Nitrogen isotopic fractionation in the synthesis of photosynthetic pigments in Rhodobacter capsulatus and Anabaena cylindrica

Nitrogen isotopic discrimination was investigated for a purple nonsulfur bacterium, Rhodobacter capsulatus, during photoheterotrophic and photoautotrophic growth with N2 and ammonia, and compared with nitrogen assimilation in a cyanobacterium. In R. capsulatus, the Δ15N values for biomass during growth with N2 fixation and ammonia assimilation, relative to the N-source, ranged from −1.8 to −0.8‰ and −12.3 to −10.2‰, respectively. In the cyanobacterium, Anabaena cylindrica, the Δ15N value for biomass resulting from growth on molecular nitrogen was −0.8 to −1.7‰ relative to N-source. These values are similar to those observed for other microorganisms. Bacteriochlorophyll a in R. capsulatus was depleted in 15N relative to biomass, whereas chlorophyll a in A. cylindrica was relatively enriched in 15N.