Ectobiotic and Endobiotic Bacteria Associated with Eutreptiella sp. Isolated from Long Island Sound

Diversity and functional association of bacteria with Eutreptiella sp. was investigated. 16S rDNA analysis of ectobiotic bacteria revealed various lineages of Alphaproteobacteria and abundant Gammaproteobacteria, specifically Marinobacter. Antibiotic treatment yielded axenic cultures, and experiments based on them indicated that ectobiotic bacteria likely provide vitamin B12 and other growth-enhancing factors for the alga. Further, DAPI staining and transmission electron microscopy revealed endobiotic bacteria in the cytoplasm of algal cells. 16S rDNA analysis showed that the bacteria belonged to one species that was most closely related to Rickettsiales endosymbionts of other organisms and phylogenetically affiliated with a new group of aquatic Rickettsiales. Observations from a diel experiment indicated that the endobiotic bacteria reproduced asynchronously with Eutreptiella sp. and had no adverse effects on lipid production (bioenergetics) or growth of the host alga. Our study reveals a diverse microbiome associated with this euglenoid alga, offering a system for studying the roles of algae-bacteria associations.