Fluorescence Induction and Relaxation (FIRe), K. Cayemitte, Nov 26, 2024

Maxim Gorbunov

Gorbunov, Maxim Y., and Paul G. Falkowski. “Using chlorophyll fluorescence kinetics to determine photosynthesis in aquatic ecosystems.” Limnology and Oceanography 66.1 (2021): 1-13.

Gorbunov, Maxim Y., and Paul G. Falkowski. “Using chlorophyll fluorescence to determine the fate of photons absorbed by phytoplankton in the World’s oceans.” Annual Review of Marine Science 14 (2022): 213-238.

Phytoplankton physiology and chlorophyll variable fluorescence: how it works and how we can do better

Variable fluorescence

1. Based on measurements of fluorescence on microsecond to mili second
2. First phase-100 microseconds (powerful excitation to close action centers)
3. Fully closed by the second phase
4. F_o - baseline fluorescence, some reaction centers are open and some are closed started conditions
5. F_m - saturating light so all reaction centers close and all energy is released as fluorescence
6. F_v - variable fluorescence
7. F_v/F_m - main parameter, quantum efficiency of photosystem 2
8. You want to get an additional parameter to supplement the information
9. As part of energy photochemistry, some is lost as heat and as fluorescence A. In chlorophyll, you have antennas that turn light into energy B. How much of the energy C. Thermal dissipation (photoacoustics- impossible to use in the ocean), Fluorescence, Photosynthetic efficacy = 1 (energy budget) D. FIRe gives you a sense of whether systems are working or not
10. The large size of the antenna = proportional to Alpha
11. Cross section slow rise of alpha-model photosynthetic rates of variable photosynthesis
12. At the end of the second impulse, turn off and look at the relaxation of photosystem II of F_m to F_o (using large flashes of light) - longer time scale (milliseconds)

Second-lifetime measurements

1. Fluorescence is short picoseconds
2. Picosecond time scale
3. Very fast phenomena
4. First process of photosynthesis to get info about other stages
5. Million times faster of kinetic (higher resolution)

Light Curves

1. Photosynthesis is a function of irradiance
2. ETR - electron transport rates (photosynthetic rates) number of elections per second per reaction center
3. Initial slope and maximum photosynthesis = complete picture A. The initial slope is controlled by F_v/F_m- maximum quantum yield f photochemistry in PSII, controlled by photosynthetic turnover rates - how fast the entire chain of secondary photochemical reactions, rate limiting process - limits and controls the amount of light absorbed
4. Low PAR - the initial slope is limited light absorption
5. High PAR - controlled by photosynthetic turnover rate - How much the energy absorbed and stored can be used in photosynthesis
6. Maximum Turnover rates (1/tau) is important to measure/model PP rates in water column-integrated rates

Rationale For using FIRe variable fluorescence to derive photosynthetic rates

1. Higher photosynthesis, less fluorescence, if heat dissipation
2. This is the reason we measure F_v/F_m over heat dissipation
3. Under low light 30% quantum yield of fluorescence is usually small compared to the quantum yield of photosynthesis (70%), The quantum yield of heat is usually 5%
4. Under low light, the ideal 60% is dissipated as heat, only 30% is used in photosynthesis
5. This switch is due to nutrients
6. Under more and more light cells have active photoprotective pigment= more energy dissipated as heat

Nonphotochemical quenching (NPQ) - Thermal dissipation when you increase light = photoprotective mechanism, fluorescence goes down and competes with what dissipation

1. NPQ is not the whole thermal dissipation
2. NPQ is an increase in thermal dissipation
3. NPQ affects F_m, F’, and F_o - these all become close to each other when they get closer
4. So most of the dissipated energy is heat
5. How would we measure PS from this? A. ETR(e)= E (how much light is absorbed by photosystem II) x Sigma PSII (functional absorption cross-sectional from photosystem II x the number of photosystems that are open

Limation of this amplitude-based F~v~ technique

1. Photosynthetic rates are not measured directly from F_v signals, the rates are models
2. Many parameters in the model have many sources of errors

Kinetic analysis

1. Deduced from RIE relaxations record under ambient light
2. Photosynthetic rates (1/tau)
3. Low light= fast
4. Photosynthetic rates approach photosynthetic turn which is how you get tau

Using F_v/F_m

1. F_v/F_m could be used as an indicator of nutrient stress A. Relationship better F_v/F_m and growth rates is highly non-linear B. Not strong
2. A new kinetic approach was better for predicting nutrient stress in phytoplankton A. Based on the photosynthetic turnover rate
3. Conclusion A. think beyond F_v/F_m and look at all photosynthetic parameters B. Use a combination of amplitude-based and kinetic analyses in the new mimi-FIRE to offer a significant improvement in the capability of F_v for assessment of absolute photosynthetic rates, growth, NPP, and relation to N and Fe stress in the ocean
   1. F_m can be a proxy for chlorophyll or algae- yes but you may have saturation at a certain point
   2. F_o can be used to determine inactive reaction centers
   3. F_v, F_m, F_v/F_m
   4. Sigma - cross-sectional photosynthetic rates
   5. Tau - photosynthetic turnover C. If coral is acclimated to low light, P_max may not be D. The most accurate picture if you make 2 measurements- shaded colony and high light acclimated colony for a full picture E. Measuring reaction center recovery time could be a parameter for health F. For thermal stress in corals, tau will be affected by thermal stress so we can examine how they react G. Low temp

Other considerations

1. What is the extent of nutrient limitation in tropics and how it effects zoox growth rates?
2. Relates to upwelling conditions which bring high nutrient conditions
3. Subject to hugh nutrient and cool temps- simulate upwelling
4. Subject to low nutrient arm water in lab