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Exploring the influence of atmospheric CO2 and O2 levels on the utility of nitrogen isotopes as proxy for biological N2 fixation - PubMed

  • ️Mon Jan 01 2024

Exploring the influence of atmospheric CO2 and O2 levels on the utility of nitrogen isotopes as proxy for biological N2 fixation

Nicola Wannicke et al. Appl Environ Microbiol. 2024.

Abstract

Biological N2 fixation (BNF) is traced to the Archean. The nitrogen isotopic fractionation composition (δ15N) of sedimentary rocks is commonly used to reconstruct the presence of ancient diazotrophic ecosystems. While δ15N has been validated mostly using organisms grown under present-day conditions; it has not under the pre-Cambrian conditions, when atmospheric pO2 was lower and pCO2 was higher. Here, we explore δ15N signatures under three atmospheres with (i) elevated CO2 and no O2 (Archean), (ii) present-day CO2, and O2 and (iii) future elevated CO2, in marine and freshwater, heterocytous cyanobacteria. Additionally, we augment our data set from literature for more generalized dependencies of δ15N and the associated fractionation factor epsilon (ε = δ15Nbiomass - δ15NN2) during BNF in Archaea and Bacteria, including cyanobacteria, and habitats. The ε ranges between 3.70‰ and -4.96‰ with a mean ε value of -1.38 ± 0.95‰, for all bacteria, including cyanobacteria, across all tested conditions. The expanded data set revealed correlations of isotopic fractionation of BNF with CO2 concentrations, toxin production, and light, although within 1‰. Moreover, correlation showed significant dependency of ε to species type, C/N ratios and toxin production in cyanobacteria, albeit it within a small range (-1.44 ± 0.89‰). We therefore conclude that δ15N is likely robust when applied to the pre-Cambrian-like atmosphere, stressing the strong cyanobacterial bias. Interestingly, the increased fractionation (lower ε) observed in the toxin-producing Nodularia and Nostoc spp. suggests a heretofore unknown role of toxins in modulating nitrogen isotopic signals that warrants further investigation.IMPORTANCENitrogen is an essential element of life on Earth; however, despite its abundance, it is not biologically accessible. Biological nitrogen fixation is an essential process whereby microbes fix N2 into biologically usable NH3. During this process, the enzyme nitrogenase preferentially uses light 14N, resulting in 15N depleted biomass. This signature can be traced back in time in sediments on Earth, and possibly other planets. In this paper, we explore the influence of pO2 and pCO2 on this fractionation signal. We find the signal is stable, especially for the primary producers, cyanobacteria, with correlations to CO2, light, and toxin-producing status, within a small range. Unexpectedly, we identified higher fractionation signals in toxin-producing Nodularia and Nostoc species that offer insight into why some organisms produce these N-rich toxic secondary metabolites.

Keywords: biological nitrogen fixation; carbon:nitrogen ratios; cyanobacteria; nitrogen isotopic fractionation; nodularin.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig 1
Fig 1

Atmospheric evolution through Earth’s history, showing the first geochemical evidence of key evolutionary processes of life. Redrawn and modified based on data from Goldblatt (51). Information about evolutionary events is reviewed by Lyons et al. (10). In the Archean (4–2.5 Ga), when biological N2 fixation evolved, pCO2 was probably two orders of magnitude higher than it is today, while pO2 was at least six orders of magnitude lower. GOE, Great Oxygenation Event; NOE, Neoproterozoic Oxygenation Event. The oldest geochemical evidence of free O2, thought to have arisen from oxygenic photosynthesis, is indicated.

Fig 2
Fig 2

Box plots of epsilon observed in the laboratory during BNF for the different cyanobacteria investigated in this study. Dark gray box plots indicate toxin (nodularin) producing cyanobacteria, while light gray denotes non-nodularin-producing strains. The spread of the box encompasses the mean 50% of the data (= interquartile range) from the lower Q1 to the upper Q3 (25%–75%).

