Against the current: upstream behavior in diatoms

Abstract

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 custom-made digital holographic microscope to investigate the locomotion of the pennate diatom Navicula cf parapontica under varying water flow conditions. Our findings demonstrates that Navicula perceives and actively counteracts water currents. As flow rates increased up to 500 nL/s, cells actively moved upstream and frequently adjusted their orientation positioning themselves partially oblique to the flow, probably to maximize mechanical resistance. This active behavior allowed the diatoms to maintain a stable locomotion velocity despite about 5-fold increase in the flow rate. This active rheotaxis likely serves as a strategy to resist resuspension and passive dispersal. These findings reveal a behavioral trait that might play significant role in how benthic diatom communities maintain their position in the sediments, ultimately influencing bentho-pelagic coupling and biogeochemical processes.