Houghton J, Fike D, Druschel G, Orphan V, Hoehler T M, Des Marais D J
Department of Earth and Planetary Sciences, Washington University, St. Louis, MO, USA.
Geobiology. 2014 Nov;12(6):557-74. doi: 10.1111/gbi.12113.
Modern laminated photosynthetic microbial mats are ideal environments to study how microbial activity creates and modifies carbon and sulfur isotopic signatures prior to lithification. Laminated microbial mats from a hypersaline lagoon (Guerrero Negro, Baja California, Mexico) maintained in a flume in a greenhouse at NASA Ames Research Center were sampled for δ(13) C of organic material and carbonate to assess the impact of carbon fixation (e.g., photosynthesis) and decomposition (e.g., bacterial respiration) on δ(13) C signatures. In the photic zone, the δ(13) C org signature records a complex relationship between the activities of cyanobacteria under variable conditions of CO2 limitation with a significant contribution from green sulfur bacteria using the reductive TCA cycle for carbon fixation. Carbonate is present in some layers of the mat, associated with high concentrations of bacteriochlorophyll e (characteristic of green sulfur bacteria) and exhibits δ(13) C signatures similar to DIC in the overlying water column (-2.0‰), with small but variable decreases consistent with localized heterotrophic activity from sulfate-reducing bacteria (SRB). Model results indicate respiration rates in the upper 12 mm of the mat alter in situ pH and HCO3- concentrations to create both phototrophic CO2 limitation and carbonate supersaturation, leading to local precipitation of carbonate minerals. The measured activity of SRB with depth suggests they variably contribute to decomposition in the mat dependent on organic substrate concentrations. Millimeter-scale variability in the δ(13) C org signature beneath the photic zone in the mat is a result of shifting dominance between cyanobacteria and green sulfur bacteria with the aggregate signature overprinted by heterotrophic reworking by SRB and methanogens. These observations highlight the impact of sedimentary microbial processes on δ(13) C org signatures; these processes need to be considered when attempting to relate observed isotopic signatures in ancient sedimentary strata to conditions in the overlying water column at the time of deposition and associated inferences about carbon cycling.
现代层状光合微生物垫是研究微生物活动如何在石化之前创造和改变碳和硫同位素特征的理想环境。在美国国家航空航天局艾姆斯研究中心温室中的水槽里维持的来自高盐泻湖（墨西哥下加利福尼亚州格雷罗内格罗）的层状微生物垫，被采集用于分析有机物质和碳酸盐的δ(13)C，以评估碳固定（如光合作用）和分解（如细菌呼吸）对δ(13)C特征的影响。在光合带，δ(13)C有机特征记录了在二氧化碳限制的可变条件下蓝细菌活动之间的复杂关系，同时绿色硫细菌利用还原性三羧酸循环进行碳固定也有显著贡献。碳酸盐存在于垫子的一些层中，与高浓度的细菌叶绿素e（绿色硫细菌的特征）相关，并且其δ(13)C特征与上覆水柱中的溶解无机碳（-2.0‰）相似，有小幅度但可变的降低，这与硫酸盐还原细菌（SRB）的局部异养活动一致。模型结果表明，垫子上部12毫米处的呼吸速率改变了原位pH值和HCO3-浓度，导致光合二氧化碳限制和碳酸盐过饱和，从而导致碳酸盐矿物的局部沉淀。测量的SRB随深度的活动表明，它们对垫子中分解的贡献因有机底物浓度而异。垫子光合带以下δ(13)C有机特征的毫米级变化是蓝细菌和绿色硫细菌之间优势转移的结果，总体特征被SRB和产甲烷菌的异养改造所叠加。这些观察结果突出了沉积微生物过程对δ(13)C有机特征的影响；在试图将古代沉积地层中观察到的同位素特征与沉积时上覆水柱的条件以及相关的碳循环推断联系起来时，需要考虑这些过程。
