The response of the photosynthetic apparatus of three marine algae to growth irradiance variations

The response of the photosynthetic apparatus of three marine algae, Macrocystis pyrifera (Phaeophyta), Cylindrotheca fusiformis (Bacillariophyta) and Dunaliella salina (Chlorophyta) to growth irradiance was investigated. The stoichiometry of Photosystem II (PSII) and PSI reaction centers varied in response to growth irradiance. In Macrocystis, the PSII/PSI ratio varied from 1.4 in winter to 2.5 in summer surface blades. Cylindrotheca exhibited a more extreme response to growth irradiance. High-light grown cells displayed a PSII/PSI ratio of 1.3 and low-light cells had a PSII/PSI ratio of 3.9. Under moderate light, the PSII/PSI ratio in Dunaliella was 1.4. In very high light cells, the PSII/PSI ratio was 3.9. Light-harvesting chlorophyll (Chl) antenna size, or absorption cross section, also varied in response to growth irradiance. In Macrocystis, {dollar}-{dollar}20 m blades showed a larger absorption cross section of PSII (by about 85%) and PSI (by about 50%) compared to surface blades. Cylindrotheca also showed larger PSII (100%) and larger PSI (30%) absorption cross section in low light versus high light grown cells. In Dunaliella, PSII exhibits heterogeneity of Chl antenna size. In moderate light grown cells, approximately 60% of the total PSII centers were PSII{dollar}alpha{dollar} (containing 500 Chl molecules) and 40% were PSII{dollar}beta{dollar} centers (containing 125 Chl). At very high light intensities, PSII{dollar}alpha{dollar} centers were absent and the proportion of PSII{dollar}beta{dollar} centers was less than 5% of the total PSII. The remainder (95%) of the PSII centers contained only 50 Chl a molecules (PSII-50). The Chl antenna of PSI showed a reduction in size with growth irradiance: from 220 Chl molecules in low light to 150 Chl in moderate light, 110 Chl in high light and only 95 Chl in very high light thylakoids. In summary, the response of the photosynthetic apparatus of marine algae to growth irradiance involves changes in the stoichiometry and chlorophyll antenna size of the photosystems. Both parameters are adjusted and optimized in response to irradiance to improve photosynthetic efficiency.