Distribution of excitation energy in photosynthesis: a study of heat loss, photo chemical activity and fluorescence emission in intact calls of cyanobacterium.
Abstract
Photosynthetic state transitions were investigated in the
cyanobacterium Synechococcus sp. PCC 6301 by studying
fluorescence emission, heat loss, and PS I activity in intact cells
brought to state 1 and state 2. 77K fluorescence emission spectra
were modelled with a sum of 6 components corresponding to PBS, PS
II, and PS I emissions. The modelled data showed a large decrease in
PS II fluorescence accompanied with a small increase in the PS I
fluorescence upon transition to state 2 for excitation wavelengths
absorbed by both PBS and ChI ll.. The fluorescence changes seen with
ChI .a. excitations do not support the predictions of the mobile PBS
model of state transition in PBS-containing organisms. Measurements
of heat loss from intact cells in the two states were similar for both
ChI it. and PBS excitations over three orders of magnitude of laser
flash intensity. This suggests that the PBS does not become decoupled
from PS II in state 2 as proposed by the PBS detachment model of
state transition in PBS-containing organisms. PS I activity
measurements done on intact cells showed no difference in the two
states, in contrast with the predictions of all of the existing models of
state transitions.
Based on these results a model for state transition In PBScontaining
organisms is proposed, with a PS II photoprotectory
function.