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 N₂ fixation by cultured marine cyanobacteria. This result suggests that nitrogen supply via N₂ fixation may be characterized by a low δ¹⁵N, down to ca. -2‰ in marine environments, where the average δ¹⁵N is higher (at ca. 6‰) 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.