Impact of host species on assembly, composition, and functional profiles of phycosphere microbiomes

Abstract

Microalgal microbiomes play vital roles in the growth and health of their host, however, their composition and functions remain only partially characterized, especially across microalgal phyla. In this study, a natural seawater microbiome was introduced to three distinct, axenic species of microalgae, the haptophyte Isochrysis galbana, the chlorophyte Tetraselmis suecica, and the diatom Conticribra weissflogii (previously Thalassiosira), and its divergence and assembly was monitored over 49 days using 16S rRNA amplicon and metagenomic analyses. The microbiomes had a high degree of host specificity in terms of taxonomic composition and potential functions, including CAZymes profiles. Rhodobacteraceae and Flavobacteriaceae families were abundant across all microalgal hosts, but I .galbana microbiomes diverged further from T. suecica and C. weissflogii microbiomes. I .galbana microbiomes had a much higher relative abundance of Flavobacteriaceae, whereas the two other algal microbiomes had higher relative abundances of Rhodobacteraceae. This could be due to the mixotrophic nature of I. galbana affecting the carbohydrate composition available to the microbiomes, which was supported by the CAZymes profile of I. galbana microbiomes diverging further from those of T. suecica and C. weissflogii microbiomes. Finally, the presence of denitrification and other anaerobic pathways was found exclusively in microbiomes of C. weissflogii potentially resulting from anoxic microenvironments in aggregates formed by this diatom during the experiment. These results underline the deterministic role of the microalgal host species on microbiome composition and functional profiles along with other factors, such as trophic mode of the microalgal host. ImportanceAs the main primary producers of the oceans, microalgae serve as cornerstones of the ecosystems they are part of. Additionally, they are increasingly used for biotechnological purposes such as the production of nutraceuticals, pigments, and antioxidants. Since the bacterial microbiomes of microalgae can affect their hosts in beneficial and detrimental ways, understanding these microbiomes is crucial to both ecological and applied roles of microalgae. The present study advances the understanding of microalgal microbiome assembly, composition, and functionality across microalgal phyla, which may inform modeling and engineering of microalgal microbiomes for biotechnological purposes.