Effect of phosphorus limitation on elemental composition and stable carbonisotope fractionation in a marine diatom growing under different CO2 concentrations

Blooms of the marine diatom Skeletonema costatum were initiated in closed-s ystem batch cultures with P-deficient medium under two different initial co ncentrations of dissolved molecular CO2 ([CO2,aq]: 20.6 and 4.5 mu mol L-1) . Algal C : N : P ratios strongly increased with decreasing P concentration . In the exponential growth phase, C : N ratios were 1.3 mol mol(-1) higher in the low relative to the high [CO2,aq] treatment. There was no [CO2,aq] effect on C : N : P ratios during P-limited growth. Carbon isotope fraction ation (epsilon (p)) was 2-3 parts per thousand higher in the high [CO2,aq] treatment. With growth rate decreasing due to P limitation, epsilon (p) inc reased in both [CO2,aq] treatments by 2-3 parts per thousand despite decrea sing [CO2,aq]. Under these conditions the effect of decreasing growth rate on isotope fractionation strongly dominated over that of declining CO2 avai lability. When extrapolated to the natural environment, these results imply that systematic changes in algal growth, as occurring during the course of phytoplankton blooms, may affect algal isotope fractionation. These result s severely complicate the interpretation of carbon isotope measurements in suspended and sedimentary organic matter.