(Table 4) Distribution of bacterial populations in sediments of ODP Hole 128-798B

Sediment whole-round cores from a dedicated hole (798B) were obtained for detailed microbiological analysis, down to 518 m below the seafloor (mbsf). These sediments have characteristic bacterial profiles in the top 6 mbsf, with high but rapidly decreasing bacterial populations (total and dividing bacteria, and concentrations of different types of viable heterotrophic bacteria) and potential bacterial activities. Rates of thymidine incorporation into bacterial DNA and anaerobic sulfate reduction are high in the surface sediments and decrease rapidly down to 3 mbsf. Methanogenesis from CO2/H2 peaks below the maximum in sulfate reduction and although it decreases markedly down the core, is present at low rates at all but one depth. Consistent with these activities is the removal of pore-water sulfate, methane gas production, and accumulation of reduced sulfide species. Rates of decrease in bacterial populations slow down below 6 mbsf, and there are some distinct increases in bacterial populations and activities that continue over considerable depth intervals. These include a large and significant increase in total heterotrophic bacteria below 375 mbsf, which corresponds to an increase in the total bacterial population, bacterial viability, a small increase in potential rates of sulfate reduction, and the presence of thermogenic methane and other gases. Bacterial distributions seem to be controlled by the availability of terminal electron acceptors (e.g., sulfate), the bioavailability of organic carbon (which may be related to the dark/light bands within the sediment), and biological and geothermal methane production. Significant bacterial populations are present even in the deepest samples (518 mbsf) and hence it seems likely that bacteria may continue to be present and active much deeper than the sediments studied here. These results confirm and extend our previous results of bacterial activity within deep sediments of the Peru Margin from Leg 112, and to our knowledge this is the first comprehensive report of the presence of active bacterial populations from the sediment surface to in excess of 500 mbsf and sediments > 4 m.y. old.本研究获取了专孔798B的全取心沉积物柱样，用于开展精细微生物学分析，采样深度达海底以下518米（meters below seafloor，mbsf）。该沉积物柱0~6 mbsf层段具有特征性的细菌群落分布特征：总细菌与分裂态细菌数量、不同类型可存活异养细菌的浓度均处于较高水平，但随深度增加快速下降，同时伴随潜在的细菌代谢活性。胸苷掺入细菌DNA的速率与厌氧硫酸盐还原速率在表层沉积物中较高，并随深度增加快速下降至3 mbsf处。由CO₂/H₂生成甲烷的甲烷生成速率在硫酸盐还原活性峰值之下出现峰值，尽管随柱样深度增加显著下降，但除一个深度外，其余所有深度均维持较低的甲烷生成速率。与上述代谢活性相呼应的现象包括：孔隙水硫酸盐的消耗、甲烷气体生成以及还原态硫化物类物质的积累。6 mbsf以下，细菌种群数量的下降速率放缓，且在相当长的深度区间内，细菌种群数量与代谢活性出现若干显著升高的现象。其中包括375 mbsf以下总异养细菌数量的显著大幅升高，这一现象与总细菌种群数量上升、细菌存活率提升、硫酸盐还原潜在速率小幅升高，以及热成因甲烷与其他气体的出现相对应。细菌的分布似乎受以下因素调控：末端电子受体（terminal electron acceptors，如硫酸盐）的可获得性、有机碳的生物可利用性（该因素可能与沉积物内的明暗纹层相关），以及生物成因与地热成因甲烷的生成作用。即便在最深层的518 mbsf样品中，仍存在数量可观的细菌种群，因此细菌可能在远超本次研究沉积物的更深层位中依然存活并具有活性。本研究结果验证并拓展了我们此前关于秘鲁陆坡深海沉积物（大洋钻探第112航次）中细菌活性的研究结论。据我们所知，本次研究是首份涵盖从沉积物表层至500 mbsf以上、年龄超过4百万年的沉积物中活性细菌种群分布的综合报告。