Depending on the type of reactor and the substrate used, it can be difficult to maintain monocultures of high-lipid-producing strains. For example, mixotrophic cultivation in open ponds, using wastewater as the nutrient source, will result in a significant reduction in the cost of production. However, under such conditions, monocultures of oleaginous strains are likely to be outcompeted by faster growing species of microalgae or cyanobacteria. Therefore, it is important to explore the use of naturally occurring mixed cultures in wastewaters. Lipid content of mixed cul­tures in municipal wastewater was reported to be 11.3 %, and as high as 29 % when grown with anaerobic digester effluent (Woertz et al. 2009). Griffiths (2009) reported a fatty acid methyl ester content of as high as 23.4 % after in situ transesterification of a mixed culture grown in municipal wastewater.

Mixed cultures of selected strains of microalgae, as well as mixed cultures of microalgae and yeasts have also been investigated. Mixed cultures can be designed for efficient absorption of light (combining strains with different light absorption spectrum) or better nutrient utilization (combining strains with different nutrient preference). Competition for nutrients can lead to a limitation of nutrients, such as P and N, which can induce oil accumulation. Mixtures of Chlamydomonas and Chlorella, Scenedesmus and Chlorella, or a combination of three strains Chla­mydomonas, Chlorella, and Scenedesmus were investigated for efficient oil pro­duction (Bhatnagar et al. 2011). In a mixed culture of Chlorella and yeast under mixotrophic cultivation with molasses as the carbon source, the biomass, lipid content per cell, and lipid productivities were higher than the values obtained in a monoculture of either of the strains (Leesing et al. 2012).