Effects of cell size and specific growth rate on stable carbon isotope discrimination by two species of marine diatom

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 delta(13)C Of dissolved inorganic carbon (DIG), showed no relationship to specific growth rate within species. When examined interspecifically there was some evidence that growth rate or cell size affected the C-13/C-12 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 pmol photon m(-2) s(-1). These data are in agreement with a hypothesis stating that faster-growing diatoms should be enriched in C-13. However, at the highest growth irradiance of 60 mu mol m(-2) s(-1), Delta values were higher in C. calcitrans than in D. brightwellii. Source delta(13)C values varied between individual cultures and demonstrated the importance of directly measuring the delta(13)C of DIC. The value of physiological data in fully interpreting the stable carbon isotope ratios of diatoms is also discussed.