Fig. 6. Scheme of main supposed ways
of energy deactivation in the PSI of thermophilic cyanobacteria upon heating
above 60oC. Reactions activated and suppressed during heating are
marked by thick and dotted lines, respectively. Carotenoids are inactivated
upon heating to 60oC (reaction 1). Heating from 60 to 80oC
gradually inactivates forward electron transport to iron-sulfur clusters of
[4Fe-4S] type (reaction 2) and further electron transfer to ferredoxin and
oxygen (reaction 3). The rates of backward electron transfer in PSI (reactions
4 and 5), as well as the rate constant of triplet-singlet conversion in excited
chlorophyll molecules (reaction 7), rise. These thermodependent changes in PSI
enhance both the generation of singlet oxygen (reaction 6) and fluorescence
yield (reaction 8).