Screen and Optimization of an Aptamer for Alexandrium tamarense-A Common Toxin-Producing Harmful Alga

Among all the paralytic shellfish toxins (PSTs)-producing algae, Alexandrium tamarense is one of the most widespread harmful species posing a serious threat to marine resources and human health. Therefore, it is extremely important to establish a rapid and accurate monitoring method for A. tamarense that can provide early warnings of harmful algal blooms (HABs) caused by this alga and limit the contamination due to PSTs. In this study, an ssDNA library was first obtained by whole cell systematic evolution of ligands by exponential enrichment after 18 consecutive rounds of iterative screening. After sequencing in combination with subsequent multiple alignment of sequences and secondary structure simulation, the library could be classified into 2 families, namely, Family1 and Family2, according to sequence similarity. Flow cytometry was used to test the affinity and cross-reactivity of Ata19, Ata6, Ata25 and Ata29 belonging to Family2. Ata19 was selected to be modified by truncation, through which a new resultant aptamer named as Ata19-1-1 was obtained. Ata19-1-1 with a K_D of 75.16 ± 11.10 nM displayed a much higher affinity than Ata19. The specificity test showed that Ata19-1-1 has the same discrimination ability as Ata19 and can at least distinguish the target microalga from other microalgae. The observation under a fluorescence microscopy showed that the A. tamarense cells labeled with Ata19-1-1 are exhibiting bright green fluorescence and could be easily identified, factually confirming the binding of the aptamer with target cells. In summary, the aptamer Ata19-1-1 produced in this study may serve as an ideal molecular recognition element for A. tamarense, which has the potential to be developed into a novel detection method for this harmful alga in the future.