![]() ![]() In this respect the red algae (Rhodophyta) appear unique among photosynthetic plants. Although the chlorophylls (and carotenoids) are present in quantities comparable to the green algae, their function is apparently not that of a primary light absorber this role is taken over by the phycobilins. In all these red algae, photosynthesis is almost minimal at 435 mµ and 675 mµ, where chlorophyll shows maximum absorption. perforata, with increasingly more phycocyanin and less phycoerythrin: the action spectra reflect this, with increasing activity in the orange-red region (600 to 640 mµ) where phycocyanin absorbs. In the genus Porphyra, there is a series P. Such algae include Delesseria, Schizymenia, and Porphyrella. In red algae containing chiefly phycoerythrin, the action spectrum closely resembles that of the water-extracted pigment, with peaks corresponding to its absorption maxima (495, 540, and 565 mµ). The photosynthetic rates are high in the spectral regions absorbed by the water-soluble "phycobilin" pigments (phycoerythrin and phycocyanin), while the light absorbed by chlorophyll and carotenoids is poorly utilized for oxygen production. Action spectra for a wide variety of red algae, however, show marked deviations from their corresponding absorption spectra. In green and brown algae, light absorbed by both chlorophyll and carotenoids seems photosynthetically effective, although some inactive absorption by carotenoids is indicated. Coilodesme (a brown alga) shows almost as good correspondence, including the spectral region absorbed by the carotenoid, fucoxanthin. Ulva and Monostroma (green algae) show action spectra which correspond very closely to their absorption spectra. These were exposed to monochromatic light, of equal energy, at some 35 points through the visible spectrum (derived from a monochromator). Before we introduce the two photosystems, however, we need to describe the light-gathering antennas and the energy needs of photosynthesis.A polarographic oxygen determination, with tissue in direct contact with a stationary platinum electrode, has been used to measure the photosynthetic response of marine algae. But the principle of the experiment is the same as that of Engelmann's experiments.Īction spectra were very important for the discovery of two distinct photosystems operating in O2-evolving pho-tosynthetic organisms. Today, action spectra can be measured in room-sized spectrographs in which a huge monochroma-tor bathes the experimental samples in monochromatic light. These were the regions illuminated by blue light and red light, which are strongly absorbed by chlorophyll. Discrepancies are found in the region of carotenoid absorption, from 450 to 550 nm, indicating that energy transfer from carotenoids to chlorophylls is not as effective as energy transfer between chlorophylls.īacteria congregated in the regions of the filaments that evolved the most O2. In the example shown here, the action spectrum for oxygen evolution matches the absorption spectrum of intact chloroplasts quite well, indicating that light absorption by the chlorophylls mediates oxygen evolution. If the pigment used to obtain the absorption spectrum is the same as those that cause the response, the absorption and action spectra will match. An action spectrum is measured by plotting a response to light such as oxygen evolution, as a function of wavelength. The absorption spectrum is measured as shown in Figure 7.4. This action spectrum gave the first indication of the effectiveness of light absorbed by accessory pigments in driving photosynthesis.įIGURE 7.8 Action spectrum compared with an absorption spectrum. Engelmann projected a spectrum of light onto the spiral chloroplast of the filamentous green alga Spirogyra and observed that oxygen-seeking bacteria introduced into the system collected in the region of the spectrum where chlorophyll pigments absorb. TheįIGURE 7.9 Schematic diagram of the action spectrum measurements by T. A population of O2-seeking bacteria was introduced into the system. Engelmann used a prism to disperse sunlight into a rainbow that was allowed to fall on an aquatic algal filament. Engelmann in the late 1800s (Figure 7.9). Some of the first action spectra were measured by T. Often an action spectrum can identify the chromophore (pigment) responsible for a particular light-induced phenomenon. For example, an action spectrum for photosynthesis can be constructed from measurements of oxygen evolution at different wavelengths (Figure 7.8). An action spectrum depicts the magnitude of a response of a biological system to light, as a function of wavelength. ![]() The use of action spectra has been central to the development of our current understanding of photosynthesis. ![]()
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