Dynamic response of AFA to light/dark transitions

Paper 2

The transient response for these cultures going from light to dark and dark to light showed a lag that was not consistent with previous reports (Figure 5). This response is much different than the post-illumination enhancement of respiration rate described by (Beardall et al. ****). The response shows that the O2 concentration in the cuvette continues to climb after the lights are turned off, reaching a value almost 1 nmol mL^-1 higher than when the lights were on. Eventually the steady state O2 consumption rate resumes. A regression on this O2 consumption line intersects the time = 8 value at an O2 concentration higher than when the light was on, indicating that there is some "extra" oxygen introduced to the cuvette.

Figure 5. An example of the transient response. The cells were in the light until time = 8 minutes and then in the dark.

estimating internal O2 from these data and the transient response

Raven estimates for permeability, internal and external concentrations not much different

figure 5 46data compared to xls model

verify with model time course - very rapid

compared to actual transient data

L to D, continued O2 production after light is shut off

electrode response

other cause - hypothesize H2O2 production and SOD

Bader 1994 - O2/H2O2 cycling in cyanobacteria -- not seen in higher plants, ??what was the time scale??

hypothesis 1 - permeability

calculations on permeability, and model show that it would take very high concentrations of O2 in the cell

 

figure ** - Cartoon of permeability hypothesis

figure ** Simulation of permeability.

photo_rate 5 nmol min^-1 resp_rate -1 nmol min^-1 P 2.00E+02 nmol cm^-1 min^-1 cell_vol 6.00E-04 mL cell_area 1.20E-04 cm^2 dry_weight 3.50E-05 g mL^-1 change_fraction 20

hypothesis 2 - H2O2

ROS as by product of Mehler rxn build up and go to H2O2 which diffuses out into the media

figure ** - Cartoon of H2O2 hypothesis

 

figure ** Simulation of H2O2 production and destruction.

addition of catalase

measurement of H2O2 shows that there is this amount in the media

importance of H2O2

hypothesized mechanism adapted from Bagchi and Kleiner

NH40H build up
NH3 uncoupling of PSI to make more H2O2
production of NO2 which is less toxic than either NH3 or H2O2

require more respiration, slows down e- transport to NADP+ and causes some e- to be dumped onto O2 through SOD to H2O2

 

Effects of stress on respiration

increased oxygen in the cell or oxidative stress might trigger something like the oxyR response in other eubacteria ??? (Neidhardt et al 1990 - page 355) which triggers about 12 genes for protection from H2O2 and other similar oxidants

NO2 effect