Fossilized carotenoid hydrocarbons provide a window into the physiology and biochemistry of ancient microbial phototrophic communities for which only a sparse and incomplete fossil record exists. However, accurate interpretation of carotenoid-derived biomarkers requires detailed knowledge of the carotenoid inventories of contemporary phototrophs and their physiologies. Here we report two distinct patterns of fossilized C40 diaromatic carotenoids. Phanerozoic marine settings show distributions of diaromatic hydrocarbons dominated by isorenieratane, a biomarker derived from low-light-adapted phototrophic green sulfur bacteria. In contrast, isorenieratane is only a minor constituent within Neoproterozoic marine sediments and Phanerozoic lacustrine paleoenvironments, for which the major compounds detected are renierapurpurane and renieratane, together with some novel C39 and C38 carotenoid degradation products. This latter pattern can be traced to cyanobacteria as shown by analyses of cultured taxa and laboratory simulations of sedimentary diagenesis. The cyanobacterial carotenoid synechoxanthin, and its immediate biosynthetic precursors, contain thermally labile, aromatic carboxylic-acid functional groups, which upon hydrogenation and mild heating yield mixtures of products that closely resemble those found in the Proterozoic fossil record. The Neoproterozoic–Phanerozoic transition in fossil carotenoid patterns likely reflects a step change in the surface sulfur inventory that afforded opportunities for the expansion of phototropic sulfur bacteria in marine ecosystems. Furthermore, this expansion might have also coincided with a major change in physiology. One possibility is that the green sulfur bacteria developed the capacity to oxidize sulfide fully to sulfate, an innovation which would have significantly increased their capacity for photosynthetic carbon fixation.

Understanding the causal mechanisms of past marine deoxygenation is critical to predicting the long-term Earth systems response to climate change. However, the processes and events preceding widespread carbon burial coincident with oceanic anoxic events remain poorly constrained. Here, we report a comprehensive biomarker inventory enveloping Oceanic Anoxic Event 2 that captures microbial communities spanning epipelagic to benthic environments in the southern proto-North Atlantic Ocean. We identify an abrupt, sustained increase in primary productivity that predates Oceanic Anoxic Event 2 by ∼220 ± 4 thousand years, well before other geochemical proxies register biogeochemical perturbations. During the event, recurrent photic zone euxinia triggered a major marine microbial reorganization accompanied by a decrease in primary production. These findings highlight how organic carbon burial drivers operated along a continuum in concert with microbial ecological changes, with antecedent, localized increases in primary production destabilizing carbon cycling and promoting the progressive marine deoxygenation leading to Oceanic Anoxic Event 2.

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids that are frequently employed as paleoenvironmental proxies because of the strong empirical correlations between their relative abundances and environmental temperature and pH. Despite the ubiquity of brGDGTs in modern and paleoenvironments, the source organisms of these enigmatic compounds have remained elusive, requiring paleoenvironmental applications to rely solely on observed environmental correlations. Previous laboratory and environmental studies have suggested that the globally abundant bacterial phylum of the Acidobacteria may be an important brGDGT producer in nature. Here, we report on experiments with a cultured Acidobacterium, Solibacter usitatus, that makes a large portion of its cellular membrane (24 ± 9% across all experiments) out of a structurally diverse set of tetraethers including the common brGDGTs Ia, IIa, IIIa, Ib, and IIb. Solibacter usitatus was grown across a range of conditions including temperatures from 15 to 30°C, pH from 5.0 to 6.5, and O2 from 1% to 21%, and demonstrated pronounced shifts in the degree of brGDGT methylation across these growth conditions. The temperature response in culture was in close agreement with trends observed in published environmental datasets, supporting a physiological basis for the empirical relationship between brGDGT methylation number and temperature. However, brGDGT methylation at lower temperatures (15 and 20°C) was modulated by culture pH with higher pH systematically increasing the degree of methylation. In contrast, pH had little effect on brGDGT cyclization, supporting the hypothesis that changes in bacterial community composition may underlie the link between cyclization number and pH observed in environmental samples. Oxygen concentration likewise affected brGDGT methylation highlighting the potential for this environmental parameter to impact paleotemperature reconstruction. Low O2 culture conditions further resulted in the production of uncommon brGDGT isomers that could be indicators of O2 limitation. Finally, the production of brGTGTs (trialkyl tetraethers) in addition to the previously discovered iso‐C15‐based mono‐ and diethers in S. usitatus suggests a potential biosynthetic pathway for brGDGTs that uses homologs of the archaeal tetraether synthase (Tes) enzyme for tetraether synthesis from diethers.

Studying tropical hydroclimate and productivity change in the past is critical for understanding global climate dynamics. Northwest Australia is an ideal location for investigating Australian monsoon dynamics, the variability of the Indonesian Throughflow (ITF), and their impact on past productivity and warm pool evolution, which remain poorly understood during the 40 kyr world in the mid-early Pleistocene. In this study, we present multi-proxy records from International Ocean Discovery Program (IODP) Site U1483 in the Timor Sea spanning the last 2000 ka, including orbitally-resolved records from the 40 kyr world between 2000 and 1300 ka. Our results suggest that northwest Australia underwent a step of increased aridification and that productivity in the Timor Sea declined during the transition from ~ 1700 to ~ 1400 ka. We attribute this aridification to the reduced moisture supply to this region caused by the ITF restriction and warm pool contraction. We ascribe the declined productivity to a decrease in the nutrient supply of the Pacific source water associated with global nutrient redistribution. At orbital timescale, multiple mechanisms, including sea level changes, monsoon, and the Intertropical Convergence Zone (ITCZ) dynamics, and variations in the ITF and Walker circulation could control variations of productivity and terrigenous input in the Timor Sea during the 40 kyr world. Our bulk nitrogen and benthic carbon isotope records suggest a strong coupling to biogeochemical changes in the Pacific during this period. This research contributes to a better understanding of tropical hydroclimate and productivity changes during the 40 kyr world.

Rationale Cyclic tetrapyrroles, such as chlorophylls and their diagenetic derivatives, are structurally diverse and often chelated with certain metal species in the natural environment. A high throughput analytical method enabling quick tetrapyrrole screening in complex samples will promote the study of tetrapyrrole biogeochemistry and probably discoveries of new tetrapyrroles. Methods Total lipid extracts of biological and environmental samples were injected onto a C18 column to separate compounds with a reverse‐phase gradient. Collision‐induced dissociation (CID) was performed at different energy levels, from 40 to 200 eV, on a quadrupole time‐of‐flight mass spectrometry (QTOF‐MS) to identify cyclic tetrapyrroles in complex matrices. Results Under 200 eV CID cyclic tetrapyrroles exhibit a unique fragmentation behavior, the production of fragments larger than 300 Da. Utilizing such feature as a filter to extract product ions in the range of 300–500 Da, various cyclic tetrapyrrole derivatives are readily recognized in all tested biological and environmental samples. The 200 eV CID setup also dissociates chelated to porphyrin metals, including Cu, Fe, Mn, Ni, and V, as single‐charged ions for direct MS detection. Conclusions The 200 eV CID setup provides an efficient approach for the identification of cyclic tetrapyrroles, such as chlorophylls and fossil metalloporphyrins, in complex environmental samples. The direct detection of chelated to porphyrin metal ions with QTOF‐MS shows the potential for compound‐specific metal isotope analysis.