On-Chip Photoacoustics-Activated Cell Sorting (PA-ACS) for Label-Free and High-Throughput Detection and Screening of Microalgal Cells.

Microalgae play a crucial role in global carbon cycling as they convert carbon dioxide into various valuable macromolecules. Among them, Haematococcus pluvialis ( H. pluvialis ) is the richest natural source of astaxanthin (AXT), which is a valuable antioxidant, anti-inflammatory, and antiapoptosis agent. These benefits make AXT highly commercially valuable in pharmaceuticals, cosmetics, and nutritional industries. However, intrinsic genetic characteristics and extrinsic cultivation conditions influence biomass gains, leading to low productivity and extraction as the main techno-economic bottlenecks in this industry. Thus, detecting AXT in H. pluvialis is essential to determine the influence of multiple parameters on biocompound accumulation, enabling optimization of cultivation and enrichment of AXT-rich H. pluvialis cells. This work developed an opto-acousto-fluidic microplatform for detection, analysis, and sorting of microalgae. Via label-free monitoring and extraction of sample-induced ultrasonic signals, a photoacoustic microscopic system was proposed to provide a full-field visualization of AXT's content and distribution inside H. pluvialis cells. When employed as on-chip image-based flow cytometry, our microplatform can also offer high-throughput measurements of intracellular AXT in real time, which demonstrates similar results to conventional spectrophotometry methods and further reveals the heterogeneity of AXT content at the single-cell level. In addition, a solenoid valve-pump dual-mode cell sorter was integrated for effective sorting of cells with a maximum working frequency of 0.77 Hz, reducing the fluid response time by 50% in rising and 40-fold in recovery. The H. pluvialis cells which have more AXT accumulation (>30 μm in diameter) were 4.38-fold enriched with almost no dead empty and small green cells. According to the results, automated and reliable photoacoustics-activated cell sorting (PA-ACS) can screen AXT-rich cells and remove impurities at the terminal stage of cultivation, thereby increasing the effectiveness and purity of AXT extraction. The proposed system can be further adopted to enrich strains and mutants for the production of biofuels or other rare organic substances such as β-carotene and lutein.