Distribution of major carbon pools in a cyanobacterial mat from Salin de Giraud, Camargue, France

The fine-scale depth distribution of major carbon pools and their stable carbon isotopic signatures (d13C) were determined in a cyanobacterial mat (Salin-de-Giraud, Camargue, France) to study early diagenetic alterations and the carbon preservation potential in hypersaline mat ecosystems. Particular emphasis was placed on the geochemical role of extracellular polymeric substances (EPS). Total carbon (Ctot), organic carbon (Corg), total nitrogen (Ntot), total hydrolysable amino acids (THAA), carbohydrates, cyanobacteria-derived hydrocarbons (8-methylhexadecane, n-heptadec-5-ene, n-heptadecane) and EPS showed highest concentrations in the top millimetre of the mat and decreased with depth. The hydrocarbons attributed to cyanobacteria showed the strongest decrease in concentration with depth. This correlated well with the depth profiles of oxygenic photosynthesis and oxygen, which were detected in the top 0.6 and 1.05 mm, respectively, at a high down-welling irradiance (1441 µmol photons m**-2 s**-1). At depths beneath the surface layer, the Corg was composed mainly of amino acids and carbohydrates. A resistance towards microbial degradation could have resulted from interactions with diverse functional groups present in biopolymers (EPS) and with minerals deposited in the mat. A 13C enrichment with depth for the total carbon pool (Ctot) was observed, with d13C values ranging from -16.3 permil at the surface to -11.3 permil at 9-10 mm depth. Total lipids depicted a d13C value of -17.2 permil in the top millimetre and then became depleted in 13C with depth (-21.7 to -23.3 permil). The d13C value of EPS varied only slightly with depth (-16.1 to -17.3 permil) and closely followed the d13C value of Corg at depths beneath 4 mm. The EPS represents an organic carbon pool of preservation potential during early stages of diagenesis in recent cyanobacterial mats as a result of a variety of possible interactions. Their analyses might improve our understanding of fossilized microbial remains from mat ecosystems.

为探究高盐蓝藻席（cyanobacterial mat）生态系统中的早期成岩改造作用与碳保存潜力，本研究以法国卡马尔格地区萨兰-吉罗盐沼的蓝藻席为研究对象，测定了其主要碳库的精细深度分布特征与稳定碳同位素组成（d13C）。研究重点关注胞外聚合物（extracellular polymeric substances, EPS）的地球化学作用。总碳（Total carbon, Ctot）、有机碳（organic carbon, Corg）、总氮（total nitrogen, Ntot）、总水解氨基酸（total hydrolysable amino acids, THAA）、碳水化合物、蓝藻源烃类（8-甲基十六烷、正十七碳-5-烯、正十七烷）以及EPS的浓度峰值均出现在席体表层毫米级区间内，并随深度增加逐渐降低；其中蓝藻源烃类的浓度随深度衰减幅度最为显著。该浓度变化趋势与产氧光合作用及氧气的深度分布高度吻合：在1441 μmol光子·m⁻²·s⁻¹的高下行辐照度（down-welling irradiance）条件下，产氧光合作用与氧气的检出深度分别局限于表层0.6 mm与1.05 mm范围内。在表层以下深度区间，有机碳（Corg）主要由氨基酸与碳水化合物组成。其抗微生物降解特性可能源于与生物聚合物（如EPS）中多种官能团的相互作用，以及与席体内沉积矿物的结合作用。总碳库（Ctot）的d13C值随深度增加呈现¹³C富集现象，表层d13C值为-16.3‰，至9~10 mm深度时变为-11.3‰。表层毫米级范围内的总脂类d13C值为-17.2‰，随后随深度增加出现¹³C贫化，数值范围为-21.7‰至-23.3‰。EPS的d13C值随深度变化幅度极小（-16.1‰~-17.3‰），且在4 mm以下深度时与有机碳（Corg）的d13C值高度一致。EPS作为一类有机碳库，在现代蓝藻席的早期成岩阶段具备碳保存潜力，这源于其多种潜在的相互作用机制。对其的分析有助于深化我们对席生态系统中化石微生物残体的认知。