Cells of the red alga Porphyridium cruentum (ATCC 50161) exposed to increasing growth irradiance exhibited up to a threefold reduction in photosystems I and 11 (PSI and PSII) and phycobilisomes but little change in the relative numbers of these components. Batch cultures of P. cruentum were grown under four photon flux densities of continuous white light; 6 (low light, LL), 35 (medium light, ML), 180 (high light, HL), and 280 (very high light, VHL) microeinsteins per square meter per second and sampled in the exponential phase of growth. Ratios of PSII to PSI ranged between 0.43 and 0.54. About three PSII centers per phycobilisome were found, regardless of growth irradiance. The phycoerythrin content of phycobilisomes decreased by about 25% for HL and VHL compared to LL and ML cultures. The unit sizes of PSI (chlorophyll/P700) and PSII (chlorophyll/QA) decreased by about 20% with increase in photon flux density from 6 to 280 microeinsteins per square meter per second. A threefold reduction in cell content of chlorophyll at the higher photon flux densities was accompanied by a twofold reduction in a-carotene, and a drastic reduction in thylakoid membrane area. Cell content of zeaxanthin, the major carotenoid in P. cruentum, did not vary with growth irradiance, suggesting a role other than light-harvesting. HL cultures had a growth rate twice that of ML, eight times that of LL, and slightly greater than that of VHL cultures. Cell volume increased threefold from LL to VHL, but volume of the single chloroplast did not change. From this study it is evident that a relatively fixed stoichiometry of PSI, PSII, and phycobilisomes is maintained in the photosynthetic apparatus of this red alga over a wide range of growth irradiance.An ability to adjust the composition of the photosynthetic apparatus to achieve a more efficient harvesting oflight energy is expected to be of significant advantage to organisms which are subjected to long-term variations in the light environment (22). An extensive literature has accumulated on the physiological responses of marine algae and other photosynthetic organisms to changes in intensity and wavelength of the incident light (1,22 this phenomenon in the Rhodophyceae and, iwpaticilar, in the unicellular red alga Porphyridium cruentum (2,3,17,20,(24)(25)(26)38). These earlier studies have examined acclimation in P. cruentum primarily from the perspective of changes in function or activity, including such aspects as action spectra (38) effective absorption cross-sections (25) fluorescence yield spectra (26) and photosynthesis-irradiance curves (24). From previous work in our laboratory (24), it appears that growth under continuous light of a certain, fixed intensity results in physiological changes of advantage to P. cruentum. Cultures grown under low light intensities exhibit a very low compensation point which allows for a net carbon gain under severely limiting quantum flux. Conversely, those cultures grown under high PFD3 have a much greater photosynthetic capacity.The present w...

SynopsisA starch graft poly(methy1 acrylate) copolymer was developed having grafted side chains with molecular weight of less than 500,ooO. This material can be easily extruded into a film which shows excellent initial tensile strength and elongation. Tensile strength, however, falls off rapidly after 70 hr of water immersion at 25OC. Starch graft poly(methy1 acrylate) films show excellent susceptibility to fungal growth, some samples losing more than 40% of their weight after 22 days of incubation with Aspergillus niger. Tensile tests and scanning electron micrographs of the incubated samples, after being freed of mycelium, indicate substantial biodegradation of the starch portion of the copolymer. This material may have application as a biodegradable plastic mulch.

Acclimation of the photosynthetic apparatus to light absorbed primarily by photosystem I (PSI) or by photosystem 11 (PSII) was studied in the unicellular red alga Porphyridium cruentum (ATCC 50161). Cultures grown under green light of 15 microeinsteins per square meter per second (PSII light; absorbed predominantly by the phycobilisomes) exhibited a PSII/PSI ratio of 0.26 ± 0.05. Under red light (PSI light; absorbed primarily by chlorophyll) of comparable quantum flux, cells contained nearly five times as many PSII per PSI (1.21 ± 0.10), and three times as many PSII per cell. About 12% of the chlorophyll was attributed to PSII in green light, 22% in white light, and 39% in red light-grown cultures. Chlorophyll antenna sizes appeared to remain constant at about 75 chlorophyll per PSII and 140 per PSI. Spectral quality had little effect on cell content or composition of the phycobilisomes, thus the number of PSII per phycobilisome was substantially greater in red light-grown cultures (4.2 ± 0.6) than in those grown under green (1.6 ± 0.3) or white light (2.9 ± 0.1). Total photosystems (PSI + PSII) per phycobilisome remained at about eight in each case. Carotenoid content and composition was little affected by the spectral composition of the growth light. Zeaxanthin comprised more than 50% (mole/mole), ,8-carotene about 40%, and cryptoxanthin about 4% of the carotenoid pigment. Despite marked changes in the light-harvesting apparatus, red and green light-grown cultures have generation times equal to that of cultures grown under white light of only one-third the quantum flux.

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