Roselli, L. | bioRxiv Science

bioRxiv2026

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.

ecology↗

bioRxiv2020

Shape matters: cell geometry determines phytoplankton diversity

Size and shape profoundly influence an organisms ecophysiological performance and evolutionary fitness, suggesting a link between morphology and diversity. However, not much is known about how body shape is related to taxonomic richness, especially in microbes. Here we analyse global datasets of unicellular marine phytoplankton, a major group of primary producers with an exceptional diversity of cell sizes and shapes and, additionally, heterotrophic protists. Using two measures of cell shape elongation, we quantify taxonomic diversity as a function of cell size and shape. We find that cells of intermediate volume have the greatest shape variation, from oblate to extremely elongated forms, while small and large cells are mostly compact (e.g., spherical or cubic). Taxonomic diversity is strongly related to cell elongation and cell volume, together explaining up to 92% of total variance. Taxonomic diversity decays exponentially with cell elongation and displays a log-normal dependence on cell volume, peaking for intermediate-volume cells with compact shapes. These previously unreported broad patterns in phytoplankton diversity reveal selective pressures and ecophysiological constraints on the geometry of phytoplankton cells which may improve our understanding of marine ecology and the evolutionary rules of life.

ecology↗