Against the current: upstream behavior in diatoms

Diatoms significantly contribute to aquatic primary productivity and biogeochemical cycles, with motility playing a crucial role in their ecological success. While several factors influence their motility, the effect of water flow remains poorly understood. This study used a digital holographic microscope to investigate the locomotion of the pennate diatom Navicula cf parapontica under varying flow rates. It demonstrates, for the first time, that Navicula perceives and actively counteracts water flows. As flow rates increased up to 500 nL/s, cells consistently moved against the current and frequently adjusted their orientation to maximize resistance. This behaviour allowed the diatoms to maintain a stable locomotion velocity despite a 6.7-fold increase in flow rate. This active rheotaxis likely serves as a strategy to resist resuspension and passive dispersal. These findings reveal a behavioural trait that might play significant role in the way benthic diatom communities maintain their position in the sediments, influencing bentho-pelagic coupling and biogeochemical processes.