Interpreting lacustrine bulk sediment δ¹⁵N values using metagenomics in a tropical hypersaline lake system

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.