Lake Biwa - Scientific seminar
Prof. Shin-ichi Nakano is presenting a research on long-term changes in the water quality of lake Biwa (Japan). He is visiting Biology Centre CAS (IHB) thanks to a academic exchange program Mobility Plus Japan.
Thursday 5.10. 2017 13:00
Lecture Hall, Institute of Hydrobiology (Biology Centre CAS), Na Sádkách 7, ČB
Abstract:
Long-term changes in water quality in Lake Biwa with special reference to organic matter dynamics and microbial ecology
Shin-ichi Nakano1, Kazuhide Hayakawa2, Yoshikuni Hodoki1, Yusuke Okazaki1, Indranil Mukherjee1, Shoji D. Thottathil 3, Hiroyuki Takasu4, Shohei Fujinaga1
1 Center for Ecological Research, Kyoto University, Hirano 2-503-9, Otsu, Shiga 520-2113, Japan;
2 Lake Biwa Environmental Research Institute, Yanagasaki, Otsu, Shiga 520-0022, Japan;
3 Département des Sciences Biologiques, Université du Québec a` Montréal, Case Postale 8888, succ. Centre-ville, Montréal, Canada, H3C 3P8;
4 Graduate School of Fisheries and Environmental Sciences, Nagasaki University, Bunkyo-cho 1-14, Nagasaki, Nagasaki 852-8521, Japan
During the last three decades, the water quality of Lake Biwa, the largest freshwater lake in Japan, has been improved through collaboration among multiple stakeholders. Mysteriously, a portion of organic matter expressed by chemical oxygen demand (CODMn) in the lake has been gradually increasing every year. Some researchers have reported that the increase in CODMn might be due to the accumulation of refractory and/or semi-labile DOM, and those DOM might be autochthonously produced. We have partly clarified the microbial processes with special reference to production of those DOM in Lake Biwa, especially in the lake’s hypolimnion. In the epilimnion of the lake, phytoplankton biomass is produced through primary production, followed by sinking into the hypolimnion. In the hypolimnion, a part of the phytoplankton biomass is converted into and produced as humic-like DOM through decomposition by planktonic bacteria. Fluorescence in situ hybridization (FISH) showed that bacterial clade, CL500-11 (phylum Chloroflexi), predominates in the hypolimnion. We made further analyses on prokaryotic community composition by high throughput 16S rRNA gene amplicon sequencing which showed the dominance by members of Planctomycetes exclusively occurred in the hypolimnion. In addition, FISH on eukaryotes showed that bacterivorous kinetoplastid flagellates are the dominant eukaryotes in the hypolimnion. So, the results indicate the presence of unique microbial food webs in the hypolimnion of Lake Biwa, where humic-like DOM is produced by the hypolimnion bacterial assemblages, and those bacteria are grazed by the dominant kinetoplastids and other hypolimnion dwelling bacterivorous protists.
