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{| class="model-ib" style="float:right; margin-left:1em; margin-bottom:1em;" ! Photosystem stoichiometry (PSII/PSI) |- | '''Approach:''' immunodetection or targeted mass spectrometry of PSII/PSI proteins |- | '''Context:''' ''in situ'', incubation, lab |- | '''Spatial sc..."

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* [[Page authors|Page authors]]: [[PRIMO]]
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{| class="model-ib" style="float:right; margin-left:1em; margin-bottom:1em;"
! Photosystem stoichiometry (PSII/PSI)
|-
| '''Approach:''' immunodetection or targeted mass spectrometry of PSII/PSI proteins
|-
| '''Context:''' ''in situ'', incubation, lab
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| '''Spatial scale:''' point sample
|-
| '''Temporal scale:''' minutes to hours
|-
| '''Units:''' mol protein per cell; mol protein L<sup>-1</sup>; protein mass fraction
|-
| '''Community captured:''' bulk to group-specific
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| '''Co-measurements:''' Single Turnover Chlorophyll Fluorescence; <sup>14</sup>C or <sup>13</sup>C fixation; total protein
|}
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== 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 (F<sub>v</sub>/F<sub>m</sub>, σ<sub>PSII</sub>) to constrain PSII electron transport rates and, ultimately, carbon fixation<ref name="Campbell2003">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</ref>.

=== 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<sup>-1</sup>, 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'' <ref name="Campbell2003" />

=== Recent applications ===
* Macey et al. (2014) ''Photosynthetic protein stoichiometry and photophysiology in the high latitude North Atlantic'' <ref name="Macey2014">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</ref>

=== Common calculations/conversions ===
* ETR<sub>PSII</sub> per unit volume = [PSII] × k<sub>PSII</sub>; where k<sub>PSII</sub> (s<sup>-1</sup>) is the PSII-specific photochemical rate derived from σ<sub>PSII</sub> and irradiance.

== References ==

[[Category:Main Pages|Model types]]

PSII/PSI quantification - Revision history