Carotenoid biomarkers in Namibian shelf sediments: Anoxygenic photosynthesis during sulfide eruptions in the Benguela Upwelling System

Abstract

Aromatic carotenoid-derived hydrocarbon biomarkers are ubiquitous in ancient sediments and oils and are typically attributed to anoxygenic phototrophic green sulfur bacteria (GSB) and purple sulfur bacteria (PSB). These biomarkers serve as proxies for the environmental growth requirements of PSB and GSB, namely euxinic waters extending into the photic zone. Until now, prevailing models for environments supporting anoxygenic phototrophs include microbial mats, restricted basins and fjords with deep chemoclines, and meromictic lakes with shallow chemoclines. However, carotenoids have been reported in ancient open marine settings for which there currently are no known modern analogs that host GSB and PSB. The Benguela Upwelling System offshore Namibia, known for exceptionally high primary productivity, is prone to recurrent toxic gas eruptions whereupon hydrogen sulfide emanates from sediments into the overlying water column. These events, visible in satellite imagery as water masses clouded with elemental sulfur, suggest that the Benguela Upwelling System may be capable of supporting GSB and PSB. Here, we compare distributions of biomarkers in the free and sulfur-bound organic matter of Namibian shelf sediments. Numerous compounds—including acyclic isoprenoids, steranes, triterpanes, and carotenoids—were released from the polar lipid fractions upon Raney nickel desulfurization. The prevalence of isorenieratane and β-isorenieratane in sampling stations along the shelf verified anoxygenic photosynthesis by low-light-adapted, brown-colored GSB in this open marine setting. Renierapurpurane was also present in the sulfur-bound carotenoids and was typically accompanied by lower abundances of renieratane and β-renierapurpurane, thereby identifying cyanobacteria as an additional aromatic carotenoid source.

Hydrodynamic regimes modulate nitrogen fixation and the mode of diazotrophy in Lake Tanganyika

“…Many of these upwelling systems are also associated with gammaproteobacterial heterotrophic and often chemotactic diazotrophs 79 , like Pseudomonas 24 , 31 , 35 , 80 , 82 . Moreover, when highly stratified conditions develop in marine upwelling regions (e.g., coastal sulfidic event 83 ), niches for phototrophic green sulfur bacteria become favorable 84 , consistent with observations in Lake Tanganyika’s Northern basin (Table 1 ). The predominance of Dolichospermum in the stratified North/Central basins of Lake Tanganyika shares some parallels to the stratified ocean gyre systems that support higher abundances of large-sized cyanobacteria (e.g., Trichodesmium ) 18 .…”

Reviews and syntheses: Review of proxies for low-oxygen paleoceanographic reconstructions

“…Early sulphurization can trap biomarker signals before diagenetic reworking and can make these S-bound lipids a relatively high-fidelity archive of biomarker information. In both modern and ancient sediments, S-bound lipid distributions are often distinct from free (extractable) lipids, reflecting important aspects of environmental oxygenation such as pigments, steroid distributions, and Cor S-isotope compositions (Kohnen et al, 1991b;Wakeham et al, 1995;Kok et al, 2000;Rosenberg et al, 2018;Ma et al, 2021). Sulphide in the environment may also contribute to the stabilization of free lipids by reducing double bonds (Hebting et al, 2006).…”

Molecular Characterization of Hydrocarbons in Petroleum by Ultrahigh-Resolution Mass Spectrometry

“…Therefore, the content of olefins and aromatics should be strictly controlled in the oil processing process [1,10,11]. In the field of geochemical studies, the characterization of petroleum hydrocarbons contributes to a deeper understanding of source rocks and can better reveal their present-day state and evolutionary history [12][13][14][15][16][17], because biomarkers such as steranes, terpenes, and diamondoids preserve the carbon skeleton of the original biochemical components and record evidence of the evolution from living organic matter to sedimentary organic matter [18][19][20]. Aromatic compounds occupy a large proportion of petroleum and sedimentary organic matter.…”