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

Abstract

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 lightlimlted continuous cultures to produce a wide range of specific growth rates from 0.12 d' in D. brightwellii to 1.01 d-' in C. calcitrans. Carbon isotope discrimination (A) values, relative to source 8':'C 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 '/C 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. brightwehi. Values of A were greater in D. brightwellii at PFDs of 5. 20 and 40 pm01 photon m-2 S-'. These data are in agreement with a hypothesis stating that fastergrowing diatoms should be enriched in I3C. However, at the highest growth irradiance of 60 pm01 m-' S-', A values were higher in C. calcitrans than in D. brightwellii. Source 6'.'(: values varied between individual cultures and demonstrated the ~mportance of directly measuring the 6' of DIC. The value of physiological data in fully interpreting the stable carbon isotope ratios of diatoms is also discussed.