The production of antibiotics by plankton algae and its cflcct upon bacterial activities 285 



It is thus evident that the difference between the light and ilic dark bottles is not 

 due to Chlorella photosynthesis. In a corresponding series made simultaneously but 

 without Chlorella added the O2- consumption was the same in the light and in the 

 dark bottles. The difference was 0-03 - 0-08. This indicates that no photosynthesiz- 

 ing cells in any important number were introduced together with the bacteria, it 

 further shows that light has no direct effect on the activity of the bacteria developing 

 in the bottles. This is in accordance with the results published b\ Vaccaro and 

 Ryther (1954). 



There is no reason to give any details about the other similar experiments made, 

 using either Chlorella or the marine diatom. The results are similar. The difference 

 in oxygen consumption between light and dark bottles was between 12 and 30 times 

 higher than the oxygen production due to the photosynthesis of the algae. 



It turned out to be impossible in these experiments to use unsterilized, stored sea 

 water. Products produced by the special bacterial flora developing made the use of 

 Winkler titrations rather impossible. The water was therefore collected immediately 

 before an experiment. 



Experiments were made to investigate the influence of an addition of a substratum 

 from which a rather dense, vigorously growing Chlorella population had been filtered 

 off. Although a pronounced influence was found, the direct influence of a dilute 

 Chlorella population was pronouncedly higher. By adding 4% of such a substratum 

 at the start of an experiment lasting 2 days, only a decrease in Oz-consumption 

 of 0-09 ml/1 was found. In the corresponding experiment the influence of a very 

 dilute Chlorella population in the light bottles was 5 times as high. This is. however, 

 in perfect agreement with the statements by Pratt (1948) according to which the 

 concentration of the antobiotics produced by algae seems to be higher at a low con- 

 centration of growing Chlorella than at a higher concentration. 



The experiments presented in this article show that the presence of algae, presum- 

 ably due to the production of antibiotics, effects a reduction in the Orconsumption 

 of the bacteria in the light bottles. This reduction was of a higher order of magnitude 

 than the Oo-production due to the photosynthesis of the algae. There is no reason 

 to beUeve that the plankton algae living in oligotrophic oceanic water should not 

 behave in just the same way. Thus it is not possible to use the light and dark bottle 

 oxygen technique here. In eutrophic areas the technique is fully applicable. 



In the culture medium used in the present experiments the organic matter present 

 was easily accessible for the decomposing bacteria. It must be assumed that a number 

 of different species took part in this decomposition. In oceanic water the dissolved 

 organic matter presumably is only accessible for some few bacterium species able 

 to excrete the necessary exoenzymes. It is therefore rather probable that the effect 

 of the antibiotics produced can be still higher here. In the experiment shown in 

 Table 1, oxygen consumption was reduced by 19",. in the light bottles. In light and 

 dark bottle experiments in oligotrophic oceanic water the reduction is mostly more 

 than 50%. 



DISCUSSION 



By looking at a series of light and dark bottle experiments made in the open ocean, 

 it is evident that the rate of respiration in the dark bottles varies from one station to 

 another. Thus the values published by Riley (1939) for 3 days" expermicnts u.th 



