Natural variations in the ratios of nitrogen isotopes in biomass reflect variations in nutrient sources utilized for growth. In order to use delta 15N values of chloropigments of photosynthetic organisms to determine the corresponding delta 15N 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 N2, NO3-, and NH4+ as nitrogen sources, and were examined under different light regimes and growth conditions. We find surprisingly high variability in the isotopic difference ( delta 15Nbiomass - delta 15Nchloropigment) for prokaryotes, with average values for species ranging from -12.2 to +11.7. We define this difference as epsilon por, a term that encompasses diagenetic porphyrins and chlorins, as well as chlorophyll. Negative values of epsilon por reflect chloropigments that are 15N-enriched relative to biomass. Notably, this enrichment appears to occur only in cyanobacteria. The average value of delta epor 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., delta epor values for freshwater Chroococcales resemble those of freshwater Nostocales but differ from those of marine Chroococcales. Our measured values of epsilon por for eukaryotic algae (range = 4.7--8.7) are similar to previous reports for pure cultures. For all taxa studied, values of epsilon por do not depend on the type of nitrogen substrate used for growth. The observed environmental control of delta epor suggests that values of epsilon por could be useful for determining the fractional burial of eukaryotic vs. cyanobacterial organic matter in the sedimentary record.