ROLE OF MICROBIAL LOOP IN AQUATIC CARBON AND NITROGEN CYCLES

本研究的目的是探讨微生物食物网在淡水湖泊浮游环境中碳和氮循环中的作用。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的食物网分析。因此，我们用微生物和地球化学的方法分析了微生物循环在C和N循环中的作用。我们特别关注铵再生和细菌利用溶解性有机物（DOM）的机制。在中营养型湖Kizaki和琵琶湖的大小分级NH 4再生的测量显示，细菌和噬菌动物大大有助于总NH 4再生;高达80%的NH 4再生可归因于异养生物在<20 μ m的部分。细菌似乎是一个源，而不是在研究区的NH 4汇。为进一步研究淡水细菌对碳氮代谢的调控，测定了琵琶湖水体中溶解游离氨基酸（DFAA）和结合氨基酸（DCAA）的浓度和细菌利用率，初步结果表明，DCAA是细菌有机氮的主要来源。我们的研究结果表明，微生物循环驱动的碳和氮通量在淡水环境中的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.