DIURNAL VARIATION IN THE PHOTOSYNTHESIS OF NATURAL 

 PHYTOPLANKTON POPULATIONS IN ARTIFICIAL LIGHT 

 by 

 Robert W. Holmes and Francis T. Haxo 



Evidence for the existence of a daily period- 

 icity in photosynthesis of marine phytoplank- 

 ton has been presented In two recent papers 

 (Doty and Oguri, 1957> and Yentsch and Ryther, 

 1957) • These authors observed a diurnal per- 

 iodicity In photosynthesis in surface-water 

 samples collected at intervals throughout the 

 night and day, illuminated under constant 

 light, and kept at a constant temperature. 

 The rate of photosynthesis in the water samples 

 began to increase during the early morning 

 hours and reached a maximum at about 0800 hours . 

 This was followed by a rapid decrease. At 

 about 1800 hours a low level of photosynthesis 

 was reached and was maintained until about 2^00 

 hours when the predawn rise began culminating 

 in the 0800 maximum. 



Two preliminary experiments are described be- 

 low which were designed to study this rhythm 

 in the eastern Pacific. The techniques em- 

 ployed were similar to those of Doty and Oguri 

 (1957). Surface samples were collected at 

 two-hour intervals (three-hour intervals during 

 the second experiment) alongside a free-float- 

 ing surface buoy to which was attached, just 

 below the sea surface, a regulation U. S. Navy 

 parachute. The samples were collected in a 

 large plastic bucket and two 250-ml. aliquot s 

 immediately inoculated with 1 uc of C^- and 

 placed in the shipboard incubator . These 

 samples were subjected to constant illumina- 

 tion (about 1000 foot-candles) from daylight- 

 type fluorescent lights and kept at a tempera- 

 ture slightly exceeding (about 1°C) the sea-sur- 

 face temperature for approximately two hours. 

 After the incubation period the samples were 

 filtered through 1-in. HA Millipore filters. 

 The filters were then dried and counted in the 

 normal manner (see p. 7)- The data were 

 corrected for any slight deviation in the dura- 

 tion of the incubation period. In the second 

 experiment, dark-bottle uptake was subtracted 

 from the uptake in the illuminated bottles. 

 The results of these two experiments are Illus- 

 trated in figures 7 and 8. 



The results of both experiments clearly indicate 

 that the photosynthesis of samples collected 

 between 1800 and 0200 hours was less than that 

 observed during the remainder of the 2k-hour 

 period. The difference between the maximum 

 and minimum uptake varied by a factor of 5-8. 

 This Is somewhat less extreme than that ob- 

 served by Doty and Oguri (1957) and greater 

 than that reported by Yentsch and Ryther (1957K 

 In the first experiment (Fig. 7) the daily 

 maximum occurred between 0800 and 1000 hours 

 while in the second experiment (Fig. 8) a max- 

 imum was observed between 1200 and llK)0 hours . 

 Unfortunately, the 0900 samples were lost in 

 this second experiment for the daily maximum 

 might have occurred at about this time. 



The time Of the photosynthesis maximum cannot 

 be defined with certainty since the C^ 

 measurements were discontinuous, representing 

 averaged rates of uptake for 2-3 hour incuba- 

 tion periods of samples collected at 2-3 hour 

 intervals . The data show that the photosyn- 

 thetic activity of surface waters varies- 

 diurnally. Such a periodicity may be associat- 

 ed with concomitant changes in phytoplankton 

 standing crop or may be a manifestation of an 

 inherent photosynthetic rhythm. Samples 

 collected in Experiment No. 1 to assess the 

 first of these possibilities have not yet 

 been examined. In the second experiment 

 chlorophyll "a" determinations made at the 

 beginning of each 3-hour incubation period in- 

 dicated a fairly constant chlorophyll "a" 

 content. These preliminary results differ 

 from those of Yentsch and Ryther (1957) and 

 B. M. Shimada (personal communication) who 

 have observed a diurnal periodicity in 

 chlorophyll "a" quite similar to the photo- 

 synthetic periodicity. 



73 



