284 E- Steemann Nielsen 



As mentioned above, Pratt et al. (1944) showed that the antibiotics are given off 

 by the alga to the water. In a sample of natural sea water containing phytoplankton, 

 it must therefore be expected that the antibiotics produced are not only effective for 

 the prevention of bacterial growth on the surface of the algae themselves, but also on 

 other surfaces, thus on the inner walls of the experimental bottles. 



EXPERIMENTS 



Photosynthesis may be written CO2 + HgO^ (H2CO) + O2; respiration may 

 be written (H2CO) + 02^ CO2 + H2O. Respiration by the plankton algae, and 

 by all other organisms present, is going on both in the Hght and the dark, photo- 

 synthesis by the plankton algae is going on only in the light. 



When making experiments in which the amount of dissolved oxygen in the water 

 is measured before the experiment, and in light and dark bottles at the end of the 

 experiment, the difference in the O2 content between the initial bottles and the dark 

 bottles represents the rate of respiration by all organisms. The rate of photosyn- 

 thesis should be measured by the Og-difference in the light and in the dark bottles. 

 A necessary condition is, however, that light neither directly nor indirectly decreases 

 the rate of respiration, a condition which is not fulfilled. 



Experiments were made in which bacteria were growing together with very few 

 algae (the freshwater green alga, Chlorella pyrenoidosa or the marine diatom, Thalas- 

 siosira nana). The rate of photosynthesis per number of cells was first measured in a 

 special experiment using a sufficient concentration of the algae. The method was 

 the same as used in the main experiment. There the concentration of the algae was 

 made so low that it would have been impossible to measure the rate of photosyn- 

 thesis by the Oz-production according to the normal Winkler technique. By counting 

 the number of algae cells at the start and at the end it was possible to determine 

 growth during the experiments, and thus the total rate of photosynthesis. 



In the experiments with the marine diatom, sea water from the Sound enriched 

 with some nutrient salts was employed. In the Chlorella experiments the culture 

 solution D (OsTERLiND, 1949) was used in a slightly modified form. In both cases 

 7-8 mg glucose and 2-4 mg peptone was added per 1. The light intensity was 8,000 

 lux, the temperature 22° C. In every experiment 3 bottles of each kind was em- 

 ployed. Two Winkler titrations were made with water from each bottle. 



In order to get an incubation by bacteria, the culture solution used for the main 

 experiments was incubated with 0-5 ml/htre of freshly collected surface water either 

 from a lake near Woods Hole {Chlorella experiments) or from the Sound (the marine 

 diatom). The water was filtered through a coarse paper filter before use. 



In Table 1 is given a typical experiment. 



Table I. Chlorella pyrenoidosa 



Rate of photosynthesis (real assimilation) in preliminary experiment 1-46 ± 001 ml Oj/l per hour. 



Dilution of the algae used in the main experiment 1 : 10000. 



Duration of main experiment 72 hours — 3 light periods (44 hours together) alternating with 3 

 dark periods. 



Rate of photosynthesis (real assimilation) by Chlorella in light bottles (corrected for growth) per 

 72 hours 004 ml02/l. 



02-consumption in dark bottles per 72 hours 3-59 ::r 0-02 ml. Oj/l. 

 Oj-consumption in light bottles per 72 hours 2-92 :r 003 ml O2/I. 

 Reduction in Oz-consumption in light bottles as compared with dark bottles 0-67 ± 0-04 ml Oj/l. 



