From competition to export: Dinoflagellates can shape vertical flux through the ‘allelopathic shuttle’ mechanism

The downward flux of organic particles, or marine snow (MS), is a critical driver of microbial activity, energy transfer, and the ocean’s biological carbon pump. While MS formation is linked to the aggregation of microalgae, faecal pellets, and debris—especially following bloom termination—the role of plankton interactions in regulating this flux remains poorly understood. Although allelopathic inhibition of competitors is common across phytoplankton groups, its ecological and biogeochemical consequences—particularly in relation to MS—remain poorly understood.We investigated how the allelopathic dinoflagellate Alexandrium influence aggregation and vertical biomass export when interacting with diatom competitors. Exposure of Chaetoceros muelleri to Alexandrium minutum allelochemicals triggered rapid aggregate formation, comprising both live and dead cells, mediated by transparent exopolymer particles released during cell lysis. Notably, A. minutum cells were under-represented in aggregates, suggesting a mechanism to avoid self-aggregation.We propose the “allelopathic shuttle” concept: allelopathic species reduce competition for light and nutrients by lysing competitors and enhancing downward biomass export as MS, while maintaining their position in the euphotic zone. In contrast, interactions between Alexandrium catenella and the intrinsically aggregating diatom Thalassiosira minuscula resulted in basal aggregation unaffected in magnitude by allelochemicals, but produced bleached aggregates with significantly reduced sinking velocities.These findings reveal a dual role for allelopathy in shaping vertical organic matter fluxes—both promoting aggregate formation and altering sinking dynamics. Our work identifies the allelopathic shuttle as a novel ecological and biogeochemical mechanism, with implications for bloom termination, phytoplankton community structuring, and carbon export efficiency in the ocean.