1272 THE PIGMENT FACTOR CHAP. 32 



duced considerably more oxygen per unit cell volume per unit time than 

 the ordinary strains of Chlorella pyrenoidosa do at their optimum tempera- 

 ture. The assimilation number of such thermophilic algae at 39° C. may be 

 considerably higher, and their assimilation time correspondingly shorter, 

 than those that other strains can reach under the most favorable conditions. 



4. Chlorophyll Concentration and Yield of Photosynthesis 



in Flashing Light 



In chapter 34, we will deal with experiments on photosynthesis in 

 periodically interrupted light; we must here anticipate some of the results, 

 which concern the relation between the chlorophyll content and the maxi- 

 mum amount of oxygen that can be liberated by a single short flash of hght. 



This quantity can be interpreted in different ways. The interpreta- 

 tion that seemed most natural and was therefore the first to be discussed 

 was based on the identification of the maximum amount of substrate that 

 can be reduced by a single flash, with the quantity of this substrate available 

 in the cells, after a period of darkness, in a form suitable for immediate 

 photochemical reduction (e. g., as an acceptor-carbon dioxide compound or 

 complex, [CO2] or ACO2). In a dark-rested chloroplast, in presence of 

 sufficient carbon dioxide, each acceptor molecule must be associated with a 

 molecule of carbon dioxide. A sudden flash of light will send all these 

 molecules through the photochemical reduction stage (or stages), and thus 

 produce a quantity of primary photoproducts equivalent to the amount of 

 the available acceptor. After this, a certain interval of time may be re- 

 quired for the "recharging" of the photosynthetic apparatus, e. g., by the 

 slow formation of a new quantity of the acceptor-carbon dioxide complex 

 ("preparatory" dark reaction). The yield of oxygen, produced by a single 

 strong flash, will then be equivalent to the quantity of the carbon dioxide 

 acceptor present in the cells. 



In discussing the nature of the carbon dioxide acceptor in chapter 8, we 

 found that it is a compound the content of which in green cells is about 

 equivalent to that of chlorophyll. In chapter 34, we will find evidence 

 that this acceptor may itself be a transient product of photosynthesis, and 

 therefore decrease in concentration in the dark and increase again as photo- 

 synthesis gets under way — perhaps approaching approximate equivalency 

 with chlorophyll in the stationary state in strong light. If the maximum 

 oxygen yield per flash were limited by the available quantity of this ac- 

 ceptor, we would expect this yield, too, to be approximately equivalent to 

 the chlorophyll content of the cells; in other words, it should be of the order 

 of 0.01 mole per fiter of cells, or as much as 0.2 volume of oxygen per unit 

 volume of cells per flash. 



