Self-aggregation for sustainable harvesting of microalgae

Microalgae can be cultivated to support wastewater treatment or produce human and animal feed, fuels, and high value metabolites (e.g., cosmetics). However, the low biomass concentration achieved in photoautotrophic algal cultures means that a large volume of culture must be dewatered before processing the biomass. The simplest and cheapest method of dewatering is sedimentation. However, microalgae cells have an inherently slow settling velocity due to their small size, low density, and negative surface charges; thus, expensive chemical flocculants are often added to increase the settling efficiency and speed. Other dewatering methods can be very energy-demanding and/or require high capital and operational investment (e.g., centrifugation). Biomass dewatering therefore has environmental impacts related to equipment, energy and/or chemical uses and can account for up to 30 % of the cost of microalgal biomass production. A potential novel harvesting strategy involves triggering self-aggregation of microalgae cells, which eliminates the need for expensive chemical flocculants that contaminate the biomass and enables economic harvesting of the biomass by settling and/or cell centrifugation. This review discusses self-aggregation, reported for numerous microalgae in response to stresses such as predation, changes in temperature, light, and solute concentrations. While the mechanisms of self-aggregation are not completely understood, identification of candidate genes in Chlamydomonas, Tetradesmus and Microcystis suggests that self-aggregation can involve predator sensing, cell-cell communication signals, extracellular polysaccharides and/or cell modifications. Predator-induced self-aggregation has been proposed to involve "infochemical" transfer between the predator and microalga. To date, only infochemicals from the predator Daphnia pulex have been identified and reported to induce self-aggregation of Scenedesmus gutwinskii. We estimate that if effective on a commercial scale, the use of infochemicals could reduce the cost of chemical microalgae flocculation by three to five orders of magnitude.