ROLE OF MICROBIAL LOOP IN AQUATIC CARBON AND NITROGEN CYCLES

本研究旨在探讨淡水湖中上层环境中微生物食物网在碳氮循环中的作用。研究分为三类。(1)氨再生机理及其控制因素；(2)细菌分解和利用溶解有机氮的机理；(3)利用稳定同位素比值分析食物网。因此，我们采用跨学科方法（微生物学和地球化学方法）分析了微生物对碳和氮循环的作用。我们特别关注铵再生和细菌利用溶解有机物（DOM）的机制。中营养湖木崎湖和琵洼湖NH4再生的分级测量结果表明，细菌和细菌捕食者对NH4再生总量有重要贡献；在<20mum部分，高达80%的NH4再生归因于异养。在研究区域，细菌似乎是NH4的来源而不是汇。为了进一步研究淡水细菌对碳氮代谢的调节作用，我们测量了琵瓦湖水体中溶解游离氨基酸（DFAA）和组合氨基酸（DCAA）的浓度和细菌利用情况。初步结果表明，DCAA是细菌有机氮的主要来源，而非DFAA。研究结果表明，微生物环驱动的碳氮通量在淡水环境生态系统功能和群落动态中起着关键作用。

The purpose of this study was to examine the role of microbial food webs in the carbon and nitrogen cycling within pelagic environments of freshwater lakes. Studies were categorized in three categories. (1) Mechanism of ammonia regeneration and its controlling factor, (2) Mechanism of bacterial decomposition and utilization of dissolved organic nitrogen, and (3) Food web analysis using stable isotope ratios. We, thus, analyzed the role of microbial loo p on C and N cycles with interdisciprinary approaches (microbiological and geochemical ones). We particularly focused on mechanisms of ammonium regeneration and bacterial utilization of dissolved organic matter (DOM). Measurements of size-fractionated NH4 regeneration in mesotrophic Lakes Kizaki and Biwa revealed that bacteria and bacterivores contribute substantially to total NH4 regeneration ; up to 80% of NH4 regeneration was attributable to heterotrophs in the <20mum fraction. Bacteria appeared to be a source rather than a sink of NH4 in the study area. To further examine the regulation of carbon and nitrogen metabolism by freshwater bacteria, we measured concentrations and bacterial utilization of dissolved free (DFAA) and combined amino acids (DCAA) in Lake Biwa.Preliminary results suggested that DCAA rather than DFAA were the major source of organic nitrogen for bacteria. Our results demonstrate that carbon and nitrogen fluxes driven by the microbial loop play the key role in ecosystem functioning and community dynamics in freshwater environments.