An advantage of most wastewaters is that it contains N as well as P; the two most important nutrients required by microalgae. Microalgae, however, consume N and P in a more or less fixed ratio: the Redfield ratio; a molar ratio of N:P of 16:1. The ratio of N to P in wastewater is highly variable between wastewaters and even within a specific type of wastewater. If the N:P ratio in the wastewater exceeds 16:1, P will be limiting the growth of the microalgae, and vice versa. High or low N:P ratios may result in incomplete removal of N or P from the wastewater (Li and Brett 2013). However, studies on the stoichiometric composition of microalgae in the past two decades have indicated that the N:P Redfield ratio of 16 is not as fixed as it was once believed (Sterner and Elser 2002). When both the nutrient supply rate and microalgal growth rates are high, the N:P ratio in algae varies from 5 to 19. Conversely, when microalgal growth rates are limited by nutrients, this range is extended from less than 5 to more than 100; a strong deviation from the Redfield ratio (Geider and Roche 2002). Nutrient limitation influences the biochemical composition of the microalgae. Without nutrient limitation, microalgae tend to have a high content of proteins. When nutrients are limiting, the microalgae will accu­mulate carbohydrates or lipids (Smith et al. 2010; Markou et al. 2012a; Gonzalez — Fernandez and Ballesteros 2013). Whether carbohydrates or lipids are accumulated under nutrient starvation depends on the species of microalgae and the degree of nutrient limitation. Changes in the biochemical composition of the biomass as a result of variations in the N:P ratio of the wastewater may have important conse­quences for the valorization of the microalgal biomass. If N and P are balanced, the biomass will have a high protein content and will be attractive for animal feed production. When N or P are limiting, lipids or carbohydrates are accumulated and the biomass can be used for biodiesel, bioethanol, or biomethane production. When P concentrations in the medium are high, some microalgae can accumulate excess

P as polyphosphate granules, a phenomenon known as luxury uptake. Luxury uptake occurs when P-limited microalgae are suddenly supplied with P. This luxury uptake appears to be a transient phenomenon, as microalgae quickly convert their internal P-reserves into growth (Powell et al. 2009). The N:P ratio of microalgae is not only influenced by the N:P ratio of the culture medium, but it also differs between different species or groups of microalgae. P is mainly used in the micro­algal cells to produce rRNA, which is part of the ribosomes. Fast-growing mic­roalgae species have a high content of rRNA to produce new proteins and tend to have a high P content (Klausmeier et al. 2004). Cyanobacteria, on the contrary, have a high protein content and therefore have a higher N:P ratio than other mic­roalgae (Becker 2007; Lopez et al. 2010).