Sodium and Potassium in Normal and ParalyzedChlamydomonas

The potential effect of low cell osmolarity on cell function through decreased concentration of enzyme substrates

Some freshwater algae have lower (<130 osmol m-3) intracellular osmolarities than most others (>180 osmol m-3). Low osmolarities are related to the presence of flagella and the low energy cost of active water efflux following downhill water influx unconstrained by cell walls covering the plasmalemma, and the low resource cost of cell wall synthesis with the same mechanical degree of safety. One consequence of low intracellular osmolarity is limitation on the concentration of metabolites, that is, substrates and products of enzyme activity. Models of the flux through metabolic pathways, and hence the specific growth rate, using steady-state concentrations of enzymes and metabolites have involved organisms with intracellular metabolite osmolarities >280 osmol m-3, where the metabolite concentrations are much greater than the total osmolarity of some freshwater algae. Since the protein concentration (mol m-3) in the cells and the specific growth rates of freshwater cells with low and with higher intracellular osmolarity are highly similar, the models of trade-offs between enzyme and metabolite concentrations for cells with high intracellular osmolarity need modification for cells with low intracellular osmolarity. The soluble free-radical scavenger ascorbate can constitute as little as 0.2% of the low intracellular metabolite concentration (mol m-3) of low-intracellular-osmolarity cells.

H+- and Na+- elicited rapid changes of the microtubule cytoskeleton in the biflagellated green alga Chlamydomonas

Although microtubules are known for dynamic instability, the dynamicity is considered to be tightly controlled to support a variety of cellular processes. Yet diverse evidence suggests that this is not applicable to Chlamydomonas, a biflagellate fresh water green alga, but intense autofluorescence from photosynthesis pigments has hindered the investigation. By expressing a bright fluorescent reporter protein at the endogenous level, we demonstrate in real time discreet sweeping changes in algal microtubules elicited by rises of intracellular H⁺ and Na⁺. These results from this model organism with characteristics of animal and plant cells provide novel explanations regarding how pH may drive cellular processes; how plants may respond to, and perhaps sense stresses; and how organisms with a similar sensitive cytoskeleton may be susceptible to environmental changes.

Genetic control of flagellar structure inChlamydomonas reinhardii

Seventeen non-motile strains ofChlamydomonas reinhardiiisolated by Dr Lewin and thirty-six newly isolated strains have been examined in the electron microscope for structural abnormalities of the flagella. Fourteen of them have straight, paralysed flagella in which the central fibres are replaced by an irregular core of disorganized material. The fourteen mutations map at four unlinked loci. Some of them are leaky; light and electron microscope observations on leakiness are described. In the former case leakiness is measured by the proportion of motile cells, and in the latter by the proportion of intact centre fibres seen in transverse sections. The degree of leakiness is to some extent characteristic of particular loci.

A partial suppressor of some of these mutations has been isolated which acts on all the mutant alleles at two loci and to a much lesser extent on four out of five alleles at a third locus.