Fission Rate of Plankton Algae of the Black Sea in Cultures 131 



carried out with mixed cultures showed that some algae (Skel- 

 etonema costatum, Chaetoceros socialis forma vernalis) in the 

 period of their intensive multiplication inhibit the development 

 of Cerataidina herii,onii, Thalassioncma nitzchioicJes, Chaeto- 

 ceros curvisetus, Rhizosolcnia alata and others, while in mono- 

 cultures the latter divide with a comparatively high rate. A quick 

 disappearance of some algae in the sea is not always connected 

 with the decrease of their potential of multiplication because 

 in cultures they do not show any decrease in fission rate during 

 the same period. It is then probable that this is due to some limit- 

 ing factors. Decrease in growth may also depend on the abund- 

 ance of consumers of algae in the sea besides the changes in con- 

 tent of nutrients and the biotic relationships. Zooplankton organ- 

 isms energetically consuming plankton algae, in fact, decrease 

 the initial value for the population of the latter. Mavbe this is 

 one of the main reasons of the lower quantitative values of some 

 species of dinoflagellates and small flagellates in the open part of 

 the Black Sea. In spite of the fact that in cultures the division 

 rate of diatoms and dinoflagellates is nearly equal, the quantity of 

 Prorocenfrum micans, Exuviaella cordata, Peridiniwn triquetrum, 

 Gymnodinimn widfii and others in the sea does not exceed 300 

 thousand cells per liter while diatoms often reach 20 million cells 

 per liter. This may be an indirect confinnation of intensive graz- 

 ing of dinoflagellates. 



The experimental data show that the fission rate of diatoms 

 and dinoflagellates is diurnal: it is usually higher during daylight 

 than at darkness. The minimum fission rate was observed in 

 summer from midnight till 4 o'clock in the morning and till 7-8 

 o'clock at winter time, i.e., before daybreak. 



Since the algae were grown in unialgal cultures, but 

 not free of bacteria, it was interesting to investigate the influ- 

 ence of bacteria on algae. The number of bacteria was determined 

 by counts on a peptone-meat infusion-agar. Actively-dividing 

 algal cultures depressed bacterial development; the number of 

 bacteria sharply decreased. In weak cultures with slow algal 

 division, especially after the cells begin to die off, the number 

 of bacteria greatly increased. 



Not all the species of planktonic algae equally depressed 



