Gas Vacuoles

INTRODUCTION is attributable to the parietal distribution of the vacuoles within the cells of the particular strain of blue-green alga used in these experiments . Gas vacuoles are small, cylindrical, gas-filled vesicles which occur in certain procaryotic cells . They are found in numerous blue-green algae, many photosynthetic bacteria, some halophilic bacteria, and some planktonic freshwater bacteria (3, 7-9, 11, 13) . Because gas-vacuolate blue-green algae are often observed floating at the surface of water where high light intensity may damage cells, Lemmerman (4) first suggested that gas vacuoles may function as light-shielding organelles . Recently, it has been suggested that such light shielding (if it occurred) might be due to the optical properties and the intracellular distribution of the gas vacuoles (12, 16) . Being gas filled, these vacuoles have a refractive index much lower than that of the cytoplasm which surrounds them, and therefore they scatter light . A suspension of gasvacuolate cells becomes visibly less milky and more transparent when the gas vacuoles are collapsed . Similarly, when a milky-white, opalescent suspension of isolated gas vacuoles is subjected to sudden pressure, a completely transparent suspension results . These changes are due to a decrease in light scattering when gas vacuoles collapse . But, although light scattering by gas vacuoles could protect cells by scattering away a large portion of the incident radiation, it could also lead to increased rather than decreased irradiation of some cellular components if the light were back-scattered into these components . Whether gas vacuoles function as light shields would depend upon the distribution of the gas vacuoles inside the cells . In this paper, we present spectrophotometric evidence that the presence of inflated gas vacuoles in Nostoc muscorum does actually reduce the amount of light absorbed by the photosynthetic pigments . This reduced absorption shows that the gas vacuoles may indeed function as light shields and MATERIALS AND METHODS