Towards a better understanding of microalgae natural flocculation mechanisms to enhance flotation harvesting efficiency

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.

Effects of Co-culture on Improved Productivity and Bioresource for Microalgal Biomass Using the Floc-Forming Bacteria Melaminivora Jejuensis

“…Recently, studies have reported the application of heterotrophic cultivation methods for Chlorella (Fan et al, 2020). Studies have described microalgal species with high sedimentation and methods for improving microalgae harvesting efficiency through flocculation (Demir et al, 2020). Our research was conducted with wastewater and heterotrophic cultivation approaches in mind.…”

“…Thus, we tried to induce microalgae floc using wastewater-derived or -related bacteria. Chlorella was selected as an experimental microalgal strain due to its smaller cell size than other microalgae, contributing to low sedimentation rates (Cheng and Liu, 2020;Demir et al, 2020;Potocar et al, 2020). Low sedimentation rate is the main limitation for biomass production from Chlorella (Cheng and Liu, 2020;Demir et al, 2020;Potocar et al, 2020); therefore, improving sedimentation can increase the usefulness of Chlorella.…”

“…Chlorella was selected as an experimental microalgal strain due to its smaller cell size than other microalgae, contributing to low sedimentation rates (Cheng and Liu, 2020;Demir et al, 2020;Potocar et al, 2020). Low sedimentation rate is the main limitation for biomass production from Chlorella (Cheng and Liu, 2020;Demir et al, 2020;Potocar et al, 2020); therefore, improving sedimentation can increase the usefulness of Chlorella. Various approaches to improving sedimentation have been attempted (Cheng and Liu, 2020;Demir et al, 2020;Potocar et al, 2020).…”

Data_Sheet_1.pdf

“…Nevertheless, the improved lipid productivity in the MC condition could be attributed to the increased biomass productivity, which improved enough to overcome the lower total lipid contents, unlike the CC condition ( Figure 4A). Additionally, the co-culture biomass value was between the total lipid contents of the pure bacteria and the algae-derived biomass, which is presumed because the co-culture biomass was composed of bacterial and algae-derived biomass together (Berthold et al, 2019;Demir et al, 2020). Moreover, the improvement of lipid productivity and biomass suggests that high-quality biomass can be produced through the co-culture of bacteria and algae.…”

“…Low sedimentation rate is the main limitation for biomass production from Chlorella (Cheng and Liu, 2020;Demir et al, 2020;Potocar et al, 2020); therefore, improving sedimentation can increase the usefulness of Chlorella. Various approaches to improving sedimentation have been attempted (Cheng and Liu, 2020;Demir et al, 2020;Potocar et al, 2020). Furthermore, industrial-scale facilities, such as wastewater treatment plants, require floc-forming and pollutant removal capabilities to remove pollutants.…”