Reconstruction of the biogeochemistry and ecology of photoautotrophs based on the nitrogen and carbon isotopic compositions of vanadyl porphyrins from Miocene siliceous sediments

Abstract

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-a 2 or b 2 or bacteriochlorophylla. 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 δ 15 N value for the entire photoautotrophic community is estimated to be -2 to +1‰ from the δ 15 N 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 15 N by ∼4.8‰ and enriched in 13 C by ∼1.8‰ relative to the whole cells. This finding suggests that nitrogen utilized in the primary production was supplied mainly through N 2 -fixation by diazotrophic cyanobacteria. Based on the δ 13 C 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 Correspondence to: Y. Kashiyama (chiro@jamstec.go.jp) the carbon substrate, indicating in turn the substantial contribution of diazotrophic cyanobacteria to primary production. Based on the δ 15 N values of 17-nor-DPEP (-7.4 to -2.4‰; n=7), the δ 15 N range of chlorophylls-c-producing algae was estimated to be -3 to +3‰. This relative depletion in 15 N suggests that these algae mainly utilized nitrogen regenerated from diazotrophic cyanobacteria. Given that diatoms are likely to have constituted the chlorophylls-cproducing algae within the biogenic-silica-rich Onnagawa Formation, cyanobacteria-hosting diatoms may have been important contributors to primary production.

Ubiquity and quantitative significance of detoxification catabolism of chlorophyll associated with protistan herbivory

“…183 Ma) (Fig. S5) (10,(43)(44)(45)(46)(47)(48)(49)(50). Interestingly, fossilbased evidence suggests that this timing correlates roughly with the emergence of secondary algae containing red algae-derived plastids.…”

The origin of Cretaceous black shales: a change in the surface ocean ecosystem and its triggers

“…4 ), a derivative of chlorophylls c 1 – c 3 , 109 ) was also observed. 110 ) Surprisingly, the nitrogen isotopic composition of 17-nor-DPEP is also in the range of N 2 -fixation (Fig. 5 ).…”

“…To resolve this apparent contradiction, it has been proposed that a symbiotic association of diazotrophic cyanobacteria and algae is an important primary producer. 110 ) This symbiosis is widespread in the modern nitrate-depleted ocean. In the present-day ocean, symbiosis between diatoms and cyanobacteria is widely observed, and the association often forms extensive blooms.…”

Paleoenvironmental implications of taxonomic variation among δ15N values of chloropigments

“…The pattern of "eukaryotic" e por values near 5& even appears to hold during Cretaceous Oceanic Anoxic Events (OAEs), when productivity has been suggested to have been influenced by a relatively large increase in cyanobacterial N 2 -fixation. Other results to date include a sample of a Triassic oil shale (Chicarelli et al, 1993), two data points for the Bonarelli shale of Cretaceous OAE 2 (Ohkouchi et al, 2006), four additional samples from the Bonarelli and one from the older Selli level (Kashiyama et al, 2008a), and seven horizons from the middle Miocene (Kashiyama et al, 2008b). Most of these values are $4& (rather than a typical $5&) depleted in 15 N relative to coeval sedimentary nitrogen or kerogen; with the exception of the Triassic sample, which has a smaller apparent e por value of around 2-3&.…”

“…Since degradation of the tetrapyrrole macrocycle occurs initially at C-C methine bonds, measuring intact chlorins and porphyrins ensures a nitrogen isotopic value unaltered by diagenesis (Louda and Baker, 1986). Recent advances in analytical techniques have enabled the measurement of minute quantities of chlorins and porphyrins, rendering these methods applicable for paleoceanographic studies on even low total organic carbon (TOC) sediments (Chikaraishi et al, 2008;Kashiyama et al, 2008bKashiyama et al, , 2010Higgins et al, 2009Higgins et al, , 2010. However, these proxies can be used to constrain paleoceanographic interpretations only if the relationship between chloropigment and total biomass d 15 N values is well understood across phylogenetically diverse phytoplankton, including eukaryotic phytoplankton, as well as aerobic and anaerobic photosynthetic bacteria.…”