Marine phytoplankton impose strong selective pressures on in vitro microbiome assembly, but drift dominates selection Metagenome

Phytoplankton are known ecosystem engineers that can modulate community assembly processes in the ocean, but the universality and extent of control they exert over microbiomes remains. Here we use in vitro incubations and 16S rRNA amplicon sequencing to show the influence of phytoplankton on community successions in response to blooms. Presence of phytoplankton grown with reduced diversity cultures or supplemented with exogenously added microbiomes show 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. Different growth phases selected first for rare and abundant ASV, with increasing selection for core ASV in exponential phase and relaxing of selection during death phase as seen throughout incubations for microbiome only controls. Surprisingly, quantification of selection processes 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 dominate as drivers of community selection.