The relative form resistance of straight and helical blue-green algal filaments

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Abstract
A helical filament mutant was isolated from a clonal culture of the gas-vacuolate blue-green alga Anabaena flos-aquae which had originally been established from a single straight filament. Further mutants, lacking gas vacuoles, were isolated from the straight and helical filament cultures and their sinking velocities were compared on linear sucrose density gradients. Helical filaments showed a progressive increase in sinking velocity with length, from 4·6 μm s-1 for 10–20 cell chains to 8·1 μm s-1 for 40–50 cell chains. The sinking velocities of straight filaments of comparable length varied over a much smaller range, 3·9 to 4·5 μm s-1. The differences between the velocities of the helical and straight filaments is attributed to their shape-dependent form resistance, rather than differences in the shape or density of individual cells. The helical form may have advantages in natural waters but in culture straight filament revertants develop and out-compete the helical form.