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It has been proven that the relationship between microalgae and bacteria affects the dynamic process of harmful algal blooms (HABs). Microalgae-associated microorganisms widely exist in the phycosphere and play an essential role in algae-bacteria cross-kingdom interactions. Among these processes, quorum sensing (QS), as a communication system of bacteria, is thought to participate in algae-bacteria interactions. However, the species of QS bacteria in the phycosphere and their ecological function are still unknown. In this study, microalgae-associated microorganisms with a QS system were screened by the biosensor method and identified based on 16S rRNA gene analysis. The types and number of acyl-L-homoserine lactone (AHL) signalling molecules produced by QS bacteria were analysed by thin layer chromatography (TLC) bioautography and gas chromatography-mass spectrometer (GC-MS). The film formation, β-dimethylmercaptopropionic (DMSP) degradation and algae growth effects of QS bacteria were investigated. The results showed that 113 QS bacteria were isolated from 842 microalgae-associated bacteria. Detection of AHL molecules in 10 different species of QS bacteria showed that most of them were N -(3-Oxodecanoyl)-L-homoserine lactone (OC10-HSL), N -Octanoyl-L-homoserine lactone (C8-HSL) and N -(3-Oxooctanoyl)-L-homoserine lactone (OC8-HSL). All 10 QS bacteria had film-forming ability, and they could degrade DMSP (except strain E26). The crude metabolic extracts of the 10 QS bacteria can inhibit or promote microalgae growth to different degrees. Our study is helpful to understand the role of microalgae-associated microorganisms with the QS system in algae-bacteria interactions and community succession of HAB microalgae.

Sulfitobacter pseudonitzschiae H46, a marine bacterium associated with microalgae, exhibits algicidal activity against harmful algal bloom microalgae Chattonella marina and Alexandrium tamarense. The complete genome of S. pseudonitzschiae H46 was sequenced to investigate its algicidal mechanism further. One chromosome, eight circular plasmids and add one not circularized plasmid (Plas II) were identified, which are 4.93 Mb in length with 4871 open reading frames. The genome contains 43 transfer RNA (tRNA) genes; six ribosome RNA (rRNA) genes that can be classified into two 5S rRNA, two 16S rRNA, two 23S rRNA, and two small RNA (sRNA) genes. Genomic annotation revealed that S. pseudonitzschiae H46 has two pairs of LuxI/R-type Quorum-sensing (QS) systems. The QS systems can regulate the functions involved in host colonization and biofilm formation. Moreover, the existence of 49 Type-IV and Dddl genes in the chromosome indicated that Dimethylsulfoniopropionate (DMSP) catabolism of S. pseudonitzschiae H46 is the main interaction with marine microalgae. S. pseudonitzschiae H46 utilizes DMSP produced by microalgae as the carbon source, and this may induce algicidal activity. The complete genome sequence of S. pseudonitzschiae H46 can provide useful information on the underlying molecular mechanism between QS and algicidal activity., (© 2021. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)