Isotopic evidence for methane-based chemosynthesis in the Upper Floridan aquifer food web

Methane cycling in the carbonate critical zone

The carbonate critical zone (CZ) is characterized by extensive groundwater-surface water exchange that leads to highly variable redox states of groundwater. Changes in redox condition may cause either production or consumption of methane (CH4), thereby providing an atmospheric source or sink of this important greenhouse gas. To assess how groundwater-surface water exchange affects redox state and CH4 cycling in the carbonate CZ, we measured CH4 concentrations and 13C isotopes in water from streams, spring systems, and wells in north-central Florida. Sampled groundwater has subsurface residence times ranging from hours at a stream sink-rise system, to months following a flood recharge event into a spring vent, to decades at springs with limited point recharge. Concentrations of CH4 ranged from 0.002 to 89 μM, with an inverse relationship in springs between subsurface residence time and CH4 concentration. Where residence time is short, low CH4 concentrations result from methanotrophy linked to elevated dissolved oxygen (DO) concentrations. Following flooding, methanotrophy occurs soon after recharge and is followed by methanogenesis as groundwater becomes increasingly reducing. Groundwater extracted from wells had CH4 concentrations greater than spring water indicating CH4 is lost during flow to spring vents. CH4 concentrations covary with δ13C-CH4 values, which supports both methanogenesis and methanotrophy with changing residence times. Mean fluxes of CH4 ranged from -0.05 to 1.0 mg m-2 d-1 at spring vents, with negative values caused by CH4 uptake in water undersaturated with respect to atmospheric concentration. Most springs are dominated by methanotrophy, limiting atmospheric evasion of CH4 produced in the carbonate CZ. We estimate CH4 emissions to be 12.6 × 10-6 Tg a-1 across all Florida springs or about two orders of magnitude less than emissions from Floridan aquifer groundwater abstraction (3041 × 10-6 Tg a-1). Although CH4 is produced in the carbonate CZ, natural attenuation limits its effects on the global carbon cycle.

Subterranean carbon flows from source to stygofauna: a case study on the atyid shrimp Stygiocaris stylifera (Holthuis, 1960) from Barrow Island (WA)

Groundwater biota are crucial for the ecological functioning of subterranean ecosystems. However, while knowledge of the taxonomic diversity of groundwater invertebrates (stygofauna) is increasing, functional ecological information is still limited. Here, we investigate seldom empirically tested assumptions around stygofaunal trophic plasticity in coping with oligotrophic habitats. We focus on Barrow Island (Western Australia), an ideal natural laboratory due to the occurrence of natural oil seeps in association with aquifers. The trophic position and food source use of the endemic atyid shrimp Stygiocaris stylifera (Holthuis, 1960) were assessed via δ¹³C and δ¹⁵N stable isotope analysis (SIA). Background information on the environmental conditions was gathered through hydrochemical data and δ¹³C SIA combined with ¹⁴C data from dissolved inorganic/organic carbon and particulate organic carbon from groundwater samples. Our results indicate carbon enrichment in proximity to the natural oil seepage coupled with changes in trophic positions of S. stylifera from higher consumers/predators to biofilm grazers/decomposers. These results are consistent with an increased involvement of hydrocarbon seeps and associated microbial communities in the carbon flows and confirm potential for the trophic flexibility in stygofauna. Further investigations involving other trophic groups will help elucidate the functioning of the ecosystems at a community level.

Methane- and dissolved organic carbon-fueled microbial loop supports a tropical subterranean estuary ecosystem

Subterranean estuaries extend inland into density-stratified coastal carbonate aquifers containing a surprising diversity of endemic animals (mostly crustaceans) within a highly oligotrophic habitat. How complex ecosystems (termed anchialine) thrive in this globally distributed, cryptic environment is poorly understood. Here, we demonstrate that a microbial loop shuttles methane and dissolved organic carbon (DOC) to higher trophic levels of the anchialine food web in the Yucatan Peninsula (Mexico). Methane and DOC production and consumption within the coastal groundwater correspond with a microbial community capable of methanotrophy, heterotrophy, and chemoautotrophy, based on characterization by 16S rRNA gene amplicon sequencing and respiratory quinone composition. Fatty acid and bulk stable carbon isotope values of cave-adapted shrimp suggest that carbon from methanotrophic bacteria comprises 21% of their diet, on average. These findings reveal a heretofore unrecognized subterranean methane sink and contribute to our understanding of the carbon cycle and ecosystem function of karst subterranean estuaries.

It remains unclear how oligotrophic habitats in subterranean estuaries sustain complex ecosystems. Here, using stable isotopic evidence from organic matter and pelagic shrimp, the authors show that a microbial loop fuelled by methane and dissolved organic carbon sustains the anchialine food web.