Composition, uniqueness and variability of the epiphytic bacterial community of the green alga Ulva australis

Abstract

Green Ulvacean marine macroalgae are distributed worldwide in coastal tidal and subtidal ecosystems. As for many living surfaces in the marine environment, little is known concerning the epiphytic bacterial biofilm communities that inhabit algal surfaces. This study reports on the largest published libraries of near full-length 16S rRNA genes from a marine algal surface (5293 sequences from six samples) allowing for an in-depth assessment of the diversity and phylogenetic profile of the bacterial community on a green Ulvacean alga. Large 16S rRNA gene libraries of surrounding seawater were also used to determine the uniqueness of this bacterial community. The surface of Ulva australis is dominated by sequences of Alphaproteobacteria and the Bacteroidetes, especially within the Rhodobacteriaceae, Sphingomonadaceae, Flavobacteriaceae and Sapropiraceae families. Seawater libraries were also dominated by Alphaproteobacteria and Bacteroidetes sequences, but were shown to be clearly distinct from U. australis libraries through the clustering of sequences into operational taxonomic units and Bray-Curtis similarity analysis. Almost no similarity was observed between these two environments at the species level, and only minor similarity was observed at levels of sequence clustering representing clades of bacteria within family and genus taxonomic groups. Variability between libraries of U. australis was relatively high, and a consistent sub-population of bacterial species was not detected. The competitive lottery model, originally derived to explain diversity in coral reef fishes, may explain the pattern of colonization of this algal surface.

Figure 1

Taxonomic affiliations for sequences within U. australis and seawater 16S rRNA gene libraries. The average percentage across all U. australis and seawater libraries that were classified into particular (a) phyla, (b) family and (c) genus taxonomic groups. Sequences within phyla, family and genus groups, which represented less than 0.01% of library sequences are not displayed.

Figure 2

Rarefaction curve. The number of OTUs at 0.03 (species) detected vs the number of sequences sampled in each algal and seawater 16S rRNA gene library.

Figure 3

Venn diagrams showing the total number of OTUs and the number of shared OTUs between all U. australis (UA) and seawater (SW) libraries at 0.03, 0.05 and 0.10 cutoff.

Figure 4

Dendrograms comparing the level of similarity between 10 planktonic and six U. australis 16S rRNA gene libraries with (a) OTUs at 0.03 difference, (b) OTUs with 0.05 difference and (c) OTUs at 0.10 difference.