Significance of swimming and feeding currents for nutrient uptake in osmotrophic and interception feeding flagellates

Abstract

We introduce 2 simple models for the flow generated by a self-propelled flagellate: a sphere propelled by a cylindrical flagellum and one propelled by an external point force. We use these models to examine the role of advection in enhancing feeding rates in 3 situations: (1) osmotroph feeding on dissolved molecules, (2) interception feeding on non-motile prey particles, and (3) interception feeding on motile prey (such as bacteria). We show that the Sherwood number is close to unity for osmotrophic flagellates, as suggested by most previous models. However, a more correct representation of the flow field than that predicted by a naive sinking sphere model leads to substantially higher clearance rates for interception-feeding flagellates. We find that a short flagellum is favourable for interception feeding, but at the cost of an increase in the drag on the body of the swimming unicell. We finally demonstrate that prey motility significantly enhances prey encounter rates in interception-feeding flagellates and is, in fact, often much more important for food acquisition than the feeding current.

Prey detection in a cruising copepod

“…This is well described for cruising fish larvae that localize prey visually [9] and for cruising copepods that perceive prey by fluid signals [5], as suggested here for the detection of escaping nauplii. Flagellates that were formerly believed to be interception feeders [10] have since turned out to encounter prey by other means [11]. Small aquatic predators approaching or capturing prey have to 'trick' viscosity in various ways to overcome the boundary layer effect.…”

Feeding currents facilitate a mixotrophic way of life

“…The advective enhancement of nutrients because of swimming/feeding currents is quantified by the Sherwood number, with Sh ¼ 1 for no enhancement relative to pure diffusion (KarpBoss et al, 1996). For an organism moved by a body force (such as gravity), Sherwood numbers are close to unity for the Reynolds numbers considered here, whereas it is higher for a self-propelled organism, as suggested by theoretical models based on idealized flows (Magar et al, 2003;Magar and Pedley, 2005;Langlois et al, 2009;Bearon and Magar, 2010). Similarly, prey encounter by direct interception depends on the flow field generated by the flagellate and is higher the closer the streamlines come to the capture region on the flagellate (Langlois et al, 2009).…”

“…For an organism moved by a body force (such as gravity), Sherwood numbers are close to unity for the Reynolds numbers considered here, whereas it is higher for a self-propelled organism, as suggested by theoretical models based on idealized flows (Magar et al, 2003;Magar and Pedley, 2005;Langlois et al, 2009;Bearon and Magar, 2010). Similarly, prey encounter by direct interception depends on the flow field generated by the flagellate and is higher the closer the streamlines come to the capture region on the flagellate (Langlois et al, 2009). Interception feeding has been suggested as a common prey encounter mechanism in flagellates (Fenchel, 1982(Fenchel, , 1984Christensen-Dalsgaard and Fenchel, 2003), but feeding flows of free-swimming forms have not been described in any detail.…”

“…Some resolve to filter feeding, for example, choanoflagellates, some sessile nanoflagellates and ciliates (Sleigh, 1964;Fenchel, 1982Fenchel, , 1986Fenchel and Patterson, 1986;Pettitt et al, 2002), others are diffusion feeders that depend on prey swimming across the viscous boundary (Fenchel, 1984;Langlois et al, 2009) and others again have developed rather unique methods such as the toxic mucus traps of Alexandrium pseudogonyaulax (Blossom et al, 2012). Although interception feeding may be common among free-swimming flagellates (Fenchel, 1984), fluid dynamics have mostly been studied for sessile forms (Fenchel, 1982;Christensen-Dalsgaard and Fenchel, 2003), and our results are, to our knowledge, the first empirical quantification of the importance of feeding currents in free-swimming interception feeders.…”

“…The enhancement of nutrient delivery and the rate of prey encounter and prey capture success may, however, depend on the flow field that the flagellate generates (Langlois et al, 2009). The advective enhancement of nutrients because of swimming/feeding currents is quantified by the Sherwood number, with Sh ¼ 1 for no enhancement relative to pure diffusion (KarpBoss et al, 1996).…”