The functional biology of Strombidium sulcatum, a marine oligotrich ciliate (Ciliophora, Oligotrichina)

Abstract

he bioenergetics and swimming behaviour of the marine oligotrich ciliate Strombidium sulcatum were studied. It grows on bacteria and heterotrophic flagellates with a maximum rate around 0.1 h-. Cell yield is 3.3 X 10-5 ciliates bacterium- and 7.7 X I O – ciliates flagellate- indicating a gross growth efficiency around 0.5. Starvation of S. sulcatum is accompanied by a 10-fold reduction in cell volume with a mean survival time of about 70 h. Clearance decreases rapidly for particles smaller than about 2 pm and cannot be detected for 0.21 pm beads. Maximum clearing rate for 2.83 pm latex beads is 1.6 1 h-, close to the clearing rates found for bacteria, (1.5p1 h-) and flagellates (2 1 h-) and also to the calculated volume of water filtered (2 5 111 h-). S. sulcatum efficiently accumulates in patches of bacteria. Analysis of ciliate behaviour revealed that this is caused by changes of swimming pattern from a helical path to a closed circle in response to bacterial density and that this response is chemically induced. This modulation of swimming behaviour when encountering a bacterial patch results in a 20- fold decrease in transportation rate with only a 40 % reduction in swimming speed. Video recordings of swimming ciliates in strobe-light suggest that the membranelles can change their position to affect the axial rotation of the ciliate. Absence of axial rotation results In a circular swimming path. In addition to this kinetic response, a transient response was observed which further enhances the probability of finding and remaining in bacterial patches. It was also found that S. sulcatum changes its vertical distribution in response to nutntional status; thls is at least in part due to the lunetic response, but change in cell morphology may also play a role. Findings are interpreted as adaptations for exploiting heterogeneous environments. The scale of the patchiness which the ciliate can exploit is about 1 m.