•This review investigates the sources, cycling, and trophic modification of nitrogen.•δ15N values of amino acids are useful for estimating trophic positions of organisms.•The δ15N value of ...phenylalanine indicates the δ15N baseline of the food web.
Compound-specific isotopic analysis of amino acids (CSIA-AA) has emerged in the last decade as a powerful approach for tracing the origins and fate of nitrogen in ecological and biogeochemical studies. This approach is based on the empirical observation that source amino acids (SAAs) (i.e., phenylalanine), fractionate 15N very little (< 0.5‰) during trophic transfer, whereas trophic AAs (TAAs) (i.e., glutamic acid), are greatly (∼6–8‰) enriched in 15N during each trophic step. The differential fractionation of these two AA groups can provide a valuable estimate of consumer trophic position that is internally indexed to the baseline δ15N value of the integrated food web. In this paper, we critically review the analytical methods for determining the nitrogen isotopic composition of AAs by gas chromatography–isotope-ratio mass spectrometry. We also discuss methodological considerations for accurate trophic position assessment of organisms using CSIA-AA. We then discuss the advantages and challenges of the CSIA-AA approach using published case studies across a range of topics, including trophic position assessment in various ecosystems, reconstruction of ancient human diets, reconstruction of animal migration and environmental variability, and assessment of marine organic matter dynamics with new classification of microbial fractionation patterns. It is clear that the CSIA-AA approach can provide unique insight into the sources, cycling, and trophic modification of organic nitrogen as it flows through systems. However, this approach will be greatly improved through continued exploration into how biochemical, physiological, and ecological mechanisms affect isotopic fractionation of individual AAs. We end this review with a perspective on future work that will promote the evolution of the rapidly growing field of CSIA-AA.
•Pure culture experiments for δ15NAA values in five chemotrophic microbes.•First published data for δ15NAA in chemolithoautotrophic microbes and Archaea.•Algae- or animal-like δ15NAA fractionation in ...microbes under different N sources.•Specific metabolic pathways responsible for the δ15NAA fractionation.
The nitrogen isotopic composition of amino acids (δ15NAA) is a promising tool for estimating the trophic position and nitrogen sources of organisms in grazing food webs. The utility of δ15NAA analysis of microbial processes in the biogeochemistry remains uncertain, because information about the δ15NAA values of chemotrophic microbes are still limited. In this study, we report the δ15NAA values in five cultured heterotrophic or chemolithoautotrophic microbes (one fungus, one bacterium, and three archaea) with controlled nitrogen sources in synthetic media. When the microbes were grown on ammonium, the apparent nitrogen isotopic fractionation of amino acids relative to glutamic acid in the microbes (e.g., −3.0±1.0‰ for phenylalanine) was similar to those in algae. When the microbes were grown on a free amino acid mixture, the δ15N offsets of amino acids in the microbial cells relative to the amino acids in the culture media (e.g., +8.2±0.8‰ for glutamic acid and +0.1±0.2‰ for phenylalanine) were close to the standard isotopic fractionation between animals and their diets. These results suggest that the δ15NAA pattern can be used to distinguish two end-member metabolic modes in microbes (i.e., de novo synthesis of amino acids from inorganic nitrogen vs. assimilation of amino acids from environments). We also discussed the effects of metabolic pathways on the δ15NAA values and their fractionation with respect to the diversity of amino acid metabolism in microbes. These findings will form an important baseline for interpreting the δ15NAA values of organisms and detritus in natural environments.
This study explores the use of compound-specific nitrogen isotopes of amino acids (δ15NAA) of coupled dissolved and particulate organic nitrogen (DON, PON) samples as a new approach to examine ...relative sources, transformation processes, and the potential coupling of these two major forms of Ncycle in the ocean water column. We measured δ15NAA distributions in high-molecular-weight dissolved organic nitrogen (HMW DON) and suspended PON in the North Pacific Subtropical Gyre (NPSG) from surface to mesopelagic depths. A new analytical approach achieved far greater δ15NAA measurement precision for DON than earlier work, allowing us to resolve previously obscured differences in δ15NAA signatures, both with depth and between ON pools. We propose that δ15N values of total hydrolysable amino acids (THAA) represents a proxy for proteinaceous ON δ15N values in DON and PON. Together with bulk δ15N values, this allows δ15N values and changes in bulk, proteinaceous, and “other-N” to be directly evaluated.
