Impact of phosphorus quota and growth phase on carbon allocation in Chlamydomonas reinhardtii: an FTIR microspectroscopy study

Batch cultures of Chlamydomonas reinhardtii were used to study carbon allocation in relation to growth phase and phosphorus availability. Cultures were grown at initial phosphorus (PO4-P) concentrations of 500 g l-1 (high-P) and 50 g l-1 (low-P). Cellular carbon allocation was monitored using Fourier transform infrared (FTIR) microspectroscopy with the ratio of the band intensities at 1736 cm-1 (lipid) and the 1180-950 cm-1 region (carbohydrate) to 1652 cm-1 (amide I) used as an index of changing carbon balance. Cellular phosphorus concentrations (P quota) were measured by energy dispersive X-ray microanalysis (EDXRMA). Both treatments entered stationary phase on day 18. Increased cell counts in the high-P treatment (max. 3.0 106 cells ml-1 at stationary phase) led to a rapid decrease in external P availability to 20 g l-1 during early log phase, with a subsequent decrease in P quota from 0.5% to 0.1% DW. The fall of P quota to 0.1% led to an increase in the lipid/protein ratio (0.13 to 0.23) and carbohydrate/protein ratio (0.37 to 1.57), with ratios increasing further (lipid:protein 1.85; carbohydrate:protein 2.77) in late stationary phase. In the low-P treatment external P concentrations (20 g l-1 from day 1) restricted population growth (max. 0.75 106 cells ml-1 at stationary phase). P quotas fell to 0.1% in early log phase, with the carbohydrate/protein ratio increasing from 0.15 to 3.7 and remaining high into stationary phase while the lipid/protein ratio increased from 0.2 to 1.2. In both treatments increasing synthesis of lipid and carbohydrate storage products resulted in an increased cell volume. Transfer of P-deficient cells (late stationary) to fresh media led to a rapid stimulation of growth, a rapid reduction in lipid/protein and carbohydrate/protein ratios, and decreased cell volumes.