05. 5.5 Ma of Darkness
In most ecosystems, primary producers, typically photosynthetic organisms, are the foundation for the surrounding food web. These organisms fix carbon from the environment into sugars, and thus produce available carbon for utilization by the organisms that feed on them, and feed on those first-order consumers. But of course, there are plenty of places where there is no light, and thus, no photosynthetic primary producers. One of the more commonly known examples are the various hydrothermal vents and cold seeps, discovered in the 70's on the ocean floor (Ballard, 1977), where various methane, sulfide, and hydrocarbon fixing microorganisms serve as the basis of the elaborate ecosystems surrounding them (Tunnifcliffe, 1992). These aphotic ecosystems are of particular interest to astrobiologists, as these sorts of geologically produced environs may be common throughout our solar system and could be the foundation for extraterrestrial life that may exist (Boston et al., 2006). There are also terrestrial aphotic systems, found in caves, but for the vast majority of them are dependent on the flow of resources from exterior to the caves, from photic systems (Culver, 1982).
But, there are other systems, that can sustain themselves in novel and fascinating ways. The story of Lake Vostok, contained deep beneath the Antarctic ice, has captured the attention of the popular press (although by now it is long past old news), where the water has been isolated from the surface for millions of years (Siegert et al., 2001), but another system has a similar sort of deep history, that of Movile cave.
Movile cave (pronounced mauveelay) is located approximately one kilometer from the town of Mangalia, Romania, which sits on the coast of the Black Sea, and was discovered in 1986. Movile contains an entirely isolated system of water and air pockets, low in free oxygen, and high in sulfides and methane (Sarbu et al., 1997). The cave is filled with groundwater, so there is no flow in or out of water or nutrients from other biotic environs. Examining the sediment of the cave shows that this groundwater is not intermingling with the surrounding watershed, as evidenced by the complete absence of the radioactive isotopes typically found nearby due to the proximity of the region to Chernobyl. Within the air-pockets, mats of fungal and prokaryotic communities were found, growing on the surface of the water, and onto the cave walls. What was even more remarkable, was the presence of a number of macroinvertebrates living within these pockets as well, and studies began to elucidate that the chemoautotrophy sustaining the microbial mats were in turn the primary producers supporting the macroinvertebrates themselves. Devoid of sunlight, devoid of outside resources, a tiny community of under 50 invertebrate animal species (both above and below the surface, including Nematodes, annelids, arthropods (myriapods, insects, crustacea, and chelicerates), molluscs, flatworms, and more. A similarly small diversity of microbes, forming the microbial mats, has been sustaining itself, and by extension the prey which support the predators, in isolation, for millions of years.
So while aphotic environs exist elsewhere, this was unique, and one of a kind. Undisturbed, this system contained 48 different invertebrate species, of which a solid 33 were endemic (unique) to the Movile cave system. This includes copepods, nematodes, centipedes, and more. A really neat combination of different clades and different interactions, but fundamentally simplified by the constraints of the environ. Five and a half million years of complete isolation, a lightless biodome and experimental playground (Lascu, 1989).
Within the microbial mats nematodes which feed upon the bacteria within the mats. In an isolated environment, with limited resources, it is somewhat surprising to encounter multiple species which all occur the same ecological niche. But, in both in situ and simulated environmental experiments, it has been show that these five different species of nematode have drastically different fitnesses dependent on the density of available food, a classical reproductive binary known as r/K selection (Pianka, 1970). the letters r and K refer to variables in an equation that I'm not interested in exploring (this is a place for narratives, not mathematics), but describe two different reproductive strategies. r-selection describes an environment where there are no resource or population density limits, where competition is not a factor, and so rate of reproduction, and quantity of offspring, is most important. Conversely, K-selection describes where competition is highly important due to limited space/resources, and so it is more important to devote reproductive resources to producing highly fit individuals. It is, in essence, a dichotomy of quanitity versus quality of offspring.
When microbial densities are high, one of two species, Panagrolaimus sp. showed the fastest growth rate, showing r-selective reproduction. The quicker they get a foothold, the more successful they'll be. In Poikilolaimus sp., in nutrient limited conditions, their offspring are more efficient at utilizing resources, and so are more competitive, showing K-selective reproduction (Schroeder et al., 2010). And so, if the nutrient availability was constant, one would have a clear edge over the other, and the fluctuating state of the mats they live in allows for a cyclical variation in their respective population densities, and a mutual existence within the mats. This sort of population cycling has been observed in laboratory conditions similar to the actual cave mats, but the small scale and low complexity of such an interaction in the caves themselves is quite interesting (Muschiol et al., 2015). This simple food web then connects upwards, as these nematodes are then preyed upon by the caves' resident copepods (Muschiol et al. 2008; Muschiol ,2009).
There aren't any giant monsters, or glowing beasts, but it's still an amazing example of the sorts of ways life can make its way on this fantastically terrifyingly diverse planet.