PLANKTON 33 



division. It has not been widely realized that sex may play a role even 

 in this remote corner of the cosmos! Sexual union of two gametes to 

 produce a zygote does occur, particularly among the Phaeophyta and 

 Chlorophyta (Chlorella and Scenedesmus are notable exceptions.) Either 

 or both gametes may be flagellated, or mating may be achieved by ameboid 

 motion. Gilbert M. Smith 209 notes that algal sexuality was initially de- 

 scribed in 1858 in Oedogonium. In 1926, Jollos demonstrated that the 

 heterothallic green alga Dasycladus excretes sexual substances which in- 

 fluence the movement of gametes toward each other. Since then, much 

 experimentation has been carried on with Chlamydomonas eugametos, a 

 unicellular green alga. Motility is absent in cells grown anaerobically in 

 the dark, but it is revived by filtrates from cells grown in light. The sub- 

 stance responsible is reported to be crocin, an ester of the sugar genti- 

 obiose combined with the carotenoid pigment crocetin. There are also 

 substances affecting maleness and femaleness; these are activated by ex- 

 posure to light, weakened by dilution. The virilizing factor appears to 

 be s of r anal, the feminizing one a gentiobiose of safranal. There are all 

 degrees of sexual activity, depending upon the concentration of activating 

 substances and exposure to light. There are also varying degrees of male- 

 ness and femaleness, even intermediary forms, depending upon the ratio 

 of opposing substances. 



Similar findings were reported by Sager and Granick 211 in 1954. They 

 noted that depleting the nitrogen supply in culture media of Chlamy- 

 domonas reinhardi encourages the differentiation of ordinary cells to 

 gametes. If nitrogen is added, the process is reversed. Light apparently 

 acts only indirectly via photosynthesis, and it is not obligatory for zygote 

 formation of dark-grown nitrogen-depleted cells. 



Much has been learned about the metabolic characteristics ol algae by 

 observations of cultures. Chlorella has been widely used in photosynthetic 

 experiments especially because it can grow under such varied environ- 

 mental conditions. The rate of algal growth is often measured by "genera- 

 tion time" — the time required to double the number of cells in a culture. 41 

 There are two major phases of growth, one chiefly of cellular multiplica- 

 tion, the other of storage of reserve materials. In the first phase, proteins 

 predominate; in the second, fats and carbohydrates. The main factors 

 influencing algal growth are: available radiant energy, carbon dioxide 

 concentration, temperature, and composition of nutrient substrates. The 

 exact channeling of the various nutrients may depend upon the amount 

 of available nitrogen, or upon the presence of enzyme systems. Chlorella 



