Some microalgae flocculate spontaneously without addition of flocculants or pre­cipitation of minerals, and the phenomenon is generally referred to as biofloccu­lation. Bioflocculation can occur spontaneously in microalgae-based wastewater treatment systems, including high-rate microalgal ponds where it is often used in biomass harvesting (Park et al. 2011b; Benemann et al. 2012).

Bioflocculation is often due to a dominance of microalgae species with high settling rates, often colonial and/or large species of microalgae. For instance, high flocculation efficiencies are observed when the large chlorophyte Pediastrum is dominant (Park et al. 2011a). By recycling part of the harvested biomass, the dominance of this species in the community can be effectively maintained over long periods of time (Park et al. 2013). High flocculation efficiencies are also observed when filamentous cyanobacteria are prominent in the microalgal community (Su et al. 2011). Spontaneously flocculating microalgae can also be used to harvest other, non-flocculating species of microalgae by mixing bioflocculating and non­flocculating microalgae (Salim et al. 2011, 2012). Some bioflocculating species of microalgae such as the diatom Skeletonema appear to produce infochemicals that are capable of inducing flocculation in other microalgal species (Taylor et al. 2012). Cultures of otherwise non-flocculating species microalgae can be ‘trained’ to induce bioflocculation. For instance, Su et al. (2012) repeatedly removed all microalgae that remained in suspension and kept only the rapidly settling microalgae and after one month obtained a culture that flocculated spontaneously. A major advantage of spontaneous flocculation of microalgae is that no chemicals are added during the process and the harvested biomass is free from contaminants. In order to be able to use bioflocculation as a reliable harvesting method though, more research is needed to better understand the underlying mechanisms that cause the phenomenon.

Bioflocculation may also be caused by other microorganisms in the microalgal culture, such as bacteria or fungi. For example Lee et al. (2012c) showed that bacteria present in cultures of the chlorophyte Chlorella prompted autoflocculation. Bacteria or fungi can be cultured separately and added to a microalgal culture — induced flocculation (Zhou et al. 2012, 2013). Alternatively, bacteria or fungi can be co-cultured with the microalgae, in which case the culture medium should contain a carbon source to support heterotrophic growth of bacteria or fungi (Lee et al. 2008; Gultom and Hu 2013). Bacterial bioflocculation may be a particularly promising flocculation method in wastewater treatment systems, as wastewater often contains a carbon source to sustain bacterial growth. However, an optimal balance between heterotrophic bacterial and autotrophic microalgal production is required for optimal flocculation. This can be controlled by optimizing the ratio of organic carbon over inorganic carbon in the medium (Van Den Hende et al. 2011,

2014) .