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.