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Abstract

Phytoplankton and bacteria interactions have a significant role in
aquatic ecosystem functioning. Associations can range from mutualistic
to parasitic, shaping biogeochemical cycles and having a direct
influence on phytoplankton growth. How variations in phenotype and
sampling location, affect the phytoplankton microbiome is largely
unknown. A high-resolution characterization of the bacterial community
in cultures of the dinoflagellate Alexandrium was performed on strains
isolated from different geographical locations and at varying
anthropogenic impact levels. Microbiomes of Baltic Sea Alexandrium
ostenfeldii isolates were dominated by Betaproteobacteria and were
consistent over phenotypic and genotypic Alexandrium strain variation,
resulting in identification of an A. ostenfeldii core microbiome.
Comparisons with in situ bacterial communities showed that taxa found in
this A. ostenfeldii core were specifically associated to dinoflagellate
dynamics in the Baltic Sea. Microbiomes of Alexandrium tamarense and
minutum, isolated from the Mediterranean Sea, differed from those of A.
ostenfeldii in bacterial diversity and composition but displayed high
consistency, and a core set of bacterial taxa was identified. This
indicates that Alexandrium isolates with diverse phenotypes host
predictable, species-specific, core microbiomes reflecting the abiotic
conditions from which they were isolated. These findings enable in-depth
studies of potential interactions occurring between Alexandrium and
specific bacterial taxa.