Algae require about 30 different elements for growth (Kaplan et al. 1986). In this section, however, I will only consider those which are known to influence growth and photosynthesis most. Ever since Redfield (Redfield 1934, 1958) proposed the C:N:P ratio of 106:16:1 (known as the Redfield Ratio) as representing the average atomic ratio of phytoplankton there has been discussion on how representative it is. It is clear that there are significant variations from this ratio depending on growth conditions, between taxa and with cell size (Falkowski 2000; Geider and Roche 2002; Ho et al. 2005), but ‘healthy’ phytoplankton cells appear to have a composition close to this ratio. Quigg et al. (2003) extended the Redfield ratio to include trace elements with the average composition of phytoplankton in this study found to be C
. This ratio serves as a good working approximation of the nutrient requirements of microalgae and serves as a start for medium design and optimisation.