[M-IS11] Biogeochemistry

[MIS11-02] Biodegradation kinetics of different molecular-size classes of freshwater dissolved organic matter

*Yasuhiko T. Yamaguchi¹, Koichi Shimotori², Akio Imai², Yusuke Okazaki³, Tomoyuki Ohara¹, Miho Nakamura¹, Shoichi Hichiri¹, Takahiro Okamoto¹, Kazuhide Hayakawa¹ (1.Lake Biwa Environmental Research Institute, 2.NIES Lake Biwa Branch Office, National Institute for Environmental Studies, 3.Institute for Chemical Research, Kyoto University)

Keywords:Aquatic environments, Dissolved Organic Matter, Molecular size, Lake Biwa, Microbes

Degradation of aquatic dissolved organic matter (DOM) is a major process of the global biogeochemical cycles. Molecular sizes of DOM have been suggested to have a strong relationship to DOM reactivity (the size-reactivity continuum model: Amon & Benner, 1996, Limnol. Oceanogr. 41:41-51; Benner & Amon, 2015, Annu. Rev. Mar. Sci. 7:185–205). Yet, details and mechanisms of the link still remain poorly understood. In this study, we investigated biodegradation kinetics of different molecular-size classes of natural DOM in a deep meso/oligotrophic lake (Lake Biwa, Japan) and an inflowing river to the lake (Yasu River) by long-term dark incubations of water samples (> 200 days). Changes of molecular size distribution of dissolved organic carbon (DOC) were quantified by a custom-made high-performance size-exclusion chromatograph with total organic carbon analyzer (HPSEC-TOC: Shimotori et al. 2016, Limnol. Oceanogr. Meth. 14:637-648). Two molecular size classes of DOC were identified by HPSEC-TOC: high-molecular-weight (HMW) DOC (weight-averaged molecular weights (M_w) ~ 150kDa) and low-molecular-weight (LMW) DOC (M_w ~ 2kDa). HMW DOC was all degraded within 200 days, while LMW DOC contained semi-labile (LMW SLDOC) and refractory (LMW RDOC) fractions in this time scale. Thus, the DOM examined in this study can be modeled by mixing of the three components (1. HMW DOC: ~10%, 2. LMW SLDOC: ~30%, 3. LMW RDOC: ~60%). HMW DOC showed higher degradation rates than LMW SLDOC (k-d: 0.02-0.04 and 0.01-0.02, respectively). LMW SLDOC and LMW RDOC showed almost the same M_w values (~2kDa). These results suggest that, while DOM reactivity significantly differs depending on its molecular size on a large MW scale (~150kDa vs ~2kDa), this relationship would not be applicable to the finer MW scale (within the LMW fraction).