species in different proportions. Most algal species fall into one of four 

 basic color groups - green, golden-brown, red, and blue-green. The color 

 group is determined by the apparent color of the pure algal culture.' The 

 spectral characteristics of the primary pigments contained in algae are shown 

 in figure 1 (ref. 6). Algal pigments have the following natural colors: 

 chlorophyll a and chlorophyll b are green, carotenoids are orange, phycoerythrin 

 is red, and phycocyanin is blue. All algae contain chlorophyll a; in addition, 

 green algae contain chlorophyll b, golden-brown algae contain carotenoids and 

 fucoxanthin (not shown in fig. 1), red algae primarily contain phycoerythrin, 

 and blue-green algae primarily contain phycocyanin. There are similarities 

 between the spectral absorption characteristics of the primary pigments and 

 the absorption spectra of algae, as can be seen in figure 2 (ref. 7). As would 

 be expected, the absorption features are not as distinct as those of the indi- 

 vidual extracted pigments. This limits the use of specific pigment absorption 

 features in identification of algal species. The primary spectral difference 

 between the green and golden-brown algae is the presence of fucoxanthin in the 

 golden-brown color group. This pigment allows spectral differentiation between 

 these two color groups. Blue-green algae, which are not represented in fig- 

 ure 2, have an absorption spectrum indicative of their primary pigment, 

 phycocyanin. 



The exchange of energy between pigments in algae is presented in an ele- 

 mentary form in figure 3. All pigments can absorb appropriate wavelengths of 

 light directly, and the energy is then transferred toward chlorophyll a. Since 

 this process is less than 100-percent efficient for all but chlorophyll b, some 

 of the energy which is not transferred to the next pigment is lost by nonradi- 

 ative processes (usually conversion to heat), and the remainder is dissipated 

 as fluorescence by the pigment. The energy reaching chlorophyll a excites it 

 above the first singlet state of the molecule. Energy stored in the chloro- 

 phyll a is then used in photosynthesis with any remaining energy dissipated as 

 fluorescence. Since all algae contain chlorophyll a, its fluorescence proper- 

 ties are important for use in active remote sensing techniques. A typical 

 fluorescence spectrum for green and goldert-brown algae is shown in figure 4. 

 The primary fluorescence peak at 685 nm results from deexcitation of the first 

 singlet state of chlorophyll a to the ground state. The secondary peak at 

 735 nm is generally lower than the primary peak, and it is a result of a secon- 

 dary energy exchange mechanism associated with photosynthesis. Red and blue- 

 green algae also have their primary fluorescence peak at 685 nm. Thus, active 

 remote sensing techniques are based upon detection of the laser-induced fluo- 

 rescence of chlorophyll a in vivo at 685 nm. 



Normalized excitation spectra for representative species of algae in the 

 four primary algal color groups (golden-brown, green, red, and blue-green) are 

 shown in figure 5. In this figure, the fluorescence of chlorophyll a in vivo 

 was monitored at 685 nm while the wavelength of the excitation light was 

 scanned from 360 nm to 680 nm by a Perkin-Elmer fluorescence spectrophotometer. 

 The peaks of the excitation spectra were normalized to a value of 10 for com- 

 parison between color groups. Spectrophotometer studies have indicated that 

 these spectra are qualitatively representative of other algal species contained 

 in the same color groups (ref. 8). 



