USGS2011
Natural variations in the ratios of nitrogen isotopes in biomass reflect variations in nutrient sources utilized for growth. In order to use δ 15 N values of chloropigments of photosynthetic organisms to determine the corresponding δ 15 N values of biomass – and by extension, surface waters – the isotopic offset between chlorophyll and biomass must be constrained. Here we examine this offset in various geologically-relevant taxa, grown using nutrient sources that may approximate ocean conditions at different times in Earth’s history. Phytoplankton in this study include cyanobacteria (diazotrophic and non-diazotrophic), eukaryotic algae (red and green), and anoxygenic photosynthetic bacteria (Proteobacteria), as well as environmental samples from sulfidic lake water. Cultures were grown using N 2 , NO 3 − , and NH 4 + as nitrogen sources, and were examined under different light regimes and growth conditions. We find surprisingly high variability in the isotopic difference (δ 15 N biomass − δ 15 N chloropigment ) for prokaryotes, with average values for species ranging from −12.2‰ to +11.7‰. We define this difference as ε por , a term that encompasses diagenetic porphyrins and chlorins, as well as chlorophyll. Negative values of ε por reflect chloropigments that are 15 N-enriched relative to biomass. Notably, this enrichment appears to occur only in cyanobacteria. The average value of ε por for freshwater cyanobacterial species is −9.8 ± 1.8‰, while for marine cyanobacteria it is −0.9 ± 1.3‰. These isotopic effects group environmentally but not phylogenetically, e.g., ε por values for freshwater Chroococcales resemble those of freshwater Nostocales but differ from those of marine Chroococcales. Our measured values of ε por for eukaryotic algae (range = 4.7–8.7‰) are similar to previous reports for pure cultures. For all taxa studied, values of ε por do not depend on the type of nitrogen substrate used for growth. The observed environmental control of ε por suggests that values of ε por could be useful for determining the fractional burial of eukaryotic vs. cyanobacterial organic matter in the sedimentary record.
Geochimica et Cosmochimica Acta↗