Assessing Nutrient Status of Microalgae Using Chlorophyll a Fluorescence

Abstract

Phytoplankton require carbon, nitrogen and phosphorus in the approximate elemental ratio 106:16:1 (Redfield 1958), although this can vary among differ- ent taxa and growth conditions (Geider and La Roche 2002; Ho et al. 2003; Sterner et al. 2008). Other nutri- ents, including iron, calcium, manganese, copper and zinc are also required in smaller and varying amounts (Morel and Hudson 1985). Silica can also be limiting to the growth of diatoms (Carrick and Lowe 2007; Shipe et al. 2007). If the concentration of a particular nutrient drops in real terms or in relation to the other key nutrients, it may limit the growth rate, maximum biomass and/or species composition of a phytoplankton community. Considering that oceanic phytoplankton account for up to 50% of the total global primary production (Behrenfeld et al. 2001), an understanding of the limitations to the growth of phytoplankton is of considerable importance, and increasingly so, as carbon cycling dominates scientific and political discussions. The availability of nutrients is one potential limitation, with light being the other major limiting factor. Excepting carbon (C), nitrogen (N) and phosphorus (P) are the principal nutrients likely to be in limiting supply (Birch and Gordon 1981; Lean and Pick 1981; Schindler 1977; Sundareshwar et al. 2003; Wynne and Berman 1980), and are traditionally thought to be limiting in marine and freshwater ecosystems respectively, because they are found in ratios below what is typically found in nutrient replete phytoplankton cells (Hecky and Kilham 1988). More recent studies have found, however, that areas such as the North Pacific Sub-tropical Gyre and regions of the Mediterranean Sea are in fact P-limited (Karl 1999; Krom et al. 1991), while freshwater systems such as Lake Victoria may be N-limited (Guildford and Hecky 2000). In areas such as the equatorial Pacific and Southern Ocean, which are deemed ‘high nutrient, low chlorophyll’ (HNLC) regions, iron is a significant limiting resource. (Behrenfeld and Bale 1996; Boyd et al. 1999; Martin and Fitzwater 1988; Timmermans et al. 1998). Davey et al. (2008) have also noted that P-depletion is occur- ring in the North Atlantic, where aeolian dust inputs are relieving Fe-limitation and thus allowing nitrogen fixation.