MEPS 143:283-288 (1996)  -  doi:10.3354/meps143283

Effects of cell size and/or specific growth rate were studied in 2 species of marine diatom, the large-celled Ditylum brightwellii and the smaller Chaetoceros calcitrans. Cells were grown as light-limited continuous cultures to produce a wide range of specific growth rates from 0.12 d^-1 in D. brightwellii to 1.01 d^-1 in C. calcitrans. Carbon isotope discrimination (Delta) values, relative to source delta¹³C of dissolved inorganic carbon (DIC), showed no relationship to specific growth rate within species. When examined interspecifically there was some evidence that growth rate or cell size affected the ¹³C/¹²C ratios of the diatoms. At each photon flux density (PFD) used for growth, the specific growth rate of C. calcitrans was at least twice that of D. brightwellii. Values of Delta were greater in D. brightwellii at PFDs of 5, 20 and 40 µmol photon m^-2 s^-1. These data are in agreement with a hypothesis stating that faster-growing diatoms should be enriched in ¹³C. However, at the highest growth irradiance of 60 µmol m^-2 s^-1, Delta values were higher in C. calcitrans than in D. brightwellii. Source delta¹³C values varied between individual cultures and demonstrated the importance of directly measuring the delta¹³C of DIC. The value of physiological data in fully interpreting the stable carbon isotope ratios of diatoms is also discussed.

Carbon isotope discrimination · Cell size · Chaetoceros calcitrans · Ditylum brightwellii · Photon flux density · Specific growth rate