224 Marine Microbiology 



olism below 200 m. But we do not know anything about the 

 concentration of organic substances in the aphotic layer, and 

 very little about the kinds of substances that small flagellates 

 prefer. 



Another important and necessary factor is the occurrence of 

 deep currents, renewing nutrients and carrying oxygen. Unfor- 

 tunately, these are rarely measured, and theoretical calculations 

 give only some probabilities on such currents. We shall tiy a very 

 provisional reconstruction on the influence of currents in tlie 

 deep Mediterranean. 



Only general hypotheses are available now, but several facts, 

 based on observations of more than 1,000 water samples, seem 

 to confirm our theoretical views. 



Heterotrophic Nutrition 



I have observed several times (3 times off Monaco and seven 

 times off Algeria) the presence of young Cyclococcolithiis (pal- 

 melloid stages of 6 to 12 /-». ) on decaying dead Copepods or Tuni- 

 cates suspended in the sea. Using the inverted microscope, I 

 thought at first that some dead body had fallen under free pal- 

 melloid flagellates, but these cells were firmly attached to the 

 Copepod body. It seems almost sure that young stages of 

 Cyclococcolithiis are able to assimilate organic substances of ses- 

 ton. They may also consume bacteria, as several flagellates are 

 known to do. 



Figure 2 shows a probable relation between the density of 

 seston (particles of decaying matter) and the density of flagel- 

 lates: 



Our figure is based upon 252 water-samples, collected by 

 the Calypso in July 1955, between 200 and 4,000 m. Seston, per 

 ml of water, is estimated by counting after sedimentation, using 

 an inverted microscope. Of course, the large masses of seston 

 are not present in a small volume, but the little particles (4 to 

 90 /x) are dense enough and regularly distributed; their number 

 per ml varying from 3 to 1,230 in Calypso samples. 



It seems clear that, when seston is rare (3 to 10 x^arti- 

 cles/ml), flagellates are also rather rare. Probably dissolved or- 

 ganic matter is much more abundant when particulate organic 

 pellets are numerous. 



