Nitrogen (N) is often a limiting nutrient in lacustrine systems, and bulk organic matter stable isotope ratios of N (δ15N) are widely used in lake sediment studies to interpret N source inputs and lake trophic status. Although records of lacustrine sedimentary δ15N can provide critical information relating to past environmental change, often productivity interpretations from δ15N and lacustrine fossil records yield conflicting interpretations. Furthermore, components of the internal N cycle have substantial isotopic fractionation factors, and likely wield an enormous influence on bulk lacustrine sedimentary δ15N values. Yet apart from cyanobacteria N-fixation, few studies link specific microbial, N-related activity to δ15N variability in lake sediment records. Here, we assess the relationship between lacustrine sedimentary δ15N and microbiome profiles analyzed from extracted sediment DNA using metagenomics. In a ~ 1600-year-long sediment record from a hypersaline lake located on Kiritimati, Republic of Kiribati (1.9° N, 157.4° W), both δ15N and the taxonomy annotations from five unique metagenomes vary with depth. Despite the relatively high abundance of Cyanobacteria, Bacteroidetes, and N-fixation genes in the uppermost sediment, we find the highest δ15N values of the sediment record there. These high values are likely due to denitrification, supported by a relatively high abundance of denitrification genes and taxa responsible for denitrification, such as those found in family Chromatiaceae within the Gamma-proteobacteria. In the deep sediment, N-related biochemical processes are likely suppressed considering the low energy, low nutrient subsurface environment. Low δ15N values observed in deeper sediments co-occur with genes for assimilatory nitrate reduction and ammonification. Thus, metagenomics provides greater clarity with respect to the specific, microbial processes that alter primary δ15N signatures in the subsurface sediment.
Bibliographical noteFunding Information:
This research was funded by ACS-PRF 57417-DNI2 and NSF-EAR 1602590 to JLC. We thank the UIUC Roy J. Carver Biotechnology Center, specifically the High-throughput Sequencing and Genotyping Unit, for DNA sample analysis. We thank A. Wyman, M. Higley, and C. Karamperidou for field assistance and B. Fouke and M. Christie for useful comments and advice.
© 2020, Springer Nature B.V.
Copyright 2020 Elsevier B.V., All rights reserved.
- Hypersaline lakes
- N cycle