Regenerative Astaxanthin Extraction from a Single Microalgal (<i>Haematococcus pluvialis</i>) Cell Using a Gold Nano-Scalpel

Milking of microalgae, the process of reusing the biomass for continuous production of target compounds, can strikingly overcome the time and cost constraints associated with biorefinery. This process can significantly improve production efficiency of highly valuable chemicals, for example, astaxanthin (AXT) from Haematococcus pluvialis. Detailed understanding of the biological process of cell survival and AXT reaccumulation after extraction would be of great help for successful milking. Here we report extraction of AXT from a single cell of H. pluvialis through incision of the cell wall by a gold nanoscalpel (Au-NS), which allows single-cell analysis of wound healing and reaccumulation of AXT. Interestingly, upon the Au-NS incision, the cell could reaccumulate AXT at a rate two times faster than the control cells. Efficient extraction as well as minimal cellular damage, keeping cells alive, could be achieved with the optimized shape and dimensions of Au-NS: a well-defined sharp tip, thickness under 300 nm, and 1–3 μm of width. The demonstration of regenerative extraction of AXT at a single cell level hints toward the potential of a milking process for continuous recovery of target compounds from microalgae while keeping the cells alive

Magnetic-Nanoflocculant-Assisted Water–Nonpolar Solvent Interface Sieve for Microalgae Harvesting

Exploitation of magnetic flocculants is regarded as a very promising energy-saving approach to microalgae harvesting. However, its practical applicability remains limited, mainly because of the problem of the postharvest separation of magnetic flocculants from microalgal flocs, which is crucial both for magnetic-flocculant recycling and high-purity microalgal biomasses, but which is also a very challenging and energy-consuming step. In the present study, we designed magnetic nanoflocculants dually functionalizable by two different organosilane compounds, (3-aminopropyl)­triethoxysilane (APTES) and octyltriethoxysilane (OTES), which flocculate negatively charged microalgae and are readily detachable at the water-nonpolar organic solvent (NOS) interface only by application of an external magnetic field. APTES functionalization imparts a positive zeta potential charge (29.6 mV) to magnetic nanoflocculants, thereby enabling microalgae flocculation with 98.5% harvesting efficiency (with a dosage of 1.6 g of dMNF/g of cells). OTES functionalization imparts lipophilicity to magnetic nanoflocculants to make them compatible with NOS, thus effecting efficient separation of magnetic flocculants passing through the water-NOS interface sieve from hydrophilic microalgae. Our new energy-saving approach to microalgae harvesting concentrates microalgal cultures (∼1.5 g/L) up to 60 g/L, which can be directly connected to the following process of NOS-assisted wet lipid extraction or biodiesel production, and therefore provides, by simplifying multiple downstream processes, a great potential cost reduction in microalgae-based biorefinement