Impact of hypoosmotic challenges on spongy architecture of the cytoplasm of the giant marine alga Valonia utricularis.

The ultrastructure of the several micrometers thick cytoplasmic layer of the giant marine alga Valonia utricularis displays characteristics which are apparently linked with the capability of this alga to regulate turgor pressure. Transmission and scanning electron microscopy of cells prefixed in different ways, including a protocol that allows prefixation of the alga in a turgescent state, revealed a highly dendritic network of cytoplasmic strands connecting and enveloping the chloroplasts and the nuclei. Innumerable vacuolar entities are embedded in the network, giving the cytoplasm a spongy appearance. Vacuolar perfusion of turgor-pressure-clamped cells with prefixation solution containing tannic acid presented evidence that these vacuolar entities together with the huge central vacuole form a large unstirred continuum. In contrast to the tonoplast, the plasmalemma followed smoothly the lining of the cell wall, even at the numerous cell wall ingrowths. Sucrose, but not polyethylene glycol 6000, induced chloroplast clustering. Acute hypoosmotic treatment (established by reduction of external NaCl or by replacement of part of the external NaCl by equivalent osmotic concentrations of sucrose or polyethylene glycol 6000) resulted in a local relocation of the chloroplasts and cytoplasm towards the central vacuole. This effect did not occur when the relatively low reflection coefficients of these two osmolytes were taken into account. The increase in spacing between the spongy cytoplasm and the plasmalemma by chloroplast relocation (viewed by confocal laser scanning microscopy) was associated with a speckled appearance of the affected surface area under the light microscope. As indicated by electron microscopy, hypoosmotically induced chloroplast relocation resulted from disproportionate swelling of the vacuolar entities located close to the plasmalemma. The cytoskeleton in the cytoplasm and the mucopolysaccharide network in the central vacuole apparently resisted swelling of these compartments. This finding has the important consequence that relevant hydrostatic pressure gradients can be built up throughout the entire multifolded vacuolar space. This gradient could represent the trigger for turgor pressure regulation which is manifested electrically first in the tonoplast.