Functional diversity and host-specificity of macroalgal surface-associated marine bacteria

Multicellular eukaryotic organisms live in constant interaction with microorganisms thatcolonise their surfaces. In the marine environment, macroalgae are colonised bycomplex communities of bacteria, however the mechanisms that drive the assembly andstability of those bacterial communities are poorly understood. This thesis exploresbacterial communities associated to diverse macroalgal species by using 16S rRNAamplicon sequencing and metagenomics to access their taxonomic composition andfunctionality.To understand the influence of biogeography on bacterial community composition,samples of three green macroalgal species belonging to the genus Ulva (U. australis, U.rigida and U. ohnoi) and seawater collected from different locations were analysed. Ahigh taxonomic variability was detected between the communities associated withdifferent individuals of the same host-species, but despite this, a core of functions wasconsistently present and enriched in all communities. This indicated that taxonomicallydistinct bacteria are providing the same set of functions in different macroalgal species.Local and host-specific conditions also played a role in the selection and assembly ofcommunities on Ulva.To define how community composition is influenced by different kind of surfaces, thetaxonomic and functional composition of planktonic and various surface-associatedbacterial communities were compared. A partitioning of the diversity revealed a hightaxonomic variability between samples, but also showed that most of the functionaldiversity could be found in any given surface. This shows that the functionality to live on a surface can be contained in any given community and that selection occurs onspecific aspects of the surface.In order to define the stability of a macroalgal holobiont during environmental changes,individuals of the green alga Caulerpa taxifolia was exposed to low pH and hightemperature conditions. The bacterial community composition was found to berelatively stable, with only one bacterium increasing in abundance. Moreover, themacroalgal host had an increased growth rate, which suggested that this particularholobiont can resist changing conditions associated to future climate change scenarios.This thesis has contributed to the understanding of mechanisms and factors that drivethe selection and assembly of bacterial communities on marine surfaces