ACTION SPECTRA FOR THE APPEARANCE OF DIFFERENCE ABSORPTION BANDS AT 480 AND 520 mμ IN ILLUMINATED CHLORELLA CELLS AND THEIR POSSIBLE SIGNIFICANCE TO A TWO‐STEP MECHANISM OF PHOTOSYNTHESIS

In the difference absorption spectrum of thin, actively growing* aerobic suspensions of Chlorella pyrenoidosa, both the 480 mμ (negative) and the 520 mμ (positive) bands are produced by light absorbed in chlorophyll b and chlorophyll a; the ratio of absorption changes caused by equal number of incident quanta of 650 mμ light and those of 680 mμ light, is about 1.2. Both effects are partially inhibited by DCMU. Upon replacing air with argon, the effects are increased several fold and become relatively insensitive to DCMU. The increase is stronger in the absorption region of chlorophyll a, than in that of chlorophyll b; the ratio of the absorption changes, caused by equal numbers of 650 mμ and 680 mμ quanta decreases to about 0.8, for both effects. Variable (as regards the exact ratios of absorption changes), but parallel results for 480 and 520 mμ bands were obtained with cultures having low quantum yield of photosynthesis. This parallelism in the behavior of the 480 mμ and the 520 mμ band suggests that at least part of these two bands have a common origin. However, many observations suggest that both difference bands may have a multiple origin; as a working hypothesis, this origin is discussed in terms ofthree reactions: Reaction A—Photoreduction of chlorophyll a in system II; Reaction B—Photooxidation of chlorophyll b in system II; and Reaction C—Photooxidation (perhaps of a carotenoid) in system I.