Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA.
Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan, USA.
Appl Environ Microbiol. 2023 May 31;89(5):e0187022. doi: 10.1128/aem.01870-22. Epub 2023 Apr 24.
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.
现代微生物垫是原生代生态系统的潜在类似物，但只有少数研究描述了与原生代环境相关的低氧条件下的垫。在这里，我们使用蛋白质稳定同位素指纹图谱（P-SIF）来确定美国休伦湖低氧中岛水坑（MIS）底部微生物垫中自养生物、异养生物和混合营养生物的蛋白质碳同位素（δC）值。我们还测量了垫内胞外多糖（EPS）的糖部分的 δC 值，以探索蓝藻分泌物与异养生物合成碳吸收之间的关系。我们的结果表明，与硫酸盐还原菌（异养生物）相比，蓝藻（自养生物）是 C 消耗型的，而与硫氧化菌（自养生物或混合营养生物）相比，蓝藻是 C 富集型的。我们还发现 EPS 的戊糖部分比 EPS 的己糖部分系统地富集 C。我们假设这些同位素模式反映了蓝藻的代谢途径，特别是磷酸酮酶，在低氧垫环境中相对活跃，而不是在充氧垫环境中活跃。这导致低氧垫中存在同位素异质性更高的 C 源。虽然这可能部分解释了在原生代垫相中观察到的同位素可变性，但需要进一步的工作来系统地描述与蓝藻分泌物合成相关的同位素分馏。异养微生物的 δC 组成由其有机碳源的 δC 组成决定。在现代和古代光合微生物垫中，光合分泌物是异养生物最有可能的有机碳源。我们测量了现代原生代环境类似物中自养生物、异养生物和混合营养细菌的 δC 值以及最丰富的光合作用分泌物（胞外多糖）的 δC 值。有了这些数据，未来的研究将能够更好地估计现代环境以及保存在岩石记录中的原生代环境中异养生物最可能的碳源。
