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Summary: Buoyancy regulation in light-limited continuous cultures of Aphanizomenon flos-aquae was studied. Gas vesicle collapse did not occur during growth under a light-dark cycle. Only cultures with growth rates less than 20% of the maximal growth rate were positively buoyant. The loss of buoyancy at higher growth rates was due to a lower gas vesicle content in the cells. Carbohydrate accumulation was the main factor which caused buoyancy loss if the cells were shifted from low to high photon flux densities. In these experiments turgor-induced gas vesicle collapse occurred only in cultures adapted to long light periods, and several hours after the cells had lost buoyancy due to ballast increase. The results are discussed in relation to adaptation patterns in photosynthesis and carbon metabolism caused by intermittent light.
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