PSII/PSI quantification
Template:BreadcrumbsPrimaryProduction
| Photosystem stoichiometry (PSII/PSI) |
|---|
| Approach: immunodetection or targeted mass spectrometry of PSII/PSI proteins |
| Context: in situ, incubation, lab |
| Spatial scale: point sample |
| Temporal scale: minutes to hours |
| Units: mol protein per cell; mol protein L-1; protein mass fraction |
| Community captured: bulk to group-specific |
| Co-measurements: Single Turnover Chlorophyll Fluorescence; 14C or 13C fixation; total protein |
Method Overview
The reaction center proteins of photosystem II (D1, encoded by psbA) and photosystem I (PsaA/PsaB) are quantified by immunodetection (Western blotting using global antibodies) or targeted proteomics by mass spectrometry. The PSII:PSI stoichiometry reflects light-acclimation state: high-light-acclimated cells typically have lower PSII:PSI ratios than low-light-acclimated cells. Community-level photosystem stoichiometry can be linked to variable fluorescence measurements (Fv/Fm, σPSII) to constrain PSII electron transport rates and, ultimately, carbon fixation[1].
Scale of measurement
Each filtered sample provides a point measurement. Protein turnover is slower than mRNA, so photosystem protein concentrations integrate the physiological state over hours to days. Diurnal patterns in PSII:PSI ratios have been observed in some taxa.
Data generated
Absolute or relative concentrations of D1, PsaA, and PsaB proteins per cell or per unit volume. The PSII:PSI ratio is used to characterize light-acclimation state. When combined with carbon fixation data, the protein-specific rate of carbon fixation can be derived.
Units & currency
Units are mol protein per cell, mg protein L-1, or protein mass fraction. The currency is protein (carbon fixation is derived).
Sample size
Typical samples are ~1 L in volume.
Repositories & databases
Limitations
Peptide standards and antibodies may not capture the full diversity of D1, PsaA, and PsaB variants across the community. Post-translational modifications affect antibody recognition and activity. Total protein per liter is difficult to measure accurately in dilute open-ocean waters. Models linking photosystem stoichiometry to carbon fixation rates are still being developed and validated.
Example Applications & Protocols
Classic examples
- Campbell et al. (2003) Analysing photosynthetic complexes in uncharacterized species or mixed microalgal communities using global antibodies [1]
Recent applications
- Macey et al. (2014) Photosynthetic protein stoichiometry and photophysiology in the high latitude North Atlantic [2]
Common calculations/conversions
- ETRPSII per unit volume = [PSII] × kPSII; where kPSII (s-1) is the PSII-specific photochemical rate derived from σPSII and irradiance.
References
- ↑ 1.0 1.1 Campbell, D. A., Cockshutt, A. M., & Porankiewicz-Asplund, J. (2003). Analysing photosynthetic complexes in uncharacterized species or mixed microalgal communities using global antibodies. Physiologia Plantarum, 119(3), 322–327. https://doi.org/10.1034/j.1399-3054.2003.00175.x
- ↑ Macey, A. I., Ryan-Keogh, T., Richier, S., Moore, C. M., & Bibby, T. S. (2014). Photosynthetic protein stoichiometry and photophysiology in the high latitude North Atlantic. Limnology and Oceanography, 59(6), 1853–1864. https://doi.org/10.4319/lo.2014.59.6.1853