Abstract
This study addresses the causes of the metabolic depression observed when examining the metabolism of hypogean versus epigean organisms. We examined the two current hypotheses regarding the cause of metabolic cave adaptation, a paucity of food and low oxygen availability, both necessary for ATP production, by first determining if the hypogean environment examined, Edwards Aquifer, was resource limited. Stable isotope analyses indicate that there is extensive microbial chemolithoautotrophic production providing resources for the hypogean organisms. δ13C values (≤30‰ )were well below that of terrestrial biome indicating that C in the aquifer originates from chemolithoautotrophic inorganic carbon fixation, not photosynthetically derived material resulting from terrigenous sources. Data suggest the artesian system is a complex geochemical ecosystem providing inorganic energy sources from both methane and sulfates. Metabolism, examined via key aerobic and anaerobic proxies, and organismal proximate composition indicated there was no difference between metabolic rates and energy storage of Palaemonetes antrorum (stygobitic) and Palaemonetes kadiakensis (epigean). This indicates that resources within the oxic aquifer are not limited. We demonstrate that it is necessary for one, or both, of these selective pressures to be present for metabolic cave adaptation to occur.
Highlights
Scientists have long been intrigued by the adaptations of subterranean organisms to their stygian environment
The stygobitic species had highly significant lighter values of δ13C, δ15N and δ34S (Table 1) and the scatter and magnitude of these differences is shown in Figure 1.The two outlying δ13C values denote a different energy source for these individuals and their exclusion emphasizes the magnitude of the differences between the two species’ trophic biology
The outliers denote a principally terrestrial input of C from C3 photosynthetic source with δ13C values similar to surface aquatic amphipods reported in the vicinity of Movile Cave (Sarbu 2000) but with much lower δ15N values
Summary
Scientists have long been intrigued by the adaptations of subterranean organisms to their stygian environment. Subterranean aquatic environments are considered extreme on several accounts. Such habitats are in continuous total darkness and so lack direct photosynthetic energy input being mostly dependent on a slow flux of allochthonous energy input in the form of organic carbon derived from the surface, else, as is increasingly being reported, in the form of inorganic molecules derived by chemoautotrophy. Subterranean aquatic systems may be dysoxic or anoxic with regions of toxic hydrogen sulfide. Some systems, such as anchialine systems and the saline parts of the Edwards aquifer, have distinct haloclines which mark the clines conducive to microbial chemolithoautotrophic production (Hutchins et al 2011). The structural diversity of such systems (Pohlman 2011) and the contained microbial and metazoan communities (Sarbu 2000, Seymour et al 2007) are still poorly understood as noted by Humphreys (1999), they may be sought by analogy with adaptations to anoxic sediments (Fenchel and Finlay 1995)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