These novel measurements suggest three main conclusions. First, the δ15NAA signatures of both surface and mesopelagic HMW DON suggest mainly heterotrophic bacterial sources, with mesopelagic HMW DON bearing signatures of far more degraded material compared to surface material. These results contrast with a previous proposal that HMW DON δ15NAA patterns are essentially “pre-formed” by cyanobacteria in the surface ocean, undergo little change with depth. Second, different δ15NAA values and patterns of HMW DON vs. suspended PON in the surface NPSG suggest that sources and cycling of these two N reservoirs are surpisingly decoupled. Based on molecular δ15N signatures, we propose a new hypothesis that production of surface HMW DON is ultimately derived from subsurface nitrate, while PON in the mixed layer is strongly linked to N2 fixation and N recycling. In contrast, the comparative δ15NAA signatures of HMW DON vs. suspended PON in the mesopelagic also suggest a possible PON source for some HMW DON in the mid-water column. Together, these results suggest that conversion of relatively labile ON to less labile DON by heterotrophic bacteria (a “microbial nitrogen pump”) may be the key pathway for production and alteration of DON in both the surface and the mesopelagic oligotrophic ocean. Finally, in contrast to THAA, δ15N values of the other-N were substantially less affected by heterotrophic alteration, which may be consistent with a larger than expected contribution of amino sugars, or other less labile nitrogenous organic molecules.
The Maunder Minimum (A.D. 1645–1715) is a useful period to investigate possible sun—climate linkages as sunspots became exceedingly rare and the characteristics of solar cycles were different from ...those of today. Here, we report annual variations in the oxygen isotopic composition (δ¹⁸O) of tree-ring cellulose in central Japan during the Maunder Minimum. We were able to explore possible sun—climate connections through high-temporal resolution solar activity (radiocarbon contents; Δ¹⁴C) and climate (δ¹⁸O) isotope records derived from annual tree rings. The tree-ring δ¹⁸O record in Japan shows distinct negative δ¹⁸O spikes (wetter rainy seasons) coinciding with rapid cooling in Greenland and with decreases in Northern Hemisphere mean temperature at around minima of decadal solar cycles. We have determined that the climate signals in all three records strongly correlate with changes in the polarity of solar dipole magnetic field, suggesting a causal link to galactic cosmic rays (GCRs). These findings are further supported by a comparison between the interannual patterns of tree-ring δ¹⁸O record and the GCR flux reconstructed by an ice-core ¹⁰Be record. Therefore, the variation of GCR flux associated with the multidecadal cycles of solar magnetic field seem to be causally related to the significant and widespread climate changes at least during the Maunder Minimum.
The El Niño–Southern Oscillation (ENSO) appears to strongly influence East Asian Summer Monsoon (EASM) rainfall, but the relatively short instrumental rainfall record hinders the development of a ...longer-term understanding of this teleconnection. To partially overcome this issue, here we reconstruct precipitation from tree-ring oxygen isotopes (δ18O) in central Japan during AD 1612–1935. Our results indicate that tree-ring cellulose δ18O is significantly correlated with May–June (MJ) precipitation, allowing us to investigate the link between the EASM summer rainfall and ENSO over the past 400 years. Time- and frequency-domain comparison of the tree-ring δ18O record and recent ENSO reconstructions reveal a common high-frequency (3–8 year) variability that characterized the mid-17th, late 18th and late 19th centuries. Similar analyses of instrumental MJ precipitation and several ENSO indexes during the 20th century indicate that this high-frequency oscillation reappeared from AD 1980. Comparison of ENSO and Pacific Decadal Oscillation (PDO) indexes indicates that the ENSO–EASM teleconnection is strong when ENSO variance is high, and the PDO phase may modulate the ENSO–EASM relationship over the past 400 years.
