light. The fluorescence yield rises instantaneously to the initial level 0. 

 It then rises to level I in 20 to 50 msec, and remains constant or decreases 

 slightly for a brief period before rising to the peak P after 0.25 to 1 sec. 

 Within 1 to 2 sec, it decreases to the steady-state level S. The fluorescence 

 at is the only part of the curve which was found to be directly proportional 

 to the intensity of the excitation light. Unlike the other parts of the curve, 

 it is not affected by preillumination of the algae. An additional result found 

 by Munday and Govindjee (refs. 25 and 28) was that the time to peak P is 

 inversely proportional to the intensity of the excitation light. This has 

 important implications for laser light excitation of algae because with high- 

 intensity illumination the fluorescence curve could go directly to P without 

 establishing (private communication from Govindjee, University of Illinois, 

 Urbana , Illinois). The qualitative shape of the curve shown in figure 1M was 

 confirmed by E. V. Browell, 0. Jarrett, Jr., F. Farmer, and C. A. Brown, Jr., 

 using Dunaliella euchlora . A Perkin-Elmer fluorescence spectrophotometer with 

 a flowing sample cell was used to permit exposure times greater than 2 msec 

 for the algae. Current research into fluorescence emission of algae using 

 picosecond laser excitation pulses has revealed that there is a decrease in 

 the fluorescence cross section with increasing pulse intensity (ref. 30). 

 Also, there is a change in this cross section if a series of picosecond excita- 

 tion pulses separated by several nanoseconds are used because of changes which 

 occur in the algae after each pulse. Thus, data obtained with excitation times 

 shorter than a nanosecond or greater than several milliseconds are not directly 

 applicable to flash-lamp-pumped laser excitation pulses of 300 to 500 nsec. 

 The determination of the fluorescence nature of chlorophyll a in vivo in the 

 submicrosecond range is very important in determining the relationship between 

 chlorophyll a in vivo concentration and fluorescence yield for flash-lamp- 

 pumped laser excitation applications. 



Nutrient and age effects .- Since algae go from an exponential growth phase 

 to a stationary growth phase as a result of exhaustion of one of the nutrients 

 in the medium, it is primarily the limiting nutrient which causes changes in 

 fluorescence properties of the algae. Blasco's analysis of the data obtained 

 from investigations performed in northwestern Africa (ref. 27) showed that when 

 a culture reaches the stationary growth phase, the ratio of fluorescence to 

 chlorophyll a concentration increases. These results are shown in figure 15. 

 Nitrates are directly linked to pigment formation within cells, and as a result, 

 the ratio of fluorescence to chlorophyll a concentration increases immediately 

 after the cells have exhausted the nitrates in the medium. Phosphate-limited 

 cultures which were tested by Blasco did not exhibit an immediate change in 

 fluorescence until 6 days after most of the phosphate had been depleted by the 

 algae. In both cases the exhaustion of nutrients affects the growth of pig- 

 ments and, in turn, reduces the photosynthetic activity which allows more 

 energy to be dissipated by fluorescence. The change in the ratio of fluores- 

 cence to chlorophyll a concentration can be over 100 percent for both nitrate- 

 and phosphate-exhausted algal cultures. 



Water temperature effect .- There are conflicting reports in the literature 

 of the influence of temperature on the fluorescence characteristics of algae. 

 Lorenzen (ref. 29) reported a large decrease in fluorescence when the tempera- 

 ture of the algal medium was increased from 12° C to 35° C in a short period 

 of time. Blasco (ref. 27) argues that the sudden change in temperature caused 



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