INORGANIC IONS 



953 



plicated b}^ changes in the ionic dissociation of these weak acids (in addition 

 to the conipUcations caused by the dissociation of carbonic acid). 



Figure 27.13 shows the effect of pB. on carbon dioxide reduction by 

 Chromatium in 1% thiosulfate. The curve, obtained in phosphate buffers, 

 shows a shght maximum of P at pH 6.3, followed by a sharp decrease. The 

 effect disappears at low light intensities (as do all effects not connected 

 with the primary photoprocess). With hydrogen as reductant, the effect 

 was quite different (fig. 27.14); the rate, P (in strong light), increased 

 steadily with increasing alkalinity. 



; : 079 X lO** erg/cm^ sec 



70 

 pH 



Fig. 27.1.5. Effect of yjH on fluorescence of Chromatium (after Wassink, Katz and 

 Dorrestein 1942): {A) 5% CO2, 1% thiosulfate, 29° C, strong light; (5) 5% CO2, 

 \b% H2, 25° C, strong and weak light. 



One factor that may contribute to the decline of P with increased pH in 

 thiosulfate is the decrease in concentration of undissociated H2S2O3 mole- 

 cules; this may be the main species that penetrates the cells and is used 

 there as the reductant. According to figure 27.12, at 1% thiosulfate, the 

 concentration of the reductant is not a "limiting" factor at pH 6.3. Whether 

 it could become such a factor when alkalinity is higher, remains to be inves- 

 tigated. 



The increase in P with pH, as observed with hydrogen as reductant, 

 could perhaps be looked upon as new evidence of the direct participation 

 of HCOs" ions in i)hotosynthesis — if one were inclined to give weight to 

 such evidence. Alternatively and more plausibly, both the decline of P 



