Page 106 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 89

FIGURE 2.62 Swimming pattern of Euglena gracilis.

Algae use their motility (be it sinking or swimming) to generate movement relative to the water
and hence replenish the boundary layer with nutrients. Depending on the size of the organism, the
motive for swimming must differ, however, because its effects differ significantly. For small algae
in the 1–10 mm range, diffusion is about 100 times more effective in supplying nutrients than
movement. This is often expressed as the Sherwood number (S):

S ¼ (Time for transport by diffusion) (Time for transport by movement) 1
(2:3)
1 1
2
1
¼ (L D ) (L u ) ¼ (L u) 1 D
where L is the distance over which the nutrient is to be transported, u the water velocity, and D the
diffusion constant.
2
For scale of the order of 1 mm the ratio is 10 . Diffusion is about 100 times faster than move-
ment. Hence, in this world of low Reynolds numbers, nothing is gained by trying to reduce the diffu-
sion barrier by generating turbulent advection. In this context the only possible advantage to the alga of
undertaking locomotion is that it might encounter nutrients in a higher concentration. For this purpose,
a helical swimming path is more useful than a straight one inspite of the longer distance for the same

FIGURE 2.63 Swimming pattern of Bigelowiella sp.