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.

水平基因转移 (HGT) 被认为是微生物进化和生态位适应的重要驱动力，并且已在体外证明在生物膜群落中经常发生。然而，HGT 发生的程度以及在更复杂和天然的生物膜系统中转移的功能在很大程度上仍然未知。为了解决这个问题，我们在这里调查了 HGT 和普通海带（大型藻类） Ecklonia radiata生物膜群落中基因功能的富集与周围海水中的微生物群落相比。我们发现大型藻类生物膜中的 HGT 主要由同一类或同目细菌成员之间的转移主导，并且经常涉及营养转运、糖和叶绿单宁降解以及应激反应的基因，所有这些功能都被认为对生活在其中的细菌有益这个特殊的利基市场。HGT 似乎不受移动基因元件的驱动，而是表明涉及非特异性 DNA 摄取（例如自然转化）。与浮游生物群落成员相比，受 HGT 影响的基因功能与生物膜群落中富集的基因功能之间也存在较低的重叠。这表明细菌在E. radiata中生存所需的大部分功能生物膜可能来自共生体的垂直或环境传播。这项研究增强了我们对进化和生态过程在推动生物膜群落的群落组装和基因组多样性方面的相对作用的理解。