Hideo Oyagi

Ubiquity and quantitative significance of bacterioplankton lineages inhabiting the oxygenated hypolimnion of deep freshwater lakes

The oxygenated hypolimnion accounts for a volumetrically significant part of the global freshwater systems. Previous studies have proposed the presence of hypolimnion-specific bacterioplankton lineages that are distinct from those inhabiting the epilimnion. To date, however, no consensus exists regarding their ubiquity and abundance, which is necessary to evaluate their ecological importance. The present study investigated the bacterioplankton community in the oxygenated hypolimnia of 10 deep freshwater lakes. Despite the broad geochemical characteristics of the lakes, 16S rRNA gene sequencing demonstrated that the communities in the oxygenated hypolimnia were distinct from those in the epilimnia and identified several predominant lineages inhabiting multiple lakes. Catalyzed reporter deposition fluorescence in situ hybridization revealed that abundant hypolimnion-specific lineages, CL500-11 (Chloroflexi), CL500-3, CL500-37, CL500-15 (Planctomycetes) and Marine Group I (Thaumarchaeota), together accounted for 1.5–32.9% of all bacterioplankton in the hypolimnion of the lakes. Furthermore, an analysis of single-nucleotide variation in the partial 16S rRNA gene sequence (oligotyping) suggested the presence of different sub-populations between lakes and water layers among the lineages occurring in the entire water layer (for example, acI-B1 and acI-A7). Collectively, these results provide the first comprehensive overview of the bacterioplankton community in the oxygenated hypolimnion of deep freshwater lakes.

Physicochemical characteristics and their formation mechanism of Lake Waku-ike, Japan

Lake water differs from river and groundwater in terms of stagnation; therefore, the circulation system of lake water is important from the stand point of ecosystem-level environmental preservation. Lake Waku-ike hasan imperfect circulation system (OcHIAI 1960). The strong metalimnion is formed during the period from spring to autumn, whereas full circulation occurs briefly in winter. During the stratified period, the metalimnion isolates two contrasting vertical structures with a high concentration ofhydrogen sulfide in the deeper layer. Limnological studies on meromictic lakes have mainly investigated imperfect circulation systems in coastal brackish lakes (MORI 1975): a similar circulation is expected to occur even in freshwater lake. However, the formation mechanism ofthe stagnant layer in fresh water Lake Waku-ike is considerably different from that in a brackish lake. In the present study, the circulation mechanism of water in Lake Waku-ike is investigated by using physicochemical