Microalgae have been considered as one of the most promising biomass feedstocks for various industrial applications such as biofuels, animal/aquaculture feeds, food supplements, nutraceuticals, and pharmaceuticals. Several biotechnological challenges associated with algae cultivation, including the small size and negative surface charge of algal cells as well as the dilution of its cultures, need to be circumvented, which increases the cost and labor. Therefore, efficient biomass recovery or harvesting of diverse algal species represents a critical bottleneck for large-scale algal biorefinery process. Among different algae harvesting techniques (e.g., centrifugation, gravity sedimentation, screening, filtration, and air flotation), the flocculation-based processes have acquired much attention due to their promising efficiency and scalability. This review covers the basics and recent research trends of various flocculation techniques, such as auto-flocculation, bio-flocculation, chemical flocculation, particle-based flocculation, and electrochemical flocculation, and also discusses their advantages and disadvantages. The challenges and prospects for the development of eco-friendly and economical algae harvesting processes have also been outlined here.

Haematococcus pluvialis, in the dormant aplanospore (cyst) status after 30 d of cultivation, accumulates high levels of a superpotent antioxidant, astaxanthin, which has been demonstrated to have enormous therapeutic benefits. However, owing to the robust structure of its trilayered cell wall, the recovery of astaxanthin from the cyst cells remains an energy-intensive process. In the present study, a novel strategy utilizing a short-period germination based on the natural life cycle of H. pluvialis was developed as an energy-efficient pretreatment for the extraction of astaxanthin using ionic liquids (ILs) as green solvents. The germination resulted in damage and deconstruction of the cyst cell wall, and thereby facilitated the extraction of astaxanthin by ILs at room temperature. By this natural pretreatment with 1-ethyl-3-methylimidazolium ethylsulfate for a very short reaction time of 1 min, a high astaxanthin yield of 19.5 pg per cell was obtained, which was about 82% of a conventional volatile organic solvent extraction by strong, 30 000 psi French-pressure-cell homogenization. The maximal astaxanthin-extraction yield from H. pluvialis cells was observed for 12–18 h germination. The germination rate furthermore could be improved by manipulating the nutritional composition (especially the nitrate concentration) of the culture medium. In light of these results, it can be posited that natural germination following the principles of green chemistry can be a uniquely simple method of robust microalgal cyst cell pretreatment and extraction of astaxanthin with room-temperature ILs.