Microbial life inhabiting subseafloor sediments plays an important role in Earth's carbon cycle. However, the impact of geodynamic processes on the distributions and carbon-cycling activities of ...subseafloor life remains poorly constrained. We explore a submarine mud volcano of the Nankai accretionary complex by drilling down to 200 m below the summit. Stable isotopic compositions of water and carbon compounds, including clumped methane isotopologues, suggest that ~90% of methane is microbially produced at 16° to 30°C and 300 to 900 m below seafloor, corresponding to the basin bottom, where fluids in the accretionary prism are supplied via megasplay faults. Radiotracer experiments showed that relatively small microbial populations in deep mud volcano sediments (10
to 10
cells cm
) include highly active hydrogenotrophic methanogens and acetogens. Our findings indicate that subduction-associated fluid migration has stimulated microbial activity in the mud reservoir and that mud volcanoes may contribute more substantially to the methane budget than previously estimated.
Integrated Ocean Drilling Program (IODP) Expedition 329 made major strides toward fulfilling its objectives. Shipboard studies documented (1) fundamental aspects of habitability and life in this very ...low activity subseafloor sedimentary ecosystem and (2) first-order patterns of habitability within the igneous basement. A broad range of postexpedition studies will complete the expedition objectives. Throughout the South Pacific Gyre (SPG; Sites U1365–U1370), dissolved oxygen and nitrate are present throughout the entire sediment sequence, and sedimentary microbial cell counts are lower than at all previously drilled IODP/ Ocean Drilling Program (ODP)/Deep Sea Drilling Program (DSDP) sites. In contrast, at Site U1371 in the upwelling zone just south of the gyre, detectable oxygen and nitrate are limited to the top and bottom of the sediment column, manganese reduction is a prominent electron-accepting process, and cell concentrations are higher than at the same depths in the SPG sites throughout the sediment column. Geographic variation in subseafloor profiles of dissolved and solid-phase chemicals are consistent with the magnitude of organic-fueled subseafloor respiration declining from outside the gyre to the gyre center.
Chemical profiles in the sedimentary pore water and secondary mineral distributions in the basaltic basement indicate that basement alteration continues on the timescale of formation fluid replacement, even at the sites with the oldest basement (84–120 Ma at Sites U1365 and U1366).
doi: 10.2204/iodp.sd.15.01.2013
Integrated Ocean Drilling Program (IODP) Expedition 329 made major strides toward fulfilling its objectives. Shipboard studies documented (1) fundamental aspects of habitability and life in this very ...low activity subseafloor sedimentary ecosystem and (2) first-order patterns of habitability within the igneous basement. A broad range of postexpedition studies will complete the expedition objectives.Throughout the South Pacific Gyre (SPG; Sites U1365–U1370), dissolved oxygen and nitrate are present throughout the entire sediment sequence, and sedimentary microbial cell counts are lower than at all previously drilled IODP/Ocean Drilling Program (ODP)/Deep Sea Drilling Program (DSDP) sites. In contrast, at Site U1371 in the upwelling zone just south of the gyre, detectable oxygen and nitrate are limited to the top and bottom of the sediment column, manganesereduction is a prominent electron-accepting process, and cell concentrations are higher than at the same depths in the SPG sites throughout the sediment column.Geographic variation in subseafloor profiles of dissolved and solid-phase chemicals are consistent with the magnitude of organic-fueled subseafloor respiration declining from outside the gyre to the gyre center.Chemical profiles in the sedimentary pore water and secondary mineral distributions in the basaltic basement indicate that basement alteration continues on the timescale of formation fluid replacement, even at the sites with the oldest basement (84–120 Ma at Sites U1365 and U1366).
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