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 ( σ O 2 ) 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. σ O 2 at the laser wavelength (596 nm) increased from 38 Å 2 for cells grown in high light to 115 Å 2 for cells grown in low light. The 3-fold variation in σ O 2 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 ( σ Chl ) is 0.29 Å 2, 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 (PSU O 2 ) 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.