AME 83:281-293 (2019)  -  DOI: https://doi.org/10.3354/ame01918

ABSTRACT: The direct proximity of phytoplankton cells is a habitat distinct from the surrounding water. Colonizers of this habitat may form strong or weak associations with the host cells, depending on the mode of interaction and the prevailing abiotic conditions. We analyzed the bacteria and eukaryotes associated with the cyanobacterium Nodularia spumigena using Illumina Mi-Seq sequencing of bloom samples from the Baltic Sea, samples cultured for 2.5 mo and 2.5 yr after segregation, and a >3 yr old previously established lab culture. Significant differences in the composition of the associated communities between the different samples and a diversity that decreased significantly with increasing cultivation time demonstrated that most associations were labile and that culture conditions are a key driver of the associated community composition. However, present in all samples were 12 associated bacterial operational taxonomic units (OTUs) and 1 eukaryotic OTU, indicating the stable and possibly specific associations of the respective organisms with N. spumigena. Among the ubiquitous OTUs were 4 genera of Alphaproteobacteria (Roseibacterium, Hyphomonas, Brevundimonas, and Marivita), 1 genus of Betaproteobacteria (Limnobacter), and the fungus Malassezia, which were all highly abundant, suggesting key roles in associations of N. spumigena. Our findings suggest that the microbial community associated with N. spumigena contains a few stably associated taxa that play key roles in the associations, whereas the majority of accompanying organisms are more loosely associated, and their composition is apparently determined by the prevailing abiotic conditions.

KEY WORDS: Phytoplankton-bacteria interactions · Phytoplankton associations · Cyanobacteria · Nodularia spumigena · Succession · Interaction