Ecological aspects of carbon and nitrogen isotope ratios of cyanobacteria

Abstract

Variation in the δ13C of a cultured marine cyanobacterium (Agmenellum quadruplicatum, strain PR-6) is described with an emphasis on the relationships between growth rate, pCO2, and δ13C. An average nitrogen isotope fractionation of 1.0017 was obtained during N2fixation by cultured marine cyanobacteria. This result suggests that nitrogen supply via N2fixation may be characterized by a low d15N, down to ca. -2%₀ in marine environments, where the average δ15N is higher (at ca. 6%₀) for major inorganic nitrogenous compounds such as nitrate. The natural abundances of nitrogen and carbon isotope ratios for cyanobacteria collected from the McMurdo Dry Valleys, Antarctica, were characterized by extremely low δ15N and widely ranging δ13C. Summarizing these and other data obtained, an isotopic map of cyanobacteria and other plankton in aquatic ecosystems was constructed and the ecological implications are discussed. Our results suggest that the δ13C of marine phytoplankton is positively correlated with sea surface temperature and can be a useful parameter for estimating in situ growth rates in the open ocean. © 2012, The Plankton Society of Japan, The Japanese Association of Benthology. All rights reserved.