Abstract
Phytoplankton are known ecosystem engineers that modulate ocean community assembly processes, but the universality and extent of their microbiome control remains unclear. We used in vitro incubations and 16S rRNA 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. Presence of 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 where core organisms were enriched under exogenously added phytoplankton. Progression through phytoplankton growth phases selected first for rare and abundant organisms, with increased selection for core members during exponential phase and relaxing of selection during 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, we confirm the role phytoplankton play in community assembly using Southern Ocean derived model organisms but demonstrate that stochastic processes still predominately drive community selection.