa, 1997; Porst et al., 1997).

    Photoinhibition can be quantified by oxygen exchange (Häder and Schäfer, 1994) or by PAM (pulse amplitude modulated) fluorescence measurements developed by Schreiber et al. (1986) and based on transient changes of chlorophyll fluorescence. Surface-adapted macroalgae, such as several brown (Cystoseira, Padina, Fucus) and green algae (Ulva, Enteromorpha), show a maximum of oxygen production at or close to the surface (Herrmann et al., 1995b; Häder and Figueroa, 1997); whereas algae adapted to lower irradiances usually thrive best when exposed deeper in the water column (the green algae Cladophora, Caulerpa, most red algae) (Häder and Figueroa, 1997). It is interesting to note that respiration is inhibited to a far smaller degree than photosynthesis.

    PAM fluorescence allows the determination of the photochemical and non-photochemical quenching (Büchel and Wilhelm, 1993). Recently, an underwater PAM instrument was developed for in situ measuring the quantum yield of fluorescence, which promises advances in the knowledge on ecophysiology of macroalgae. The increase in nonphotochemical quenching is related to the violaxanthin cycle, which is believed to quench excess excitation energy both in algae and in higher plants (Demmig-Adams and Adams, 1992; Häder and Figueroa, 1997). Even algae harvested from rock pools, where they are exposed to extreme irradiances, show signs of photoinhibition after extended periods of exposure (Fig. 4.4). Deep-water algae and those adapted to shaded conditions are inhibited even faster when exposed to direct solar radiation. Large differences were also found in the recovery between high light-adapted and protected species. A considerable proportion of photoinhibition is due to PAR (400-700 nm). Exclusion studies were carried out to determine the effects of solar UV-B and UV-A (Herrmann et al., 1995a). Increasing exposure to solar radiation resulted in a shift of the compensation point to higher irradiances. The compentation point defines the irradiance at which photosynthetic oxygen production and respiratory oxygen consumption balance each other. Exclusion of UV-B partially reduced the effects. This trend increased when about half or all of the UV-A radiation was excluded (Schott filters WG 360 and 395).

    Chronic photoinhibition occurs when algae are exposed to excessive irradiance. The inhibition is characterized by photodamage of PS II reaction centers and subsequent proteolysis of the D1 protein (Critchley and Russell, 1994). In contrast, dynamic photoinhibition is readily reversible and follows a diurnal pattern with the lowest quantum yield around or soon after noon (Hanelt et al., 1994; Häder and Figueroa, 1997). The lowest light compensation point for photosynthesis has been reported in Arctic and Antarctic algae (Gómez et al., 1995; Wiencke, 1996; Gómez and Wiencke, 1996).

    The long-term effects of solar UV on the primary productivity of macroalgae still need to be evaluated. Shallow water specimens in coral reefs undergo a 50% reduction in photosynthetic efficiency during the middle of the day and show a complete recovery by late afternoon. Both UV-A and UV-B cause depression of the photosynthetic rate in the brown alga Laminaria digitata (Foster and Lüning, 1996).

Fig. 4.4 Photosynthetic quantum yield measured on site using a PAM fluorimeter in the Mediterranean brown alga Padina pavonica harvested from 0 m (closed bars) and 6 m depth (open bars) at 1-h intervals (from Häder, 1997c).

    Recently, different methods for measuring light absorption in macroalgae have been compared (Mercado et al., 1996). The absorption determined by using an integrating sphere and by the opal-glass technique in a spectrophotometer in thin macroalgae was intercalibrated. García-Pichel (1995) has developed a scalar irradiance fiber-optic microprobe for the measurement of ultraviolet radiation at high spatial resolution.

    The photoprotective mechanism of the xanthophyll cycle has been investigated mostly in microalgae (Schubert et al., 1994) and to less extent in macroalgae, e.g., the green alga Ulva lactuca (Grevby, 1996) and the brown algae Dictyota dichotoma (Uhrmacher et al., 1995) and Lobophora variegata (Franklin et al., 1996a). Red algae did not show the xanthophyll cycle.

    Another mechanism for protection against UV radiation (UV-A and UV-B) is the production of screening pigments such as carotenoids or UV-absorbing mycosporine-like amino acids (MAAs, Tab. 4.1). MAAs have been found in green, red and brown algae from tropical, temperate and polar regions. Since these substances are chemically very stable they accumulate in the sediment of lakes and can be used of a permanent record for past ultraviolet radiation environments (Leavitt et al., 1997). In tropical algae, enhanced levels of carotenoids and UV-absorbing compounds were detected in tissues from the canopy compared to tissues from understory locations in turf-forming rhodophytes (Beach and Smith, 1996a,b). Current research indicates that solar UV-B is a stress factor for macroalgae and seagrasses even at current levels; therefore further increases in UV-B may reduce biomass production and changes in species composition in macroalgae ecosystems.


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