The role of the environmental medium of a marine microalga culture (Tetraselmis suecica) in membrane fouling phenomenon is quantified for two ultrafiltration units: one generating a classical tangential plane flow and another involving a swirling decaying flow induced by a tangential inlet. Compared to the plane unit, the swirling configuration increases the performances of the ultrafiltration process (augmentation of 20% in terms of limiting flux). Interactions between the culture medium and cells depend on the module design. Furthermore, experiments emphasize the significant role of the particles whatever the module design: more than 70% of the total hydraulic resistance could be due to the microalgal cells, while about 30% of membrane fouling is relevant to soluble materials.