Development of high-precision nitrogen isotope analysis as a new index of dissolved organic matter sources in aquatic environments

2023 Fiscal Year Final Research Report

• Project/Area Number: 18K18189
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 63010:Environmental dynamic analysis-related
• Research Institution: Lake Biwa Environmental Research Institute
• Principal Investigator: Yamaguchi Yasuhiko 滋賀県琵琶湖環境科学研究センター, 総合解析部門, 主任研究員 (50726221)
• Project Period (FY): 2018-04-01 – 2024-03-31
• Keywords: 溶存有機物 / 窒素同位体比 / 分子サイズ / アミノ酸 / 琵琶湖

Outline of Final Research Achievements

Degradation of aquatic dissolved organic matter (DOM) is a major process of the global biogeochemical cycles. A strong relationship between molecular sizes and the reactivity of DOM has been proposed (the size-reactivity continuum model). However, the details and mechanisms of this relationship remain poorly understood. We investigated changes in the molecular size distribution of DOM in a deep meso/oligotrophic lake (Lake Biwa, Japan) using long-term dark incubations (> 400 days) and a size-exclusion chromatograph with total organic carbon analyzer (SEC-TOC). High molecular weight (HMW) DOM and low molecular weight (LMW) DOM showed different degradation characteristics. We also developed a novel method to selectively isolate HMW and LMW DOM by coupling ultrafiltration and solid phase extraction. The HMW and LMW DOM samples isolated from Lake Biwa showed significantly different nitrogen isotopic compositions, suggesting different DOM sources.

Free Research Field

生物地球化学

Academic Significance and Societal Importance of the Research Achievements

本研究では、分子サイズに着目して、DOMの分解特性分布把握と起源解明のための手法開発を進めた。琵琶湖湖水の高分子DOMと低分子DOMでは、分解特性が大きく異なり、窒素同位体比の差から、起源も異なることが示唆された。分子サイズ別のDOM分解速度定数の経験式から、天然湖水中でのDOMの分解および生産フラックスを見積もることが可能になり、DOMの環境中での詳細な機能評価が可能になった。また、高純度に分子サイズ別分画したDOM試料を回収できる手法を開発できたことで、起源解明のための詳細な化学分析が可能になった。