Characteristics of two noncarotenogenic light sensitive mutants of Scenedesmus obliquus strain D₃ Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/cj82kc62s

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  • Two light sensitive mutants of the green alga Scenedesmus obliquus strain D₃ have been characterized. The photodynamic nature of their sensitivity has been established, the subchloroplast site of photodamage has been investigated, and their noncarotenogenic character has been verified. The two mutants possess several similarities, although mutant L. S. -41 demonstrates a more extreme damage in high intensity light than does mutant L. S. -4. Mutant L. S. -41 suffers the photodynamic loss of photosynthesis with an accompanying bleaching of chlorophyll. The damage to the photosynthetic electron transport system is general, comprising an initial loss of photosystem I followed by a later impairment of photosystem II. Associated with photosystem II impairment is the disappearance of recognizable α-tocopherol and plastoquinone A, which suggests the photoprotective role of these chloroplast components. Fluorescence measurements indicate that generalized irreversible disruption of the photosynthetic thylakoids is the end result of prolonged exposure to intense light. A normal complement of carotenoids was demonstrated in mutant L. S. -41 both initially and following high intensity irradiation, confirming the mutant's noncarotenogenic nature. Mutant L. S. -4 suffers a similar photodynamic impairment of photosynthesis, but this mutant gains chlorophyll content during irradiation. The damage within the photosynthetic mechanism is very specific. Results indicate an initial impairment of cyclic photophosphorylation, followed by an interruption of acyclic electron transfer immediately on the reducing side of cytochrome f. Subsequently, damage occurs to the cytochrorne f, plastocyanin, and P700 components of electron transfer. Unlike mutant L. S. -41, mutant L. S. -4 is able to recover from photoimpairment to a certain degree. Photosynthetic capacity can be fully restored under dark aerobic conditions, but those characteristics of the electron transport system dependent upon tight coupling of the two photosystems do not fully recover. Recovery may be blocked by anaerobic conditions or inhibitors of protein synthesis which interfere with the normal function of 70 S ribosome. Carotenoids, quinones, and α-tocopherol all appear in normal levels in the mutant.
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