Tracking down carbon inputs underground from an arid zone Australian calcrete

Abstract

Freshwater ecosystems play a key role in shaping the global carbon cycle and maintaining the ecological balance that sustains biodiversity worldwide. Surficial water bodies are often interconnected with groundwater, forming a physical continuum, and their interaction has been reported as a crucial driver for organic matter (OM) inputs in groundwater systems. However, despite the growing concerns related to increasing anthropogenic pressure and effects of global change to groundwater environments, our understanding of the dynamics regulating subterranean carbon flows is still sparse. We traced carbon composition and transformations in an arid zone calcrete aquifer using a novel multidisciplinary approach that combined isotopic analyses of dissolved organic carbon (DOC) and inorganic carbon (DIC) (δ 13 C DOC , δ 13 C DIC , 14 C DOC and 14 C DIC) with fluorescence spectroscopy (Chromophoric Dissolved OM (CDOM) characterisation) and metabarcoding analyses (taxonomic and functional genomics on bacterial 16S rRNA). To compare dynamics linked to potential aquifer recharge processes, water samples were collected from two boreholes under contrasting rainfall: low rainfall ((LR), dry season) and high rainfall ((HR), wet season). Our isotopic results indicate limited changes and dominance of modern terrestrial carbon in the upper part (northeast) of the bore field, but correlation between HR and increased old and 13 C-enriched DOC in the lower area (southwest). CDOM results show a shift from terrestrially to microbially derived compounds after rainfall in the same lower field bore, which was also sampled for microbial genetics. Functional genomic results showed increased genes coding for degradative pathways-dominated by those related to aromatic compound metabolisms-during HR. Our results indicate that rainfall leads to different responses in different parts of the bore field, with an increase in old carbon sources and microbial

Rainfall as a trigger of ecological cascade effects in an Australian groundwater ecosystem

“…Overall, our results from genetic analyses on stygofaunal gut microbiomes suggest that the inflow of OM at Sturt Meadows is exploited by microbes which are the potential direct (through biofilm grazing) and indirect (via POC assimilation) diet sources of primary consumers, amphipods and copepods. A previous investigation on carbon inputs in water indicated a shift in microbial taxonomic assemblages coupled with increased degradative pathways after rainfall (HR) 22 . In line with our work, Reiss et al 9 reported that rainfall inflows, coupled with increased inputs of organic matter mediate changes in microbial diversity, abundances and respiration rates.…”

“…Indeed, further species-specific investigations are required to elucidate the mechanisms of these interactions and bring crucial comprehension of the dynamics sustaining groundwater biodiversity. (Table 1); (1b) as a result of the rainfall infiltration, phosphates dilute, carbonates are released (higher alkalinity) 23 and old POC gets to the water; (2) ammonia concentrations increase as a combined effect of animal waste leaking from the surface and microbial metabolisms 22,23 ); (3) microbial biofilms consume the newly incorporated old DOC (partially derived from POC (route 3b)) 22 ; (4) biofilms decompose POC; (5) harpacticoids browse on biofilm and cyclopoids filter particulate organic matter (route 6); (7a) amphipods graze on microbial mats (and filter POC (route 7b)) and fuel the carbon to the upper trophic levels; 8, beetles larvae and adults (route 9) (top predators) exert a higher trophic pressure on amphipods after rainfall 7 . Dashed lines lead to the proportions of the carbohydrate, lipid and amino acid microbial metabolisms (diameter of the bubbles proportional to the relative abundances in cyclopoids, harpacticoids and AM1; inner orange circles under HR (b) are illustrated for comparison with the significative lower relative abundances under LR (a)).…”

“…Some studies have attempted to model groundwater ecosystem ecological functioning 76,77 , but none of them has focused on wide multidisciplinary designs. Here we combined information from previous food web investigation on stygofauna 7,78 and microbial patterns 22 with the information gathered via isotopic (carbon and nitrogen) and genetic data on stygofaunal gut biomes from this study (Fig. 5).…”

“…Following the incubation, the analytical procedure was adapted from Saccò et al 22 and DNA extraction was carried out using DNeasy Blood and Tissue Kit (Qiagen; Venlo, Netherlands) and eluted off the silica column in 30-50 μL AE buffer. The quality and quantity of DNA extracted from each stygobitic pool was measured using www.nature.com/scientificreports/ quantitative PCR (qPCR), targeting the bacterial 16S gene.…”

“…The threshold for dropping a taxonomic assignment to LCA was set to perc_identity ≥ 96 and the difference between the % of identity of the two hits when their query coverage is equal was set to 1. Results on the microbial taxonomic diversity detected in ground water samples from a previous study on carbon inputs in the aquifer 22 were incorporated in this work to allow qualitative comparison with the stygofaunal gut diversity.…”