Tracing carbon fixation in phytoplankton-compound specific and total 13C incorporation rates.

Measurement of total primary production using ¹³C incorporation is a widely established tool. However, these bulk measurements lack information about the fate of fixed carbon: the production of major cellular compounds (carbohydrates, amino acids, fatty acids, and DNA/RNA) is affected by for instance nutrient availability as their C:N:P requirements differ. Here, we describe an approach to combine established methods in gas chromatography/isotope ratio mass spectrometry (GC/C-IRMS) and recently developed methods in liquid chromatography/IRMS (LC/IRMS) to trace stable isotope incorporation into neutral carbohydrates, amino acids, and fatty acids, and compare their production to total carbon fixation rates. We conducted a trial study at stations in the North Sea where different nutrients were limiting. There was variation in the fate of fixed carbon at these sites. The majority of fixed carbon (64-71%) was incorporated into neutral carbohydrates, followed by amino acids (19-32%) and fatty acids (4-9%). The sum of carbon fixation into these three fractions accounted for 81-116% of total carbon fixation. P- and N-limitation increased the biosynthesis of storage lipids and storage carbohydrates while N-limitation decreased synthesis of amino acid proline with a concurrent increase in glutamic acid + glutamine. This new approach provides the capability to determine direct effects of resource limitation and the consequences for the physiological state of a phytoplankton community. It may thereby enable us to evaluate the overall quality of phytoplankton as a food source for higher trophic levels or trace consumption of phytoplankton through the food web. [ABSTRACT FROM AUTHOR]