Decrease of polypeptides in the PS I antenna complex with increasing growth irradiance in the red alga Porphyridium...

Area of Science:

• Photosynthesis research
• Algal physiology
• Plant biochemistry

Background:

• Thylakoids from red algae (Porphyridium cruentum) show increased Photosystem I (PS I) activity with higher growth light.
• Photosystem stoichiometry remains largely unchanged despite varying irradiance levels.
• This suggests adaptive changes in PS I's light-absorbing capacity.

Purpose of the Study:

• To investigate the reasons behind increased PS I activity in red algae under high light.
• To determine if changes in chlorophyll antenna size or polypeptide composition of PS I holocomplexes are responsible.
• To explore the role of light-harvesting complex I (LHC I) in this adaptation.

Main Methods:

• Isolation of thylakoids and PS I holocomplexes from Porphyridium cruentum grown at different irradiances (10 and 280 μmol photons·m⁻²·s⁻¹).
• Measurement of PS I activity and chlorophyll (Chl) to P700 ratios.
• Analysis of polypeptide composition using SDS-PAGE and immunological cross-reactivity with LHC II antisera.

Main Results:

• PS I activity was 26% higher in high-light (VHL) acclimated cells compared to low-light (LL) cells.
• PS I holocomplexes from VHL cells had a lower Chl/P700 ratio (132±9) than LL cells (165±4).
• VHL-complexes showed a decrease in three specific polypeptides (19.5, 21.0, 22 kDa) associated with LHC I.

Conclusions:

• The observed decrease in PS I chlorophyll antenna size under high irradiance is attributed to modifications within the LHC I complex.
• Red algae possess a unique LHC I complex that differs from LHC II found in other organisms.
• No evidence suggests a Chl-binding antenna complex associated with PS II core complexes in this species.