Responses of the Microalga <i>Chlorophyta</i> sp. to Bacterial Quorum Sensing Molecules (<i>N</i>‑Acylhomoserine Lactones): Aromatic Protein-Induced Self-Aggregation

Bacteria and microalgae often coexist during the recycling of microalgal bioresources in wastewater treatment processes. Although the bacteria may compete with the microalgae for nutrients, they could also facilitate microalgal harvesting by forming algal-bacterial aggregates. However, very little is known about interspecies interactions between bacteria and microalgae. In this study, we investigated the responses of a model microalga, <i>Chlorophyta</i> sp., to the typical quorum sensing (QS) molecules <i>N</i>-acylhomoserine lactones (AHLs) extracted from activated sludge bacteria. <i>Chlorophyta</i> sp. self-aggregated in 200 μm bioflocs by secreting 460–1000 kDa aromatic proteins upon interacting with AHLs, and the settling efficiency of <i>Chlorophyta</i> sp. reached as high as 41%. However, <i>Chlorophyta</i> sp. cells were essentially in a free suspension in the absence of AHLs. Fluorescence intensity of the aromatic proteins had significant (<i>P</i> < 0.05) relationship with the <i>Chlorophyta</i> sp. settleability, and showed a positive correlation, indicating that aromatic proteins helped aggregate microalga. Transcriptome results further revealed up-regulation of synthesis pathways for aromatic proteins from tyrosine and phenylalanine that was assisted by anthranilate accumulation. To the best of our knowledge, this is the first study to confirm that eukaryotic microorganisms can sense and respond to prokaryotic QS molecules.