ALGAL PHOTOSYNTHETIC MEMBRANE COMPLEXES AND THE PHOTOSYNTHESIS‐IRRADIANCE CURVE: A COMPARISON OF LIGHT‐ADAPTATION RESPONSES IN CHLAMYDOMONAS REINHARDTII (CHLOROPHYTA)1

Abstract

ABSTRACTChlamydomonas reinhardtii was grown at photon flux densities (PFDs) ranging from 47 to 400 μE.m‐2 s‐1. The total cellular content of chlorophyll (Chl) was twice as high in the low light (LL) versus high light (HL) grown cells. On an equal Chl basis, photosystem II (PSII) and cytochrome f (Cyt f) content was higher in HL cells, but photosystem I (PSI) concentration displayed little variation with the light intensity during cell growth. Consequently, there was a shift in the ratio of PSII / PSI and Cyt / PSI from near unity in LL cells to greater than two in HL cells. The functional Chl antenna size of PSII and PSI ranged from 460 and 170 Chl (a + b)in HL‐grown cells to 620 and 370 Chl (a+ b)in LL‐grown cells, respectively. The initial slope of the Chl‐specific photosyn‐thesis‐irradiance (P‐I) curve was similar in LL‐ and HL‐grown cells, but the light saturated rate of photosynthesis was lower under LL. The response to low light was beneficial at the cellular level, since there was an enhancement of photosynthesis in LL. The PFD for the onset of light saturation, 1 was a factor of 2 lower in LL‐ relative to HL‐grown photosythetic membranes. Since growth PFD varied by a factor of ten, photosynthesis shifted from being light‐limited in the LL regime to light‐saturated in the HL regime. The requirement for balanced absorption of light by the two photosystems constrains the PSII / PSI ratio to near unity when growth is light‐limited, but such a constraint does not apply in HL conditions. Instead the concentration of individual electron transport complexes way be related to the pool size necessary for maximum rates of steady‐state electron transport. Thus the stoichiometry of electron transport complexes changes in response to growth PFD and this change is correlated with the response flexlbility of algal photosynthesis in diverse light environments.