Algal Exudates Promote Conjugation in Marine Roseobacters

Horizontal gene transfer (HGT) is a pivotal mechanism driving bacterial evolution, conferring adaptability within dynamic marine ecosystems. Among HGT mechanisms, conjugation mediated by Type IV secretion systems (T4SSs) plays a central role in the ecological success of marine bacteria. However, the triggers initiating conjugation events in the marine environment are not well understood. Roseobacters, abundant marine bacteria commonly associated with algae, possess a multitude of plasmids encoding T4SSs. Many Roseobacters are heterotrophic bacteria that rely on algal secreted compounds for supporting bacterial growth. Algal compounds therefore attract bacteria and promote colonization, including attachment to algal cells. Bacterial proximity, cell-to-cell contact, and attachment, can all foster HGT. Hence, we hypothesized that algal exudates, acting as chemoattractants for bacteria, may function as cues promoting bacterial HGT. Examination of various Roseobacters demonstrated that the genomic location of the T4SS impacts its functionality; a bacterial strain harboring a chromosomal-encoded T4SS does not perform conjugation, while a strain of the same species carrying a plasmid-encoded T4SS exhibits functional conjugation capability. Subsequently, we probed the influence of algal exudates on bacterial conjugation dynamics. Our findings revealed that algal exudates enhance plasmid transfer through conjugation but do not modulate the transcription of the conjugative machinery genes. These observations suggest that the bacterial responses to algal hosts evolved to correlate with an increased likelihood of encountering compatible partners for successful conjugation. Furthermore, since specific algae attract distinct bacterial populations, algae influence potential partners for genetic exchange and may shape the trajectory of bacterial evolution in the marine environment.