Marine phytoplankton impose strong selective pressures on microbiome assembly, but drift is the dominant process.

Phytoplankton are known ecosystem engineers that modulate ocean community assembly processes, but the universality and extent of their microbiome control remains unclear. We used incubations and 16S ribosomal RNA gene amplicon sequencing to test the influence of Southern and South Pacific oceans dominant phytoplankton on assembly processes and community successions in response to phytoplankton blooms. Phytoplankton grown with reduced-diversity cultures or supplemented with exogenously added microbiomes showed reduced diversity, suggesting environmental filtering. Community profiles were distinct under all culture conditions, further confirming strong selection for specific microbiomes based on phytoplankton. Analysis of core, abundant, and rare organisms in each culture condition showed a conserved response in which core organisms were enriched under conditions of exogenously added phytoplankton. Progression through phytoplankton growth phases selected first for rare and abundant organisms, with increased selection for core members during the exponential phase and relaxing of selection during the death phase, as seen throughout incubations for microbiome-only controls. Surprisingly, selection process quantification identified drift as the dominant process across all conditions and growth phases, with homogenous selection and dispersal limitation accounting for the remainder. Altogether, using Southern Ocean-derived model organisms we confirmed the role phytoplankton play in community assembly but also demonstrated that stochastic processes still predominately drive community selection.