Abstract. We determined both the nitrogen and carbon isotopic compositions of various vanadyl alkylporphyrins isolated from siliceous marine sediments of the Onnagawa Formation (middle Miocene, northeastern Japan) to investigate the biogeochemistry and ecology of photoautotrophs living in the paleo-ocean. The distinctive isotopic signals support the interpretations of previous works that the origin of 17-nor-deoxophylloerythroetioporphyrin (DPEP) is chlorophylls-c_1-3, whereas 8-nor-DPEP may have originated from chlorophylls-a₂ or b₂ or bacteriochlorophyll-a. Although DPEP and cycloheptanoDPEP are presumably derived from common precursory pigments, their isotopic compositions differed in the present study, suggesting that the latter represents a specific population within the photoautotrophic community. The average δ¹⁵N value for the entire photoautotrophic community is estimated to be –2 to +1‰ from the δ¹⁵N values of DPEP (–6.9 to –3.6‰; n=7), considering that the empirical isotopic relationships that the tetrapyrrole nuclei of chloropigments are depleted in ¹⁵N by ~4.8‰ and enriched in ¹³C by ~1.8‰ relative to the whole cells. This finding suggests that nitrogen utilized in the primary production was supplied mainly through N₂-fixation by diazotrophic cyanobacteria. Based on the δ¹³C values of DPEP (–17.9 to –15.6‰; n=7), we estimated isotopic fractionation associated with photosynthetic carbon fixation to be 8–14‰. This range suggests the importance of β-carboxylation and/or active transport of the carbon substrate, indicating in turn the substantial contribution of diazotrophic cyanobacteria to primary production. Based on the δ¹⁵N values of 17-nor-DPEP (–7.4 to –2.4‰ n=7), the δ¹⁵N range of chlorophylls-c-producing algae was estimated to be –3 to +3‰. This relative depletion in sup>15N suggests that these algae mainly utilized nitrogen regenerated from diazotrophic cyanobacteria. Given that diatoms are likely to have constituted the chlorophylls-c-producing algae within the biogenic-silica-rich Onnagawa Formation, cyanobacteria-hosting diatoms may have been important contributors to primary production.