The influence of phosphorus availability on carbon allocation and P quota in Scenedesmus subspicatus: A synchrotron-based FTIR analysis

Synchrotron-based Fourier transform infrared (FTIR) microspectroscopy was used to characterise the molecular composition of the freshwater alga Scenedesmus subspicatus, cultured at three different initial concentrations of phosphorus (PO4-P): 0.05 mg l-1, 0.5 mg l-1 and 5 mg l-1. These led, respectively, to limited algal growth due to phosphorus deficiency (low-P culture), maximum algal growth with no luxury consumption (intermediate-P) and maximum algal growth with luxury phosphorus consumption (high-P culture). In all cultures, FTIR spectra had nine major absorbance bands (wavenumber range 1760-900 cm-1), including bands at 1736 cm-1 (lipid), 1652 cm-1 (amide I) and the region from 1180 to 950 cm-1 (carbohydrate). Internal phosphorus concentrations (Qp), determined by energy dispersive X-ray microanalysis, differed markedly between low-P (typically <0.1% dry wt), intermediate-P (> 0.1%) and high-P (> 0.3%) cultures. During logarithmic growth phase, a rapid change in carbon allocation was observed in the low-P cultures, with increases in both the lipid:protein (0.1-0.34) and carbohydrate:protein (0.4-1.0) ratio. Mean cell volume increased by 60%, and the mean chlorophyll a content remained consistently low (typically <0.2 pg cell-1). The change in carbon allocation was triggered primarily by low Qp values rather than low external (culture medium) concentrations. Intermediate-P and high-P cultures showed higher chlorophyll a content (> 0.2 pg cell-1) and changes in carbon allocation only after entry into stationary phase. No increase in cell volume occurred, suggesting that a switch in carbon allocation during stationary phase (intermediate- and high-P cultures) rather than log phase (low-P culture) does not result in an increase in cell size. Entry into stationary growth phase occurred simultaneously in all three cultures and was not caused by internal (Qp) or external phosphorus depletion. Medium replacement in late stationary phase (day 35) cultures led to a rapid stimulation of growth, with a reversed carbon allocation (reduced lipid:protein and carbohydrate:protein ratios) and in low-P cultures a decrease in cell volume.