Photosynthetic units

The increases in photosynthetic pigment content that occur in algae as the light intensity at which they are grown is lowered can be due to an increase (per cell, or per unit biomass) in the number of photosyn-thetic units, or in the average size (as absorption cross-section) of the photosynthetic unit, or both.381 In most green plants - the algae as well as the angiosperms - it appears that the increase in chlorophyll content during shade adaptation is largely due to an increase in the number of photosynthetic units. This has been shown for higher plants,128 for the unicellular chlorophytes Scenedesmus obliquus400 and Dunaliella tertiolecta,383 and for the multicellular green species Ulva lactuca910 In Chlorella pyrenoidosa the five-fold increase in chlorophyll during shade adaptation was mainly due to an increased number of photosynthetic units, but there was also a 50% increase in the number of chlorophyll molecules per unit.971 In Chlamydomonas reinhardtii, Neale and Melis (1986) found an actual change in the proportion of photosystem I and photosystem II reaction centres as light intensity during growth was altered. The high-light cells, with half the chlorophyll content of the low-light cells, contained slightly less than half as many photosystem

I centres, but almost as many photosystem II centres, as the low-light cells. The photosystem II/photosystem I ratio shifted from near unity in the low-light cells to greater than two in high-light cells.

In the diatoms Skeletonema costatum383 and Chaetoceros danicus1049 it appears that the increase in cellular chlorophyll during shade adaptation is mainly due to an increase in the number of chlorophyll a molecules per photosynthetic unit, and the same appears to be true of the chrysophyte, Isochrysis galbana.1049 The diatom Phaeodactylum tricornutum, on the other hand, responds to low light by increasing the number of photosyn-thetic units per cell, without increasing the unit size.413

In the marine dinoflagellate, Glenodinium, it does seem likely that the great increase (seven-fold) in the cellular level of the peridinin-chlorophyll a protein resulting from a l2-fold decrease in growth irradiance,1080 especially when compared with the comparatively modest increase in chlorophyll (80%), is associated with a substantial increase in the number of these pigment-protein molecules per photosynthetic unit. Shade adaptation in the symbiotic dinoflagellates (zooxanthellae) of coral is seemingly due to an increase in the size, but not the number, of photosynthetic

units per cell. , In the estuarine dinoflagellate Prorocentrum mariae-lebouriae, however, shade adaptation appears to involve increases in both the size and the number of photosynthetic units.250

In the unicellular red alga Porphyridium cruentum, Levy and Gantt (1988) found that acclimation to low light intensity was accompanied by slightly more than a doubling of biliprotein content, but little change in the amount of chlorophyll or number of photosynthetic units. They concluded that adaptation of this alga to varying light levels involved changes in size of the photosystem II antenna, with little effect on photosystem I. In cases such as Gracilaria in the Adriatic Sea referred to above, where shading of macrophytic red algae brings about massive increases in phycoerythrin content, this must be accompanied by increases in the average light-harvesting capacity of the photosynthetic unit. The same is probably true in low-light-grown cultures of the cryptophyte Cryptomonas, which show a six-fold increase in phycoerythrin (relative to high-light cultures) compared with only two-fold increases in chlorophylls a and c.1351

Shade adaptation in the cyanobacterium Anacystis nidulans, comparing cells grown at 10 mmol photons m~2s_1 to those grown at 100 mmol photons m~2 s_1, was accompanied by a doubling in the number of photo-synthetic units per cell, but with no change in the number of chlorophyll molecules per photosynthetic unit: the number of phycocyanin molecules per cell, however, tripled.1413 In cultures of the very common bloom-forming cyanobacterium Microcystis aeruginosa, shade adaptation led to a 2^-fold increase in the number of photosynthetic units per cell but with little change in the number of chlorophyll molecules per unit.1107 Raps et al. (1985) found the low-light (40 mmol photons m~2s-1) cells to have a 2.6-fold greater concentration of phycobilisomes than the high-light (270 mmol photons m~2s_1) cells: phycobilisome structure and composition (phycocyanin/allophycocyanin) were the same at both light intensities. In contrast, with a marine Synechococcus strain, Kana and Glibert (1987a) found considerable changes in phycobilisome composition as growth irradiance varied. Between 700 and 30mmolphotonsm~2s_1, the phycoerythrin/phycocyanin ratio rose from 3 to 14. The phycoerythrin content of the cells increased 20-fold over this range, while chlorophyll content only doubled, suggesting a major increase in the average absorption cross-section of the photosynthetic units.

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