Modern microbial mats are potential analogues for Proterozoic ecosystems, yet only a few studies have characterized mats under low-oxygen conditions that are relevant to Proterozoic environments. Here, we use protein-stable isotope fingerprinting (P-SIF) to determine the protein carbon isotope (δC) values of autotrophic, heterotrophic, and mixotrophic organisms in a benthic microbial mat from the low-oxygen Middle Island Sinkhole, Lake Huron, USA (MIS). We also measure the δC values of the sugar moieties of exopolysaccharides (EPS) within the mat to explore the relationships between cyanobacterial exudates and heterotrophic anabolic carbon uptake. Our results show that Cyanobacteria (autotrophs) are C-depleted, relative to sulfate-reducing bacteria (heterotrophs), and C-enriched, relative to sulfur oxidizing bacteria (autotrophs or mixotrophs). We also find that the pentose moieties of EPS are systematically enriched in C, relative to the hexose moieties of EPS. We hypothesize that these isotopic patterns reflect cyanobacterial metabolic pathways, particularly phosphoketolase, that are relatively more active in low-oxygen mat environments, rather than oxygenated mat environments. This results in isotopically more heterogeneous C sources in low-oxygen mats. While this might partially explain the isotopic variability observed in Proterozoic mat facies, further work is necessary to systematically characterize the isotopic fractionations that are associated with the synthesis of cyanobacterial exudates. The δC compositions of heterotrophic microorganisms are dictated by the δC compositions of their organic carbon sources. In both modern and ancient photosynthetic microbial mats, photosynthetic exudates are the most likely source of organic carbon for heterotrophs. We measured the δC values of autotrophic, heterotrophic, and mixotrophic bacteria as well as the δC value of the most abundant photosynthetic exudate (exopolysaccharide) in a modern analogue for a Proterozoic environment. Given these data, future studies will be better equipped to estimate the most likely carbon source for heterotrophs in both modern environments as well as in Proterozoic environments preserved in the rock record.
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http://dx.doi.org/10.1128/aem.01870-22 | DOI Listing |
Food Res Int
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Department of Food Science and Technology, Shahr-e-Qods Branch, Islamic Azad University, Tehran, Iran.
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December 2024
2nd Department of Neurology, "Attikon" General University Hospital, School of Medicine, National and Kapodistrian University of Athens, 12462 Athens, Greece.
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January 2025
Department of Vascular Surgery, Jinhua Central Hospital, Jinhua City, Zhejiang Province 321000, People's Republic of China.
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December 2024
Environment and Plant Protection Institute, Chinese Academy of Tropical Agricultural Science, Haikou 571101, China.
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Department of Surgery, Faculty of Veterinary Medicine, University of Siirt, Kezer Campus, Veysel Karani District, University Street, Siirt/Türkiye.
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