Some Bacillus biofilms have been shown to spontaneously contain a subpopulation of motile bacteria capable of navigating through the matrix in three dimensions (Houry et al., 2012). These swimming bacteria vascularize the biofilms and facilitate the penetration of specific substances to the deeper layers. Microalgae are rich in molecules of interest, and are at the heart of the challenges of the 21st century. Their biofilm culture is attracting growing interest due to their high productivity potential. In this innovative study, the aim is to characterize Bacillus swimming in a Cylindrotheca closterium biofilm using swimming bacilli to further increase biomass productivity. The swimmers criss-cross a biofilm of bacteria, but are they capable of vascularizing a biofilm of microalgae, given that these microorganisms are at least three times larger? Using confocal microscopy (CLSM), an experimental method was developed to study the swimming of these bacilli. Although the microalgae biofilm becomes thicker with increasing matrix production (Fanesi et al., 2019), Bacillus swimming is still observable. Precise characterization of the swimmers requires further optimization and trajectory analyses.