10:45 AM - 11:00 AM
[MIS14-07] Effect of substrate concentration on microbial production of recalcitrant dissolved organic matter: Evaluation by molecular size analysis
Keywords:Aquatic environments, Dissolved Organic Matter, Molecular size, Lake Biwa, Microbial Carbon Pump
The accumulation of dissolved organic matter (DOM) in aquatic environments remains a pivotal unresolved issue. The long-standing 'intrinsic recalcitrance' hypothesis posits that intrinsic molecular properties of DOM (e.g., chemical structures) underlie its recalcitrance. The process by which microorganisms synthesize recalcitrant DOM (RDOM) while consuming labile organic matter has attracted significant attention as the microbial carbon pump. Conversely, the 'emergent recalcitrance' hypothesis proposes that the recalcitrance of DOM is an emergent property stemming from the ecological interplay between DOM molecules and microbial communities (Dittmar et al., 2021, Nature Rev. Earth Env., 2: 570-583). Evaluating these hypotheses is imperative for comprehending the response of DOM to environmental change. These hypotheses yield markedly disparate predictions concerning microbial production of RDOM in response to labile organic matter availability. The intrinsic recalcitrance hypothesis predicts an increase in the concentration of RDOM produced by microorganisms with an elevated supply of labile organic matter. In contrast, the emergent recalcitrance hypothesis predicts that the concentration of RDOM remains unaffected by escalating labile organic matter availability. To our knowledge, the impact of labile organic matter supply on RDOM concentration has yet to be systematically evaluated in microbial culture experiments. Assessments of the molecular size distribution of DOM generated in microbial culture experiments have also been exceedingly limited, while it is widely recognized that the reactivity of DOM varies according to molecular size. In biodegradation experiments utilizing DOM from Lake Biwa, we observed that high-molecular-weight (HMW) DOM exhibited semi-labile characteristics, degrading within tens of days, while all RDOM, persisting over hundreds of days, was identified as low-molecular-weight (LMW) DOM.
In this study, to discern between the two contrasting hypotheses, we conducted a long-term culture experiment employing natural microbial communities to elucidate the responsiveness of microbial production of RDOM to substrate concentrations. The culture media were prepared by adding glucose, a typical labile organic matter, as the sole organic carbon substrate to the artificial lake water at three distinct concentrations (3, 6, and 12 mg-C/L). Additionally, nitrogen and phosphorus nutrients (C:N:P = 96:20:1) and trace metal elements (Fe, Co, Zn, Mn, Cu, Mo) were supplemented into the culture media. Lake water, collected from the surface layer (5 m depth) of a pelagic site of Lake Biwa during April 2022, was filtered using a GF/C filter to eliminate larger particles and plankton. The microbial inoculum was mixed with the culture media in a dilution factor of 50 v/v. The bottles were prepared in duplicate and shaken in the dark at 20°C. DOM samples were collected on days 24, 49, 76, 207, and 410. The molecular size distribution of DOM was determined via a size exclusion chromatograph-total organic carbon analyzer. Organic carbon concentrations were quantified for each peak with an apparent weight-average molecular weight of ca. 150kDa, classified as HMWDOM, and ca. 2kDa, classified as LMWDOM.
The response factors to the substrate concentrations (0 if there is no response to the substrate concentrations and 1 if it changes at the same rate as the substrate concentrations) were derived from the normalized organic carbon concentration of DOM remaining on day 410. Significant response factors were observed at 0.644±0.158 for bulk DOM and 1.309±0.342 for HMWDOM (p-values for linear regression <0.02), while LMWDOM did not exhibit a significant value (p-value >0.1). Focusing solely on LMWDOM as RDOM indicates a limited response of microbial production of RDOM to the substrate concentration. The findings lend more significant support to the emergent recalcitrance hypothesis, at least within the temporal scale of this experiment.
In this study, to discern between the two contrasting hypotheses, we conducted a long-term culture experiment employing natural microbial communities to elucidate the responsiveness of microbial production of RDOM to substrate concentrations. The culture media were prepared by adding glucose, a typical labile organic matter, as the sole organic carbon substrate to the artificial lake water at three distinct concentrations (3, 6, and 12 mg-C/L). Additionally, nitrogen and phosphorus nutrients (C:N:P = 96:20:1) and trace metal elements (Fe, Co, Zn, Mn, Cu, Mo) were supplemented into the culture media. Lake water, collected from the surface layer (5 m depth) of a pelagic site of Lake Biwa during April 2022, was filtered using a GF/C filter to eliminate larger particles and plankton. The microbial inoculum was mixed with the culture media in a dilution factor of 50 v/v. The bottles were prepared in duplicate and shaken in the dark at 20°C. DOM samples were collected on days 24, 49, 76, 207, and 410. The molecular size distribution of DOM was determined via a size exclusion chromatograph-total organic carbon analyzer. Organic carbon concentrations were quantified for each peak with an apparent weight-average molecular weight of ca. 150kDa, classified as HMWDOM, and ca. 2kDa, classified as LMWDOM.
The response factors to the substrate concentrations (0 if there is no response to the substrate concentrations and 1 if it changes at the same rate as the substrate concentrations) were derived from the normalized organic carbon concentration of DOM remaining on day 410. Significant response factors were observed at 0.644±0.158 for bulk DOM and 1.309±0.342 for HMWDOM (p-values for linear regression <0.02), while LMWDOM did not exhibit a significant value (p-value >0.1). Focusing solely on LMWDOM as RDOM indicates a limited response of microbial production of RDOM to the substrate concentration. The findings lend more significant support to the emergent recalcitrance hypothesis, at least within the temporal scale of this experiment.