Surprising variability among macroalgal-associated bacteria

Macroalgae are crucial members of coastal ecosystems, contributing to primary production, offering protective environments for invertebrate species and providing living surfaces for the colonisation of microbial communities. Despite the importance of macroalgae and the importance of associated-bacterial communities in other sessile eukaryotes, the interactions between algal host-microbe and why these associations form still remain a mystery. Recently, studies have begun to ascertain the roles of individual microbial species (settlement of algal spores, antifouling and pathogenic defence), indicating the importance of these bacterial communities to the algal hosts normal function. 

This key paper by Burke et al. (2011) set out to perform an in-depth analysis of the bacterial communities of Ulva australis, then compare this with that of the surrounding seawater and finally, establish the extent of specificity in community composition between these habitats.

Bacterial DNA was extracted from the surface of U. australis fronds (sampled over space and time) and the surrounding seawater (200 litres per U. australis sample). Large 16S rRNA gene libraries of nearly full length sequences (≥1200 base pairs) were prepared from DNA extracted from all samples, sequences less than 1200 bp in length were removed. Sequences were then grouped in to operational taxonomic units (OTUs), for comparison.

An earlier publication by Tujula et al. (2010) reported a small sub-population of continually occurring species, termed ‘core microbiota’, associated with U. australis (based on DGGE (denaturing gradient gel electrophoresis) analysis). Results in this study did indicate a small number of OTUs (six) were present in all U. australis libraries, however, none of the consistently detected OTUs showed ≥97% similarity to core microbiota sequences found by Tujula et al. (2010). Furthermore, a large degree of variability was found among the U. australis libraries, the average level of similarity found to be less than 20% at the species level. Finally, as expected, U. australis-associated communities were shown to be distinct from communities present in the surrounding seawater (less than 2% of total OTUs occurring in both seawater and U. australis).

The authors conclude that the high degree of variability observed is most likely due to the deeper level of sequencing used, which identified many rare OTUs unlikely to be detected by DGGE. In opposition to the findings of Tujula et al., the results found by Burke and her colleagues suggest there does not appear to be a core microbiota present within U. australis communities. Furthermore, the authors propose that the observed variability between samples of U. australis could be explained by the lottery hypothesis, which states that species with similar functional roles, will occupy space within an ecosystem based on who gets there first. To test this theory, an analysis of the functional genes present in U. australis-associated bacterial communities is needed.

The methods used by burke and her colleagues makes this study the most comprehensive assessment of macroalgal-associated bacterial communities to date and hence, my reason for reviewing. The conclusions regarding the importance of species composition vs the function of genes present within a community interested me greatly, with potential applications in other host-microbe associations, such as corals, sponges and humans.

Tujula, N.A., Crocetti, G.R., Burke, C., Thomas, T., Holmstróm, C. and Kjelleberg, S. (2010) Variability and abundance of the epiphytic bacterial communitiy associated with a green marine Ulvacean alga. The International Society for Mircobial Ecology journal 4. 301-311. 

Burke, C., Thomas, T., Lewis, M., Steinberg, P. and Kjelleberg, S. (2011) Composition, uniqueness and variability of the epiphytic bacterial community of the green alga Ulva asutralis. The International Society for Mircobial Ecology journal  5. 590-600.