Abstract
Aquifers are important reservoirs for organic carbon. A fundamental understanding of the role of groundwater ecosystems in carbon cycling, however, is still missing. Using sediment flow-through microcosms, long-term (171d) experiments were conducted to test two scenarios. First, aquifer sediment microbial communities received dissolved organic matter (DOM) at low concentration and typical to groundwater in terms of composition (DOM-1x). Second, sediments received an elevated concentration of DOM originating from soil (DOM-5x). Changes in DOM composition were analyzed via NMR and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Carbon production, physiological adaptations and biodiversity of groundwater, and sediment prokaryotic communities were monitored by total cell counts, substrate use arrays, and deep amplicon sequencing. The experiments showed that groundwater microbial communities do not react very fast to the sudden availability of labile organic carbon from soil in terms of carbon degradation and biomass production. It took days to weeks for incoming DOM being efficiently degraded and pronounced cell production occurred. Once conditioned, the DOM-1x supplied sediments mineralized 294(±230) μgC L−1sed d−1, 10-times less than the DOM-5x fed sediment communities [2.9(±1.1) mgC L−1sed d−1]. However, the overall biomass carbon production was hardly different in the two treatments with 13.7(±4.8) μgC L−1sed d−1 and 14.3(±3.5) μgC L−1sed d−1, respectively, hinting at a significantly lower carbon use efficiency with higher DOM availability. However, the molecularly more diverse DOM from soil fostered a higher bacterial diversity. Taking the irregular inputs of labile DOM into account, shallow aquifers are assumed to have a low resilience. Lacking a highly active and responsive microbial community, oligotrophic aquifers are at high risk of contamination with organic chemicals.
Highlights
The water saturated terrestrial subsurface harbors the quantitatively most extensive freshwater ecosystems
Our results show that following a period of abiotic sorption and activation of the microbial communities, biodegradation processes in the dissolved organic matter (DOM)-5x fed sediments efficiently removed bioavailable DOM components originating from the soil leachate and turned the DOM from a soil leachate to a typical groundwater DOM in terms of relative abundance of key substructures
Apart from the comparable low concentrations of microbial cells per volume, groundwater ecosystems constitute a significant reservoir of organic carbon and biomass due to its enormous dimensions, which by far exceed those in soils and surface waters (Magnabosco et al, 2018)
Summary
The water saturated terrestrial subsurface harbors the quantitatively most extensive freshwater ecosystems. With the exclusion of light as energy source and an obviously minor contribution of chemoautotrophy, the predominantly heterotrophic communities in shallow aquifers are highly dependent on organic matter originating from the surface, received via groundwater recharge (Pabich et al, 2001; Goldscheider et al, 2006; Shen et al, 2015). While our knowledge on carbon cycling is well developed for surface waters (e.g., Drake et al, 2017; Tranvik et al, 2018), we still lack a detailed understanding of the fate of organic matter and its linkage to the heterotrophic production for groundwater ecosystems. On its way down into the aquifer, seepage water is typically depleted in dissolved organic carbon (DOC) in terms of quantity and quality (Pabich et al, 2001; Lennon and Pfaff, 2005; Shen et al, 2015). The link between dynamics in DOM concentration and composition and heterotrophic production as well as microbial biodiversity remains largely unexplored
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