Coordinated Motility of Soil Protists in Micro-Structured Habitats: Evidence for Protist-Protist Communication and Implications for Sustainable Agriculture

Protists come into contact with as much as 70 percent of carbon in the carbon cycle. In the rhizosphere, the region of soil directly under the influence of growing roots, protist communities directly influence the abundance and spatial distribution of beneficial plant symbionts. However, little research has been done to understand how individual protists interact with one another and with their natural habitat, especially at physically relevant spatial scales. Of particular importance to terrestrial ecosystems and agriculture is the potential for individual protists to coordinate motility within a physically complex microenvironment. Here we measure sequential motility decisions of individual protists within a well-defined microfluidic cell culture chamber. Euplotes vannus, a common, motile, easy-to-culture, well-studied model ciliate was selected for the study. The experimental apparatus was a PDMS device featuring an array of source wells each connected to two target wells by a branching microfluidic channel 55 µm wide and 35µm high. Just past the branch, each channel featured a heart-shaped structure (ratchet) to promote unidirectional protist movement. In the study, sequential motility decisions of 434 E. vannus were tracked in 36 replicate trials. For each trial, between 10 and 40 protists were introduced into the source well and protists were continually observed as they moved freely within the device. Once an individual protist passed a threshold point in either the “Right” or the “Left” channel, a protist motility decision was recorded. Sequential protist motility decisions were analyzed using a pairwise probit regression. Protist motility decisions were significantly different from those predicted for protists displaying random motion (P=0.001), and no significant bias was found favoring either the right or the left channel. These results suggest motility decisions of protists influence subsequent motility decisions of other protists in a community. Additional analysis among sub-groups in the experiment showed coordinated motility is most prevalent among high abundance, low motility protist communities, where low motility indicates a feeding mode for protists. Correlations between motility, feeding, and reproduction have important implications on overall system function. These results will be incorporated into a computer model of protist transport in emulated soil micromodels. In the longer term, more accurate predictions of coordinated motility can improve the accuracy of microbial community models and improve performance sustainable agriculture biotechnology.