Arthur C. Ley

Absolute absorption cross-sections for Photosystem II and the minimum quantum requirement for photosynthesis in Chlorella vulgaris

Abstract Absolute absorption cross-sections for oxygen production (σO2) were determined from the light-saturation behavior of oxygen flash yields from whole cells of Chlorella vulgaris illuminated with submicrosecond flashes of laser light. Light-saturation curves were well described by simple Poisson statistics with a single average cross-section per photosystem trap (RCII). The maximum variation about the average cross-section permitted by the data was a factor of 3. σO2 at the laser wavelength (596 nm) increased from 38 A2 for cells grown in high light to 115 A2 for cells grown in low light. The 3-fold variation in σO2 was accompanied by a 10-fold variation in total cell chlorophyll content. This behavior results, at least in part, from the partitioning of chlorophyll between Photosystem II (measured) and Photosystem I (unmeasured). The 596 nm in vivo absorption cross-section for chlorophyll in Chlorella (σChl) is 0.29 A2, independent of total cell chlorophyll content. The antenna size of RCII was calculated to range from 130 to 400 molecules of chlorophyll. At low flash energies, the relationship between the quantum requirement for oxygen production (QR), the maximum oxygen-flash yield or Emerson and Arnold number (PSUO2) and our cross-sections is QR =( PSU O 2 )· ( (σ Chl ) (σ O 2 ) . QR, found to be independent of both total cell pigmentation and RCII antenna size, was constant at 10±1 photons absorbed per oxygen molecule evolved.

Relationship of steady-state photosynthesis to fluorescence in eucaryotic algae

Abstract The change in fluorescence yield (Δφ) was measured in five species of eucaryotic algae using a ‘pump and probe’ flash technique. The half-time for the oxidation of Q, which was measured by varying the delay time between actinic (pump) and measuring (probe) flashes, averaged 400 μs and was unaffected by background irradiance between 10 12 and 10 16 quanta · cm −1 · s −1 . The absorption cross-section of PS II traps was measured by varying the intensity of the actinic flash. These cross-sections did not change (± 10%) with background irradiance. The cross-section data can be fitted to a cumulative one-hit Poisson distribution. In the steady state, the relationship between δφ and photosynthetic oxygen evolution was highly nonlinear and cannot be explained by energy transfer between PS II units. Using the criteria of Δφ, about 15% of the PS II traps remain open at light saturation of O 2 evolution. Conversely, at low irradiance levels, capable of stimulating much less than 1% of the maximum steady-state photosynthetic rate, the fluorescence yield decreases by as much as 25% from the dark-adapted value. Furthermore, the data suggest that a long-lived quencher of fluorescence is formed at moderate to continuous irradiance levels, at least 10 16 quanta · cm −2 · s −1 . Our results suggest that cyclic electron flow around PS II occurs under normal physiological conditions and is especially pronounced in chlorophytes.

The extent of energy transfer among Photosystem II reaction centers in Chlorella

Abstract We have investigated the extent of energy transfer among Photosystem II reaction centers in Chlorella vulgaris . The cells show typical non-exponential fluorescence induction kinetics in the presence of the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea. We used single-turnover flashes to determine effective absorption cross-sections for Photosystem II reaction centers (RCII) in cells which were simultaneously illuminated with a continuous background light. We varied the background irradiance to control the fraction, γ, of the total RCII closed at the time of a flash. We found that the absorption cross-section per RCII was almost completely independent of γ. Relative oxygen flash yields measured at low laser flash energies were similarly unaffected when RCII were closed with the background light. We simultaneously measured laseer flash energy saturation curves for oxygen production and for the increase in fluorescence quantum yield measured 30 μs after the laser flash. The oxygen and fluorescence saturation curves were almost identical. We conclude from these results and appropriate theoretical calculations that the difference in the probabilities for escape of excitation energy at open and closed RCII is small (under 0.25). No matter how many RCII share a common antenna, closing RCII does not greatly change the absorption cross-section of the remaining open RCII. Either the probability for escape from closed traps is small, or the probabilities for escape from open and closed traps are nearly equal.

The reversible decline of oxygen flash yields at high flash energies. Evidence for total annihilation of excitations in Photosystem II

The yield of oxygen from cells of Chlorella vulgaris illuminated for 0.5 μs with 596 nm laser light reversibly declines at flash energies much greater than those required for saturation. The decline in oxygen flash yields is specifically related to the optical cross-section per Photosystem (PS) II trap. The effect can be explained as the result of a total annihilation process which occurs either at the PS II trap (with very low probability) or in the PS II antenna (with very short lifetime). Evidence from separate experiments is discussed which suggests that the process occurs at the PS II trap. The probability of this process is about 10−4.

The Distribution of Absorbed Light Energy for Algal Photosynthesis

Photosynthesis is the process through which light energy is transformed into the chemical energy useful to biological organisms. This conversion is accomplished primarily through the enzymatic reduction of carbon from the highly oxidized CO2 to the more reduced level equivalent to formaldehyde, (CH2O)n. The reactions of carbon reduction occur in the soluble phase of the cell or chloroplast and require both reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP) [1,2]. In the simple case where only the reactions of the Calvin-Benson-Bassham Cycle [2] occur, the reduction of one molecule of CO2 requires three molecules of ATP and two of NADPH. However, in many photosynthetic organisms, other biochemical pathways, such as photorespiration of “C4” metabolism [3,4,5], change the above mentioned requirements. In addition, rapid growth or “luxury accumulation” [1] may require supplemental energy inputs. Thus, the commonly quoted ratio for ATP:NADPH: CO2 of 3:2:1 must be considered a minimum for promotion of growth.