The large springs of the Balcones Escarpment (Comal Springs in New Braunfels or San Marcos Springs, for example) are well known for their crystal clear water, but a visit to these special places provides little hint of the biological complexity farther underground.
The springs are fed by the Edwards Aquifer: home to a globally significant assemblage of strange, and poorly known species, most of which occur nowhere else on Earth. Few people have ever seen eyeless salamanders with toothpick thin legs, blind catfish (one species with teeth and one without), and a suite of invertebrates including transparent snails, a blind leach, albino beetles, and dozens of species of shrimp-like crustaceans (including actual shrimp).
In the darkness below our feet, photosynthesis, which provides the fuel for most life on Earth, cannot occur, and no green thing grows. In the absence of photosynthesis, what sustains the Edwards Aquifer underground menagerie? In an article just published in Ecology, a multidisciplinary team of researchers from Texas Parks and Wildlife, Texas State University – San Marcos, and the University of Tennessee – Knoxville provides some answers, and their findings go far beyond simply identifying what’s on the menu for obscure aquatic species.
In the late 1970’s, the world was thrilled when researchers using submersible vessels discovered deep-sea hydrothermal vents where bizarre tube worms, clams, shrimp, and a host of other species thrived in great numbers, just on the edge between the dark, freezing abyssal plain and the scalding, geothermal slurry pouring from deep within the Earth. The key to this ecosystem, was the sharp chemical gradient between those two, very different waters. The geothermal vent water is laden with chemicals such as methane and sulfide. Where these compounds come into contact with the oxygen-rich water of the deep ocean, chemical reactions occur, and these chemical reactions produce energy. Since the earliest days of life, microbes have been able to harness energy from these reactions just like plants harness energy from the sun to build organic compounds and fuel metabolic processes. In turn, those microbial communities can be consumed (or even co-opted as symbionts) by higher-level taxa. Voila! An ecosystem is born.
In the Edwards Aquifer of Central Texas, geologists and hydrologists have long known that not all water is created equal. Along the southern and eastern boundary of the aquifer, deliciously drinkable fresh water is rapidly replaced by anoxic, saline waters laden with some of the same chemical constituents found in hydrothermal vent waters (including sulfide and methane). Although not as hot, violent, or awe-inspiring as black smokers of the deep sea, this transition zone, referred to as the ‘bad water line’, supports microbial communities capable of the same, light-free carbon fixation (a process known as chemolithoautotrophy). Whether this process is important as a food source for the animals in the aquifer was one of the principal questions investigated in the new article.
Certainly, it is not the only potential food source. Along the northern and western boundary of the aquifer, surface streams loose water to the aquifer, helping to maintain spring flow, but also transporting organic matter from the surface, including plant material, algae, and even the occasional animal carcass. In many subterranean environments, this organic matter transported from the surface is the sole-energy source. In the Edwards Aquifer however, the researchers noticed an unusual pattern: the number of species present in the aquifer was greatest near the bad water line, and decreased closer to those surface streams. If these aquatic species were dependent on that surface input, why weren’t they clustered around the source?
To help answer the question, the researchers employed a technique called stable isotope analysis. At the atomic level, organic matter produced from photosynthesis looks very different than food produced via chemolithoautotrophy. Furthermore, in the case of stable isotope analysis, the old adage ‘you are what you eat’ holds true, and whether an animal is eating plant material (or other animals that were once eating plant material) or chemolithoautotrophic microbes can be revealed through the distinct atomic signature that these different food sources leave behind in the tissues of consumers. When the researchers looked at animals and organic matter from throughout the aquifer, they found the telltale signature of microbial production, particularly in food webs near the bad water line. In animals near recharge features, the photosynthetic signature was more prevalent. The researchers concluded that both photosynthetic organic matter and microbial organic matter produced at the bad water line via chemolithoautotrophy were present in the aquifer and consumed by animals, but that the importance of each to a given animal varied depending on the animal’s location within the aquifer.
The presence of two distinct food resources in the aquifer has several important impacts on the groundwater community. Looking at the specific shapes of mouthparts for several crustacean species, the researchers found evidence that some animals behaved as filter feeders, using mouthparts with long, feather-like hairs to filter organic matter from the water (like a baleen whale), while other species were scraping organic matter off of conduit walls in the aquifer. Interestingly, the filter-feeders exhibited a stronger photosynthetic signal while the scrapers showed a stronger chemolithoautotrophic signal, perhaps because the filter feeders where utilizing organic matter from the surface that was being washed through the aquifer while the scrapers were utilizing microbes growing on the walls of the aquifer. The researchers also found strong evidence of multiple food chain levels, with herbivores, predators, and predators of predators. The presence of high-level predators and animals using specialized feeding behaviors is unusual and largely unreported from underground habitats where most species are adapted to eat whatever they can find, whenever they can find it. Specialized feeding strategies are only expected where food supply is relatively constant.
Many of the species present in the aquifer, represent ancient lineages derived from marine ancestors that colonized millions of years ago. The long-term survival of those species can also be explained by chemolithoautotrophic microbes, although the researchers emphasize that such a scenario is only a hypothesis. Geologic, archeologic, and paleo-botanic evidence all point to multiple periods of extreme aridity in Texas’ past. During those times, plant production on the surface would decrease, as would the amount of surface water recharging the aquifer. Consequently, the input of plant material from the surface would also decrease. Microbial production in the aquifer, however, would continue, largely unaffected by surface aridity, allowing the deep parts of the aquifer to serve as a refuge for populations during the lean times.
This new research on the Edwards Aquifer food web has important implications not only for the conservation of Edwards Aquifer species, but also for our understanding of groundwater habitats around the world. The researchers note that chemolithoautotrophy may be much more important and more widespread than is currently understood. In the Edwards Aquifer, the presence of a dependable food source may help buffer species from natural and man-made changes to recharge and surface-derived food inputs. However, continued groundwater extraction and a growing interest in desalinization of groundwater from near the bad water line may have unpredictable impacts on microbial production. For over a century, the Edwards Aquifer has intrigued biologists, and there is no doubt that additional surprises will emerge as researchers continue to shed light on its dark secrets.