Horizontal Gene Transfer in Planktonic and Biofilm Modes. In: Rumbaugh KP, Ahmad I, editors. Antibiofilm Agents. Berlin: Springer Berlin Heidelberg; 2014. pp. 67-95. [DOI: 10.1007/978-3-642-53833-9_5]

Contribution of horizontal gene transfer to the functionality of microbial biofilm on a macroalgae

Horizontal gene transfer (HGT) is thought to be an important driving force for microbial evolution and niche adaptation and has been show in vitro to occur frequently in biofilm communities. However, the extent to which HGT takes place and what functions are being transferred in more complex and natural biofilm systems remains largely unknown. To address this issue, we investigated here HGT and enrichment of gene functions in the biofilm community of the common kelp (macroalgae) Ecklonia radiata in comparison to microbial communities in the surrounding seawater. We found that HGTs in the macroalgal biofilms were dominated by transfers between bacterial members of the same class or order and frequently involved genes for nutrient transport, sugar and phlorotannin degradation as well as stress responses, all functions that would be considered beneficial for bacteria living in this particular niche. HGT did not appear to be driven by mobile gene elements, indicating rather an involvement of unspecific DNA uptake (e.g. natural transformation). There was also a low overlap between the gene functions subject to HGT and those enriched in the biofilm community in comparison to planktonic community members. This indicates that much of the functionality required for bacteria to live in an E. radiata biofilm might be derived from vertical or environmental transmissions of symbionts. This study enhances our understanding of the relative role of evolutionary and ecological processes in driving community assembly and genomic diversity of biofilm communities.

Growing season drives the compositional changes and assembly processes of epiphytic bacterial communities of two submerged macrophytes in Taihu Lake

Large amounts of epiphytic bacteria live on the leaf surfaces of submerged macrophytes in freshwater lakes. Despite their important roles in affecting host plant's health and biogeochemical cycling, knowledge about epiphytic bacteria assembly is not sufficient. We studied epiphytic bacteria on two cohabiting plant species in Taihu Lake, China. In comparison with plant identity and geographic distance, the plant-growing season played a prominent role in driving alpha and beta diversity (compositional variations) of epiphytic bacterial communities. Phylogeny-based null model analysis revealed that the growing season also drove the relative importance of deterministic versus stochastic processes underlying bacterial community assembly. In May when both plants start growth, the deterministic processes were most prominent, while in months later than June, the stochastic processes' effects increased substantially. In addition, we found a significant positive relationship between alpha diversity and compositional stochasticity, implying that stochastic processes may have great effects on the maintenance of diversity and functioning of epiphytic bacteria in aquatic ecosystems. In summary, the growing season overwhelmed plant identity and spatial site in shaping epiphytic bacterial communities in Taihu Lake, which may suggest new clues in understanding the dynamics of epiphytic communities and their roles in large shallow lacustrine ecosystems.