Fig 3
Fig 3

Correlation of epsilon with biomass C/N ratio and toxin profile for the species tested in the laboratory study. Values of ε displaying significant Spearman correlation coefficients from Table 2 are plotted against C/N ratio (A) and nodularin production score (B).

Fig 4
Fig 4

Box- and scatter plot (single data) displaying the isotopic fractionation factor (ε) for the combined set of diazotrophs. Data obtained from this study as well as literature data were categorized and plotted in three different organismal groups, namely (A) Archaea and (B) with Bacteria plotted above the dashed line, and cyanobacterial species below. Toxin-producing Nostoc and Nodularia spp. are plotted separately from known non-toxin producers. While some strains of Anabaena produce toxins, the toxin status of the species plotted as Anabaena spp. as well as Nodularia spp. without further specification, are unknown. Anabaena variabilis ATCC29413 (64) and Anabaena cylindrica PCC7122 (65) are non-toxin producers combined in Anabaena spp. non-toxic. For detailed information on the expanded data set see Table S2 at

https://doi.org/10.5285/103983f8-b777-418c-9c4d-ba57065c91d5

. The grey dotted line in panel B represents the mean value of ε (−1.38 ± 0.95 ‰) across all bacterial, including cyanobacteria, data points.

Fig 5
Fig 5

Representation of the correlation of epsilon to environmental variables in the expanded data- set. Epsilon values with significant Spearman correlation coefficients from Table 3 were plotted to highlight the correlation to light (A) and CO2 (B) and the toxin nodularin score (C) for the expanded data set.

Fig 6
Fig 6

Box plot of epsilon for the different pCO2 atmospheres of the expanded data set (A) as well as for Nodularia spumigena (B) and Nostoc spp. (C) specifically. Dashed-dotted line in (A) represents the mean value. Different letters in the separate figures denote significant differences (P > 0.05) in ε among the three tested atmospheric conditions (AnoxHC, eCO2, and PAL) as determined by Duncan’s post hoc ANOVA statistical analysis for the whole expanded data-set (A) and non-toxic and toxic N. spumigena strains (B). Note that no significant differences were detected in non-toxic and toxic Nostoc strains (n.s., C). For detailed information on the expanded data set see Tables S2 and S3 at

https://doi.org/10.5285/103983f8-b777-418c-9c4d-ba57065c91d5

.

Fig 7
Fig 7

Geochemical data from the sedimentary rock record. (a) Nitrogen isotopes in bulk rocks and kerogen isolates (organic matter extracted from the bulk rock with hydrofluoric acid). (b) Ratios of organic carbon to total nitrogen. In both panels, samples are separated by metamorphic grade, and samples of amphibolite grade or higher are excluded, because such high metamorphic alteration is likely to have perturbed the primary signature by more than 2‰ [reviewed by reference (112)]. Plots are generated based on data from Johnson and Stüeken (113).

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References

    1. Alberty RA. 2005. Thermodynamics of the mechanism of the nitrogenase reaction. Biophys Chem 114:115–120. doi:10.1016/j.bpc.2004.11.009 - DOI - PubMed
    1. Barth P, Stüeken EE, Helling C, Rossmanith L, Peng Y, Walters W, Claire M. 2023. Isotopic constraints on lightning as a source of fixed nitrogen in Earth’s early biosphere. Nat Geosci 16:478–484. doi:10.1038/s41561-023-01187-2 - DOI
    1. Navarro‐González R, Molina MJ, Molina LT. 1998. Nitrogen fixation by volcanic lightning in the early Earth. Geophys Res Lett 25:3123–3126. doi:10.1029/98GL02423 - DOI
    1. Wong ML, Charnay BD, Gao P, Yung YL, Russell MJ. 2017. Nitrogen oxides in early Earth's atmosphere as electron acceptors for life's emergence. Astrobiology 17:975–983. doi:10.1089/ast.2016.1473 - DOI - PubMed
    1. Mather TA, Pyle DM, Allen AG. 2004. Volcanic source for fixed nitrogen in the early Earth's atmosphere. Geol 32:905. doi:10.1130/G20679.1 - DOI

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