Distinctive growth and transcriptional changes of the diatom Seminavis robusta in response to quorum sensing related compounds

In aquatic habitats, diatoms are often found in association with Proteobacteria. Species of this phylum commonly engage in cell-cell communication via N-acyl homoserine lactones (AHLs), a process called quorum sensing (QS). Recently, it has been suggested that sensing AHLs could provide diatoms with the ability to distinguish between beneficial and algicidal bacteria in their surroundings. While some microalgae can interfere with QS, e.g. by releasing AHL mimics or degrading them, molecular responses induced by AHLs in microalgae are still unclear. To this end, we treated the benthic diatom Seminavis robusta with the AHLs N-tetradecanoyl homoserine lactone (C14), N-3-hydroxytetradecanoyl homoserine lactone (OH14) and N-3-oxotetradecanoyl homoserine lactone (OXO14). One tetramic acid was also tested (TA14), which is the rearrangement product of OXO14. All treatments affected diatom growth, i.e. C14 stimulated growth, whereas treatment with OH14, OXO14 and TA14 inhibited diatom growth. Gene expression analysis following three days of treatment with C14, OXO14 and TA14 revealed differential expression of a total of 3,410 genes in response to at least one molecule compared to a control treatment. Using gene ontology enrichment analysis, it was found that the term “cAMP mediated signaling” was enriched in all three compound treatments, especially for C14. Interestingly, exposure to OXO14 and TA14 triggered a switch in lipid metabolism towards increased fatty acid degradation, while exposure to OXO14 induced downregulation of cell cycle genes, which was supported with cell growth stagnation in this treatment. In summary, expression profiles after OXO14 and TA14 treatment shared several similarities, while C14 induced distinct responses. These results indicate that S. robusta can distinguish between quorum sensing molecules with high structural similarity.