FIGURE 10.

Schematic drawing of the contraction pattern of Zoothamnium niveum. The fast contraction (520 mm s^-1) brings the colony in the sulfidic boundary layer (pink), then a slow extension movement bring it back to the oxygenated water (blue) dragging along sulfide from the boundary layer. Once extended, the cilia from the microzooids (insert) start beating again creating a toroidal vortex around the upper part of the cell (curved arrows). This current allows the cell to filter the surrounding water to gain food and, as a side effect, it also mixes the sulfide and the oxygen allowing the ectosymbionts to access both the electron donor and the electron acceptor. The beating of the cilia from all the cells of the colony also creates a general current perpendicular to the long axis of the colony (long arrows). Modified from Vopel et al. (2002).