Nitrogen isotope ratios of red tide organisms in the East China Sea: A characterization of biological nitrogen fixation

Abstract

Nitrogen isotope fractionation in the process of N 2 fixation was examined in detail by culturing the blue green alga Anabaena cylindrica. The δ 15N value of the alga was almost constant ( − 0.6 ± 0.9‰ on average) irrespective of growth conditions, similar to other N 2 fixing microorganisms previously reported. Nitrogen isotope ratios of marine plankton from the East China Sea have been studied to evaluate the contribution of N 2 fixation to the red tide organisms, predominated by the N 2 fixing blue-green alga Trichodesmium spp. The δ 15N of Trichodesmium ( − 2.1 to + 1.0‰) showed the same range of that estimated if the biological atmospheric N 2 fixation was the main nitrogen source of the red tide organisms. Mixed plankton collected by towing a phytoplankton net (96 μm) gave from 2 to 4‰ higher δ 15N than that of sorted phytoplankton. Zooplankton and fish collected from the same red tide area showed systematic enrichment of 15N along the food chain, and exhibited relatively lower δ 15N than previous results for zooplankton from other regions, reflecting biological N 2 fixation. The δ 15N of mixed plankton in the surface seawater from 32°N to 20°N latitude along 125°E longitude decreased from 6 to 1‰. A symmetrical relation was found between the horizontal distribution of δ 15N of plankton, the concentration of Trichodesmium and the trend of salinity.