Table_3_Environmental and Biological Influences on Carbonate Precipitation Within Hot Spring Microbial Mats in Little Hot Creek, CA.DOCX

Microbial mats are found in a variety of modern environments, with evidence for their presence as old as the Archean. There is much debate about the rates and conditions of processes that eventually lithify and preserve mats as microbialites. Here, we apply novel tracer experiments to quantify both mat biomass addition and the formation of CaCO3. Microbial mats from Little Hot Creek (LHC), California, contain calcium carbonate that formed within multiple mat layers, and thus constitute a good test case to investigate the relationship between the rate of microbial mat growth and carbonate precipitation. The laminated LHC mats were divided into four layers via color and fabric, and waters within and above the mat were collected to determine their carbonate saturation states. Samples of the microbial mat were also collected for 16S rRNA analysis of microbial communities in each layer. Rates of carbonate precipitation and carbon fixation were measured in the laboratory by incubating homogenized samples from each mat layer with δ13C-labeled HCO3- for 24 h. Comparing these rates with those from experimental controls, poisoned with NaN3 and HgCl2, allowed for differences in biogenic and abiogenic precipitation to be determined. Carbon fixation rates were highest in the top layer of the mat (0.17% new organic carbon/day), which also contained the most phototrophs. Isotope-labeled carbonate was precipitated in all four layers of living and poisoned mat samples. In the top layer, the precipitation rate in living mat samples was negligible although abiotic precipitation occurred. In contrast, the bottom three layers exhibited biologically enhanced carbonate precipitation. The lack of correlation between rates of carbon fixation and biogenic carbonate precipitation suggests that processes other than autotrophy may play more significant roles in the preservation of mats as microbialites.

微生物垫（Microbial mats）广泛分布于多种现代环境中，其存在的地质记录可追溯至太古宙（Archean）时期。关于最终使微生物垫岩化并保存为微生物岩（microbialites）的作用过程的速率与条件，学界尚存诸多争议。本研究采用新型示踪实验技术，同时量化微生物垫的生物量增量与碳酸钙（CaCO₃）的形成过程。采自美国加利福尼亚州小热溪（Little Hot Creek, LHC）的微生物垫内部多层结构中均赋存碳酸钙，因此该体系是探究微生物垫生长速率与碳酸盐沉积之间关联的理想研究模型。该纹层状LHC微生物垫可依据颜色与组构划分为四层，研究人员采集了垫体内部及上方的水体以测定其碳酸盐饱和状态；同时采集各层微生物垫样品，用于开展各层微生物群落的16S核糖体RNA（16S rRNA）测序分析。本研究通过将各层均质化后的微生物垫样品与碳13标记的碳酸氢根（δ¹³C标记HCO₃⁻）共同孵育24小时，在实验室中测定了碳酸盐沉积速率与碳固定速率。将上述速率与经叠氮化钠（NaN₃）与氯化汞（HgCl₂）抑制微生物活性处理的实验组对照数据进行对比，可区分生物成因与非生物成因的碳酸盐沉积差异。微生物垫顶层的碳固定速率最高（0.17% 新生有机碳/日），该层同时也是光合微生物（phototrophs）丰度最高的区域。在活体与经毒性处理的微生物垫样品的全部四层结构中，均检测到同位素标记的碳酸盐沉积。尽管顶层存在非生物成因沉积，但活体垫体样品的沉积速率可忽略不计；与之相反，下层三层则表现出生物促进的碳酸盐沉积。碳固定速率与生物成因碳酸盐沉积速率之间不存在相关性，这表明除自养作用外的其他过程，可能在微生物垫保存为微生物岩的过程中发挥更为关键的作用。