JOHN H. RYTHER 405 



surface of the vessel at all times of the day and night. Ketchum 

 (personal communication) obsewed the same behavior in Noctiluca 

 miliaris collected during a bloom at Friday Harbor. Homell and 

 Nayu'du (1923) and Nishikawa (1901) found that the red water 

 organisms remained floating at the surface of a bottle apparently 

 until they encysted; then they sank to the bottom. 



Harvey (1917) and Ketchum (unpubhshed) studied the specific 

 gravity of Noctiluca collected from patches of red w^ater. In each 

 case the organisms remained floating at the surface of the sea water 

 if the latter was diluted with fresh water until the dilution approached 

 50%, beyond which they became suspended or sank. It must be em- 

 phasized, however, that these observations were made upon cells 

 collected during typical red water conditions. It must not be assumed 

 that such low densities are typical of Noctiluca at all times, and it 

 cannot be so assumed if one subscribes to the theory that they 

 accumulate at the surface after their growth. In this connection, the 

 present author has observed a healthy culture of Noctiluca grown 

 by Dr. L. Provasoli in which the organisms were evenly distributed 

 throughout the medium. 



Pratje (1921) has suggested that Noctiluca undergoes physiological 

 changes in response to its environment, and that senescent, nondivid- 

 ing cells lose density and become buoyant in sea water. He subscribes 

 to the theory suggested above that the concentration of these organ- 

 isms at the surface does not result from active migration, but from 

 floatation. Spoehr and Milner (1949) and others have demonstrated 

 that Chlorella ceases to divide and stores fats when the nitrogen in 

 its medium becomes limiting. Fat storage in the autotrophic dino- 

 flagellates is common. Many species accumulate bright yellow or red 

 oil droplets (Graham, 1951). Noctiluca apparently does not store 

 fats, but its buoyancy is explained on the basis of the low specific 

 gravity of its cell sap possibly through the accumulation of NH4+ 

 ions (see Krogh, 1939), Such mechanisms as these are adequate to 

 account for the floatation of dinoflagellates. It remains to be demon- 

 strated that changes occur in their metabolism or osmo-regulation such 

 as to make them buoyant only at certain stages of their development. 



The accumulation of buoyant organisms at the water surface would, 

 of course, be enhanced in the absence of vertical mixing of the water, 



