Table_8_Taphonomy of Biosignatures in Microbial Mats on Little Ambergris Cay, Turks and Caicos Islands.docx

Microbial mats are taxonomically and metabolically diverse microbial ecosystems, with a characteristic layering that reflects vertical gradients in light and oxygen availability. Silicified microbial mats in Proterozoic carbonate successions are generally interpreted in terms of the surficial, mat building community. However, information about biodiversity in the once-surface-layer can be lost through decay as the mats accrete. To better understand how information about surface microbial communities is impacted by processes of decay within the mat, we studied microbial mats from Little Ambergris Cay, Turks and Caicos Islands. We used molecular techniques, microscopy and geochemistry to investigate microbial mat taphonomy – how processes of degradation affect biological signatures in sedimentary rocks, including fossils, molecular fossils and isotopic records. The top < 1 cm of these mats host cyanobacteria-rich communities overlying and admixed with diverse bacterial and eukaryotic taxa. Lower layers contain abundant, often empty, sheaths of large filamentous cyanobacteria, preserving their record as key mat-builders. Morphological remains and free lipid biomarkers of several bacterial groups, as well as diatoms, arthropods, and other eukaryotes also persist in lower mat layers, although at lower abundances than in surface layers. Carbon isotope signatures of organic matter were consistent with the majority of the biomass being sourced from CO2-limited cyanobacteria. Porewater sulfide sulfur isotope values were lower than seawater sulfate sulfur isotope values by ∼45–50‰, consistent with microbial sulfate reduction under sulfate-replete conditions. Our findings provide insight into how processes of degradation and decay bias biosignatures in the geological record of microbial mats, especially mats that formed widely during the Proterozoic (2,500–541 million years ago) Eon. Cyanobacteria were the key mat-builders, their robust and cohesive fabric retained at depth. Additionally, eukaryotic remains and eukaryotic biosignatures were preserved at depth, which suggests that microbial mats are not inherently biased against eukaryote preservation, either today or in the past.

微生物席（microbial mats）是兼具分类学与代谢多样性的微生物生态系统，其标志性的层理结构可反映光照与氧气可获得性的垂直梯度变化。元古宙（Proterozoic）碳酸盐岩序列中的硅化微生物席，通常被认为对应于形成该席体的表层群落。然而，随着微生物席的沉积加积，其原表层的生物多样性信息可能因腐解作用而遗失。为深入探究微生物席内部的腐解过程如何影响表层微生物群落的信息留存，我们对特克斯和凯科斯群岛小安布里斯礁岛的微生物席展开了研究。我们采用分子技术、显微镜观测与地球化学方法，研究微生物席埋藏学（taphonomy）——即降解过程如何影响沉积岩中的生物标志物信号，涵盖化石、分子化石与同位素记录。该类微生物席的表层（厚度不足1 cm）以蓝细菌（cyanobacteria）群落为主，其上方覆盖并混杂着多样的细菌与真核生物类群。下层则富含大型丝状蓝细菌的鞘层（多为空鞘），留存了其作为核心席构建群落的演化记录。下层微生物席中同样留存有多种细菌类群的形态遗迹与游离脂质生物标志物，同时还有硅藻（diatoms）、节肢动物（arthropods）与其他真核生物的遗迹，不过其丰度较表层更低。有机质的碳同位素信号表明，绝大多数生物量源自受二氧化碳限制的蓝细菌群落。孔隙水硫化物的硫同位素值较海水硫酸盐的硫同位素值低约45‰~50‰，这与硫酸盐充足条件下的微生物硫酸盐还原作用特征相符。本研究结果揭示了降解与腐解过程如何对微生物席地质记录中的生物标志物信号产生偏差影响，这一结论尤其适用于元古宙（25亿~5.41亿年前）广泛发育的微生物席。蓝细菌作为核心的席构建群落，其坚固且具有黏结性的结构在深层沉积物中得以留存。此外，深层沉积物中同样留存有真核生物遗迹与真核生物生物标志物信号，这表明无论在现代还是地质历史时期，微生物席本身并不会对真核生物的保存产生固有偏差。