Coccolithophores have global ecological and biogeochemical significance
as the most important calcifying marine phytoplankton group. The
structure and selection of prokaryotic communities associated with the
most abundant coccolithophore and bloom-forming species, Emiliania
huxleyi, are still poorly known. In this study, we assessed the
diversity of bacterial communities associated with an E. huxleyi bloom
in the Celtic Sea, exposed axenic E. huxleyi cultures to prokaryotic
communities derived from bloom and non-bloom conditions and followed the
dynamics of their microbiome composition over one year. Bloom-associated
prokaryotic communities were dominated by SAR11, Marine group II
Euryarchaeota, Rhodobacterales and contained substantial proportions of
known indicators of phytoplankton bloom demises such as
Flavobacteriaceae and Pseudoalteromonadaceae. Taxonomic richness of
replicated co-cultures resulting from natural communities with axenic E.
huxleyi rapidly shifted and then stabilized over time, presumably by
ecological selection favoring more beneficial populations. Recruited
microbiomes from the environment were consistently dependent on the
composition of the initial bacterioplankton community.
Phycosphere-associated communities derived from the E. huxleyi bloom
depth were highly similar to one another, suggesting deterministic
processes, whereas cultures from non-bloom conditions show an effect of
both deterministic processes and stochasticity. Overall, this work sheds
new light on the importance of the initial inoculum composition in
microbiome recruitment and elucidates the temporal dynamics of its
composition and long-term stability.