The use of the ¹³C and ¹⁵N isotopes for the simultaneous measurement of carbon and nitrogen turnover rates in marine phytoplankton1

Abstract

A stable carbon isotope ( 13 C) is used to estimate carbon assimilation by phytoplankton. Limited modifications to the existing mass spectrometric 15 N method allow the simultaneous measurement of carbon and nitrogen uptake by phytoplankton. Details of the 13 C procedure are given. Data on simultaneously obtained 15 N, 13 C, and 14 C uptake rates are presented for three species of marine algae grown under varying conditions of light and nutrients. A good agreement between 13 C and 14 C uptake rates was found (correlation coefficient of 0.91 was calculated from the data). The 13 C method should be useful in investigating the relationship between photosynthesis and inorganic nitrogen assimilation in natural phytoplankton and laboratory cultures.