When nutrient concentrations are reported for wastewater, total concentrations of nutrients are often reported. Dissolved inorganic nutrients, such nitrate and ammo­nium N and phosphate P are directly available to microalgae. Part of the total N and P in wastewater, however, may be associated with organic molecules (dissolved organic nutrients) or with particulate matter (either organic or inorganic). These nutrients are not necessarily available to microalgae. Relatively few studies have investigated the bioavailability of nutrients to microalgae in different types of wastewater.

The bioavailability of P is highly variable and may vary between 3.4 and 81 % in different types of wastewaters or surface waters (Ekholm and Krogerus 2003). The bioavailability of total P depends on the dominant P-forms that are present (Van Moorleghem et al. 2013; e. g., Li and Brett 2013). Polyphosphates and organic phosphate monoesters, for instance, have a high bioavailability. Microalgae are capable of producing phosphatase enzymes to dissociate phosphate ester bonds (e. g., Huang and Hong 1999). Phytic acid, on the contrary, has a very low bio­availability and is a major phosphate reserve in plant seeds. As it cannot be digested by livestock, it is present in high concentrations in manure from grain-fed livestock (Jongbloed and Lenis 1998). To increase the bioavailability of P from phytic acid to livestock, phytase enzymes are sometimes added to the animal feed, and this may result in a higher bioavailability of P in the manure. If the wastewater contains a lot of sediments, phosphate may be chemically bound to iron (Fe) or aluminum (Al), and this phosphate has a low bioavailability. Phosphate that is precipitated with calcium (Ca) as apatite minerals also has a low bioavailability to microalgae, despite being often detected by colorimetric phosphate analyses. Wastewater from animal manure has a high content of humic substances and these can also bind phosphate. It is assumed that phosphate is bound to oxidized Al or Fe ions that are stabilized by humic acid complexes (Gerke 2010). These humic acid-metal phos­phate complexes also have a low bioavailability to microalgae.

Less information is available about the bioavailability of organic N forms. About half of the N in animal manure, wastewater may be present as organic N (Cai et al.

2013) . Free amino acids, nucleotides, as well as urea are organic N forms that are highly bioavailable to microalgae. Peptides or proteins have a slightly lower bio­availability, and humic substances contain N and exhibit even less bioavailability (Bronk et al. 2007). Humic substances have a C:N ratio of 18-30:1 for humic acids and 45-55:1 for fulvic acids (See and Bronk 2005). Humic substances can absorb amino acids and ammonium ions, which can represent about half of N in humic substances. Part of the organic N present in wastewater is slowly made available by bacteria that live in symbiosis with microalgae (Pehlivanoglu and Sedlak 2004).