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

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