Legacy effect of repeated stimulation by indole-3-acetic acid on the rapid attachment of microalgae biofilms

Microalgae are versatile autotrophic organisms with industrial applications in carbon sequestration, wastewater treatment, and resource recovery. Under suitable conditions, some species aggregate into biofilms, and studies have shown that indole-3-acetic acid (IAA) can enhance microalgal productivity. However, the effects of repeated low-concentration IAA treatments on microalgal biofilms, a scenario often encountered in natural aquatic systems, remain largely unexplored. This study investigates the legacy effects of different IAA application frequencies on the attachment of microalgal biofilms using Chlorella vulgaris. A total IAA dose of 0.1 mg/L was divided into 1, 2, 4, and 8 applications. Results indicated that, with a constant total IAA dose, the attachment of microalgal biofilms increased with the frequency of applications. Specifically, when 0.1 mg/L IAA was applied in 8 doses over 16 days, the attachment of microalgal biofilms reached 9.2 g/m², representing a 27 % increase compared to a single high-concentration application. Experimental analysis revealed that repeated low-level stimulation upregulated genetic pathways controlling hydrophobic amino acids in microalgal cells. The number of upregulated hydrophobic amino acid genetic pathways (gene IDs) increased by 104 % with 8 applications compared to a single application. The enhanced expression of hydrophobic amino acid genes likely increases the proportion of extracellular hydrophobic proteins, reducing the energy barrier between microalgae and the receptive surface, thus promoting biofilm attachment. This study elucidates the impact of IAA application frequency on microalgal biofilm attachment, highlighting the importance of stimulation history in understanding biological responses in biofilm formation.