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

Abstract

Multicellular eukaryotic organisms live in constant interaction with microorganisms that colonise their surfaces. In the marine environment, macroalgae are colonised by complex communities of bacteria, however the mechanisms that drive the assembly and stability of those bacterial communities are poorly understood. This thesis explores bacterial communities associated to diverse macroalgal species by using 16S rRNA amplicon sequencing and metagenomics to access their taxonomic composition and functionality. 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. A high taxonomic variability was detected between the communities associated with different individuals of the same host-species, but despite this, a core of functions was consistently present and enriched in all communities. This indicated that taxonomically distinct 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 of communities on Ulva. To define how community composition is influenced by different kind of surfaces, the taxonomic and functional composition of planktonic and various surface-associated bacterial communities were compared. A partitioning of the diversity revealed a high taxonomic variability between samples, but also showed that most of the functional diversity 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 on specific 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 high temperature conditions. The bacterial community composition was found to be relatively stable, with only one bacterium increasing in abundance. Moreover, the macroalgal host had an increased growth rate, which suggested that this particular holobiont can resist changing conditions associated to future climate change scenarios. This thesis has contributed to the understanding of mechanisms and factors that drive the selection and assembly of bacterial communities on marine surfaces.