IMPLICATIONS
Our results have several useful implications. The biomechanical
mechanism underlying the ‘circular run-and-reversal’ movement behavior
of the diatom cells remains puzzling. A reasonable speculation is that
the physical constraints of boat-shaped cells with apically located
sensory receptors gliding in fluids might lead to this type of movement
trajectories (39), but this is beyond the scope of this paper. Despite
that, our work provides a clear demonstration that the statistical
properties of this unique behavior can be ‘optimized’ towards enhanced
foraging efficiency. Both theoretically and experimentally, moving
beyond the statistical descriptions of movement behaviors in previous
literature (13,16), our minimal model may thus serve as a useful
framework for follow-up studies unravelling the ecological and
evolutionary consequences of this movement behavioral plasticity in a
broader context.
One fundamental question is how diatoms would adapt their movements, at
individual and collective levels, in response to different foraging
conditions. Indeed, our observations show that the key movement
parameters revealed in our study, including reversal rate and rotational
diffusivity, are sensitive to changing resource availability (see Fig.
7). The diatom cells move with low reversal rate and high effective
diffusivity \(D\) at intermediate dSi concentrations (from 10 to 50
mg/L), whereas low and high dSi will lead to a decreased efficiency
diffusivity to cells (Fig. 7A). We attribute this to the hypothesis that
when silicon becomes the limiting factor, diatom cells increase
searching activity to meet dSi demand for survival with a higher
effective diffusivity to explore larger areas to take up dSi. It is
surprising that the peak of effective diffusivity coincides with typical
dSi concentrations of many coastal scenarios (Fig. 7A). The effective
diffusivity shows a monotonic decline with increased reversal rates
(Fig. 7B). This adaptive response suggests that diatom cells are able to
sense the local dSi concentration and adjust their reversal rate to
adapt to their physical surroundings. The searching efficiency within a
low nutrient environment is thus strongly dependent on cell movement
behaviors. Extending our results beyond dSi scavenging, there may be
other attractors server as the same role to impact motion behaviors of
diatoms. For instance, in silico comparison of experimental data
led to the suggestion that diatoms have a more efficient behavioral
adaptation to pheromone gradients as opposed to dSi (40). Our
observations thus pave the roads for follow-up work to look further into
why different movement behaviors have evolved with changing of cell body
shape among diatom species, depending on cell size and shape and in
response to different environmental stimuli.
Insights into the movement behavioral plasticity of microorganisms in
aquatic environments have been generated from disciplines such as
biophysics (41-43), but the focus of these studies has largely been on
the statistical physical causes of behavior and not on the ultimate
cause. Cases of reversal behavior were reported independently in
different species of marine bacteria (24, 44, 45), and it has been
suggested that it can contribute to increase foraging efficiency (24,
43) and group social effects (41), but similar evidence is still lacking
for motile microalgae. This study underscores the need to study the
significance of these questions in other microorganisms.