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Projections of future rates of mass loss from the Greenland Ice Sheet are highly uncertain because its sensitivity to warming is unclear. Geologic reconstructions of Quaternary interglacials can illustrate how the ice sheet responded during past warm periods, providing insights into ice sheet behavior and important tests for data-model comparisons. However, paleoclimate records from Greenland are limited: Early Holocene peak warmth has been quantified at only a few sites, and terrestrial sedimentary records of prior interglacials are exceptionally rare due to glacial erosion during the last glacial period. Here, we discuss findings from a lacustrine archive that records both the Holocene and the Last Interglacial (LIG) from Greenland, allowing for direct comparison between two interglacials. Sedimentary chironomid assemblages indicate peak July temperatures 4.0 to 7.0 °C warmer than modern during the Early Holocene maximum in summer insolation. and chironomids in LIG sediments indicate July temperatures at least 5.5 to 8.5 °C warmer than modern. These estimates indicate pronounced warming in northwest Greenland during both interglacials. This helps explain dramatic ice sheet thinning at Camp Century in northwest Greenland during the Early Holocene and, for the LIG, aligns with controversial estimates of Eemian warming from ice core data retrieved in northern Greenland. Converging geologic evidence for strong LIG warming is challenging to reconcile with inferred Greenland Ice Sheet extent during the LIG, and the two appear incompatible in many models of ice sheet evolution. An increase in LIG snowfall could help resolve this problem, pointing to the need for hydroclimate reconstructions from the region.

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are ubiquitous and well preserved sedimentary biomarkers. These compounds serve as important palaeoenvironmental indicators due to strong empirical correlations between brGDGT distributions and temperature and pH in modern environments. However, the mechanistic link between temperature, pH, and brGDGT production has been impossible to ascertain thus far due to the absence of a clear biological source for brGDGTs. Here, we report that oxygen limitation triggers brGDGT production in at least one cultured species of Acidobacteria and confirm for the first time the biosynthesis of three structural varieties of brGDGTs, including an uncharacterised isomer of brGDGT Ic. This discovery helps explain why brGDGT producers have been so difficult to identify and provides a pathway towards uncovering the genetic basis and biological function of brGDGTs, which will lead to a more comprehensive understanding of their palaeoenvironmental significance. If the oxygen effects observed here apply more broadly, the empirical calibrations for brGDGT-based temperature and pH reconstructions may currently be missing the effects of oxygen as a relevant and possibly dominant control in the environmental distributions of brGDGTs.

The rate at which microorganisms grow and reproduce is fundamental to our understanding of microbial physiology and ecology. While soil microbiologists routinely quantify soil microbial biomass levels and the growth rates of individual taxa in culture, there is a limited understanding of how quickly microbes actually grow in soil. For this work, we posed the simple question: what are the growth rates of soil microorganisms? In this study, we measure these rates in three distinct soil environments using hydrogen stable isotope probing of lipids with 2H-enriched water. This technique provides a taxa-agnostic quantification of in situ microbial growth from the degree of 2H enrichment of intact polar lipid compounds ascribed to bacteria and fungi. We find that average apparent generation times in soil are quite slow (20 to 64 days) but also highly variable at the compound-specific level (6 to 1137 days), suggesting differential growth rates between community subsets. We observe that low-biomass communities can exhibit more rapid growth rates than high-biomass communities, highlighting that biomass quantity alone does not predict microbial productivity in soil. Furthermore, within a given soil, the rates at which specific lipids are being synthesized do not relate to their quantity, suggesting a general decoupling of microbial abundance and growth in soil microbiomes. More generally, we demonstrate the utility of lipid stable isotope probing for measuring microbial growth rates in soil and highlight the importance of measuring growth rates to complement more standard analyses of soil microbial communities.

The rate at which microorganisms grow and reproduce is fundamental to our understanding of microbial physiology and ecology. While soil microbiologists routinely quantify soil microbial biomass levels and the growth rates of individual taxa in culture, there is a limited understanding of how quickly microbes actually grow in soil. For this work, we posed the simple question: what are the growth rates of soil microorganisms? In this study, we measure these rates in three distinct soil environments using hydrogen-stable isotope probing of lipids with 2 H-enriched water. This technique provides a taxa-agnostic quantification of in situ microbial growth from the degree of 2 H enrichment of intact polar lipid compounds ascribed to bacteria and fungi. We find that growth rates in soil are quite slow and correspond to average generation times of 14 to 45 d but are also highly variable at the compound-specific level (4 to 402 d), suggesting differential growth rates among community subsets. We observe that low-biomass microbial communities exhibit more rapid growth rates than high-biomass communities, highlighting that biomass quantity alone does not predict microbial productivity in soil. Furthermore, within a given soil, the rates at which specific lipids are being synthesized do not relate to their quantity, suggesting a general decoupling of microbial abundance and growth in soil microbiomes. More generally, we demonstrate the utility of lipid-stable isotope probing for measuring microbial growth rates in soil and highlight the importance of measuring growth rates to complement more standard analyses of soil microbial communities.

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