2:00 PM - 2:15 PM
[MIS09-02] Effect of cell size of riverine prokaryote on dissolved organic matter degradation
Keywords:Dissolved organic matter, Bioavailability, Optical properties, Size fraction, Land use land cover, Chubetsu River
The flux of CO2 from inland water to the atmosphere has been considered to be an important component in the global carbon cycle. It is important to understand the factors controlling organic matter degradation in riverine water to estimate the accurate flux and its change with climate change/land use change. Since more than 50% of the organic matter in river water is dissolved organic matter (DOM), it is necessary to understand the degradability of riverine DOM. Recently, it has been suggested that microbiologically refractory DOM is selectively degraded by microbial communities with large cell sizes. However, studies evaluating the link between DOM degradability and microbial cell size with microbial incubation experiments have been limited. Therefore, the aim of this study was to evaluate whether microbiologically refractory DOM in river water is selectively degraded by prokaryote with large cell sizes.
We collected water samples on August 16, 2022 from a small tributary in forest, a dam lake, a upper reach, and a channel in rice paddy fields in the Chubetsu River watershed. All sampling sites except for the dam lake had high flow rates due to heavy precipitation on the sampling day. Since microbial degradability of low molecular weight (LMW) DOM has been considered to be lower than tha of high molecular weight (HMW) DOM, water samples were filtered 0.22 μm and were subsequently separated into HMW and LMW fractions by ultrafiltration with a nominal molecular weight of 3000. The inoculums were acquired from a middle reach of the Chubetsu River and were filtered through 1.0 μm or 5.0 μm filter. The HMW and LMW DOM were mixed with each inoculum at a ratio of 9:1, and the mixtures were incubated for 30 days under 20°C and dark conditions. A control without inoculum was also incubated under the same conditions. Before and after incubation, samples were filtered using a 0.22 μm filter, and the dissolved organic carbon (DOC) concentration was measured. The optical properties of DOM were also analyzed to evaluate changes in DOM qualities which likely linked to the degradability. In addition, unfiltered samples were fixed with paraformaldehyde for bacterial counts by flow cytometry.
DOC concentrations in the LMW fractions decreased in all substrates/all treatments, and there were no significant differences among the control, <1.0 µm, and <5.0 µm treatments. The prokaryote in the LMW fractions, in general, did not increase significantly, suggesting that prokaryote did not degrade the LMW DOM, but that DOC was removed by abiotic factors. DOC concentrations and prokaryote number in the HMW fractions decreased and increased in all substrates/all treatments including the control treatments, respectively. The decreases in DOC concentration in the control treatments were the almost same extent as in the <1.0 and <5.0 µm treatments for all the HMW DOM incubations. These results suggest that microbial communities with larger cell sizes did not selectively degrade HMW DOM irrespective of differences in the quality, but that most of the HMW DOM was degraded by prokaryote smaller than 0.22 µm. Experimental results indicated that larger microbial communities did not contribute on selective degradation of refractory DOM in the Chubetsu River, and suggest that smaller microbial communities play a major role in DOM degradation.
We collected water samples on August 16, 2022 from a small tributary in forest, a dam lake, a upper reach, and a channel in rice paddy fields in the Chubetsu River watershed. All sampling sites except for the dam lake had high flow rates due to heavy precipitation on the sampling day. Since microbial degradability of low molecular weight (LMW) DOM has been considered to be lower than tha of high molecular weight (HMW) DOM, water samples were filtered 0.22 μm and were subsequently separated into HMW and LMW fractions by ultrafiltration with a nominal molecular weight of 3000. The inoculums were acquired from a middle reach of the Chubetsu River and were filtered through 1.0 μm or 5.0 μm filter. The HMW and LMW DOM were mixed with each inoculum at a ratio of 9:1, and the mixtures were incubated for 30 days under 20°C and dark conditions. A control without inoculum was also incubated under the same conditions. Before and after incubation, samples were filtered using a 0.22 μm filter, and the dissolved organic carbon (DOC) concentration was measured. The optical properties of DOM were also analyzed to evaluate changes in DOM qualities which likely linked to the degradability. In addition, unfiltered samples were fixed with paraformaldehyde for bacterial counts by flow cytometry.
DOC concentrations in the LMW fractions decreased in all substrates/all treatments, and there were no significant differences among the control, <1.0 µm, and <5.0 µm treatments. The prokaryote in the LMW fractions, in general, did not increase significantly, suggesting that prokaryote did not degrade the LMW DOM, but that DOC was removed by abiotic factors. DOC concentrations and prokaryote number in the HMW fractions decreased and increased in all substrates/all treatments including the control treatments, respectively. The decreases in DOC concentration in the control treatments were the almost same extent as in the <1.0 and <5.0 µm treatments for all the HMW DOM incubations. These results suggest that microbial communities with larger cell sizes did not selectively degrade HMW DOM irrespective of differences in the quality, but that most of the HMW DOM was degraded by prokaryote smaller than 0.22 µm. Experimental results indicated that larger microbial communities did not contribute on selective degradation of refractory DOM in the Chubetsu River, and suggest that smaller microbial communities play a major role in DOM degradation.