Abstract
In microalgae harvesting, flocculation is usually a compulsory preliminary step to further separation by sedimentation or flotation. For some microalgae species, and under certain growth conditions, flocculation can occur naturally. Natural flocculation presents many advantages as it does not require the addition of any flocculants to the culture medium and shows high efficiency rate. But because natural flocculation is so specific to the species and conditions, and thanks to the knowledge accumulated over the last years on flocculation mechanisms, researchers have developed strategies to induce this natural harvesting. In this review, we first decipher at the molecular scale the underlying mechanisms of natural flocculation and illustrate them by selected studies from the literature. Then we describe the developed strategies to induce natural flocculation that include the use of biopolymers, chemically modified or not, or involve mixed species cultures. But all these strategies need the addition of external compounds or microorganism which can present some issues. Thus alternative directions to completely eliminate the need for an external molecule, through genetic engineering of microalgae strains, are presented and discussed in the third part of this review.
Highlights
Modern life is intimately linked to the availability of fossil fuels, which continue for the moment to meet the world’s growing energy needs even though their use drives climate change (Georgianna & Mayfield )
Depending on several parameters such as the microalgae species used, or the conditions in which they are cultured, one or another mechanism takes place. This makes it an important field of research to identify and understand these mechanisms for all the different species/culturing situations, as being able to use natural flocculation, combined with flotation in harvesting processes, could be the key to reduce the costs associated with microalgae
For example in the case of co-cultures of the fungus Aspergillus flavus and C. vulgaris, it has been shown that fungal cells have a positive zeta potential, allowing their electrostatic interactions with negatively charged C. vulgaris cells (Zhang & Hu ). It is worth mentioning these examples of mixed cultures to induce flocculation of microalgae, as they represent valuable strategies to harvest cells without the addition of any flocculants or modified biopolymers, it is clear that the mechanisms of flocculation in these cases are very specific, and expand out of the ‘classic’ flocculation mechanisms that were described in the first part of this review
Summary
Modern life is intimately linked to the availability of fossil fuels, which continue for the moment to meet the world’s growing energy needs even though their use drives climate change (Georgianna & Mayfield ). This makes it an important field of research to identify and understand these mechanisms for all the different species/culturing situations, as being able to use natural flocculation, combined with flotation in harvesting processes, could be the key to reduce the costs associated with microalgae.
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