Feasting on terrestrial organic matter: Dining in a dark lake changes microbial decomposition

Abstract

Boreal lakes are major components of the global carbon cycle, partly because of sediment‐bound heterotrophic microorganisms that decompose within‐lake and terrestrially derived organic matter (t‐OM). The ability for sediment bacteria to break down and alter t‐OM may depend on environmental characteristics and community composition. However, the connection between these two potential drivers of decomposition is poorly understood. We tested how bacterial activity changed along experimental gradients in the quality and quantity of t‐OM inputs into littoral sediments of two small boreal lakes, a dark and a clear lake, and measured the abundance of operational taxonomic units and functional genes to identify mechanisms underlying bacterial responses. We found that bacterial production (BP) decreased across lakes with aromatic dissolved organic matter (DOM) in sediment pore water, but the process underlying this pattern differed between lakes. Bacteria in the dark lake invested in the energetically costly production of extracellular enzymes as aromatic DOM increased in availability in the sediments. By contrast, bacteria in the clear lake may have lacked the nutrients and/or genetic potential to degrade aromatic DOM and instead mineralized photo‐degraded OM into CO2. The two lakes differed in community composition, with concentrations of dissolved organic carbon and pH differentiating microbial assemblages. Furthermore, functional genes relating to t‐OM degradation were relatively higher in the dark lake. Our results suggest that future changes in t‐OM inputs to lake sediments will have different effects on carbon cycling depending on the potential for photo‐degradation of OM and composition of resident bacterial communities.

Think global, act local: The small‐scale environment mainly influences microbial community development and function in lake sediment

“…Although divergence rates did not differ between lakes, changes more closely tracked humification and pH of sediment pore water. These results advance previous studies that have shown both sediment pH and humification influence microbial community composition at single time points (Ruiz‐González et al ; Amaral et al ; Fitch et al ; Tripathi et al ). One explanation for the importance of humic substances is that they may provide favorable conditions for microbial growth by acting both as electron donors and acceptors (Hessen ; Lovley et al ; Torres et al ).…”

DOM degradation by light and microbes along the Yukon River-coastal ocean continuum

“…Our observations of an increasingly humic signal over time are consistent with observations of successive microbial processing to a diverse, "universal" aquatic DOM pool 50,51 as well as "universal" fluorescent signatures, consistent with similar functional processing of material by microbes to a convergent optical signature regardless of source 52 . Microbial community structure and function appear to play a role in this process, illustrated by the differences in production of humic fluorescence between size-fractionated inocula and the role of UV exposure in accelerating production of humic material 9,53 . Our results illustrate that optical signatures for DOM carry a microbial processing signature that skews toward humic-like fluorescence that www.nature.com/scientificreports/ is more distributed in excitation-emission space than precursor DOM, consistent with a DOM pool diversified by microbes, where structurally diverse fluorescent molecules do not directly overlap.…”

Chemical and microbial diversity covary in fresh water to influence ecosystem functioning

“…Our results suggest that the close molecular-level association between DOM and microbes can result in chemodiversity influencing ecosystem functioning. Different microbial communities have affinities for different carbon compounds (15,53), partly because they have different functional genes for degrading macromolecules (54). These preferences would explain how a greater diversity of organic molecules provides more niches for microbes to occupy (8,9) and increases carbon mineralization.…”