4 Summary of the results
environments to survive without additional photosynthesis. The mixotrophic strategy
is thus found to be essential for the observed agellates in a typical marine environment.
4.3 Paper III: Hydrodynamics of microbial lter feeding
Filter feeding, i.e., the capture of small particles with a feeding current through a mesh
structure, seems to be a very successful strategy for heterotrophic microbes that, as
opposed to most other unicellular species, can survive purely on prey capture.
In this paper we investigate the lter-feeding strategy of microbes with a focus on
choanoagellates that use one undulating agellum to drive ow through a collar lter.
We estimate the volume ow rate through the lter of the species Diaphanoeca
grandis with the use of µPTV in several individuals. The estimate based on the ow
measurement matches previous incubation experiments and also conrms the approximately
needed clearance rate of one million cell volumes per day for the survival in
characteristic oceanic environments.
The organism morphology and the agellar kinematics are measured from image
sequences. These characteristics are used to calculate the emerging ow eld around
D. grandis in a computational uid dynamics (CFD) simulation, where the agellum is
modelled as an undulating slender structure. Both the CFD simulation and analytical
estimates of produced forces based on slender-body theory underestimate the clearance
rate by about one order of magnitude.
In a few choanoagellate species traces of a agellar vane have been observed, which
is a thin sheet extending from the agellum and which in the related sponge cells has
been shown to span the whole collar width. When including a 5-µm-wide vane in
the simulations we obtain ow elds that t much better to the observed ones (gure
4.5). With this additional structure the ltering mechanism resembles more that of a
peristaltic pump, for which we can make a matching theoretical estimate. From these
ndings we speculate that the agellar vane exists in more species than previously
Another important aspect of lter feeding, which is my main contribution to this
study, is the choice of lter spacings which should be small enough in order to not
let important prey slip away, but it also should not be too small, since the needed
force to create a given ow rate increases dramatically for more narrow lters. This
essential trade-o leads to an optimum spacing (gure 4.6 A). In order to calculate the
optimum mesh spacing we model the lter as an array of parallel cylinders and assume
the Sheldon size spectrum for the prey, i.e., equal amounts of biomass in logarithmic
bins of particle size. The encounter rate from sieving is then calculated with the driving
force F, the integrated prey concentration for prey diameters from lter spacing l to
maximum prey diameter d, and the average lter permeability hi = l=(ahi) (section
3.7). We obtain
E = F C0