The multidimensional nutrient screen. (a) Initial Screen 1 (Screen 1.1) optimises phosphate concentration (0, 2, 10 mM KH2PO4) as well as N type (NaNO3, NH4Cl, (NH2)2CO and NH4NO3) and concentration (0 to 30 mM). The N source concentration is adjusted to account for the number of N atoms in the source (for example, NaNO3 = 1, NH4NO3 = 2). (b) Screen 2 uses a Box-Behnken design in which the nitrogen/phosphate condition that yielded the highest growth rate in Screen 1 form the midpoint of this multidimensional screen. It has three concentration levels (−1, 0 and +1) for the elements Ca, Mg, Fe, Cu, Mn, Zn, B, Se, V and Si. The trace elements Mo, Co and vitamins B1 and B12 are kept constant (see ‘Materials and methods’). Screens 1 and 2 are designed to measure the rate of change in optical density (for example, OD750) to define specific growth rates and total yield as a proxy for biomass production. (c) Screen 1.2 is the repeat of Screen 1.1 with adjusted concentrations of the elements tested in Screen 2 to demonstrate algae performance improvements.