Cave (Cueva de Villa Luz)

Introduction

In the far corners of the lush Mayan rainforest, from the deep dark spiritual world of underground caverns, there emanates a pervasive and overpowering stench of rotten eggs. This invasive odor marks one’s proximity to the sulfurous spring cave, Cueva de Villa Luz. Every Spring, this cave is visited by hundreds of Mayan harvesters (men, women and children) who enter the sacred domain to fish thousands of pounds of sardines that will sustain them until the rains bring new crops. While the Mayans reap the bounty of the caves entranceway, they dare not go beyond where the poisonous atmosphere could prove deadly. Deeper within the cave, its ceilings and walls drip sulfuric acid, its floors lie buried deep in mud and slime and at its farthest reaches, in a section deemed “Snot Heaven”, there exist vast colonies of microorganisms which dangle from the ceiling in stalactite-like formations.

Description of Niche

Where located?

Cueva de Villa Luz, also known by the locals as “Cueva de la Sardina”, is a cavern in Tabasco, Mexico, near the small town of Tapijulapa. It is located within the regional park of Kolem Jaa. Somewhat isolated, the cave may be visited via buses and cabs leaving the state’s capitol of Villahermosa, which lies approximately forty miles from the town of Tapijulapa. The cave itself is located at the end of a mile-and-a-half trail from the Alamandro River, at the edge of the Chiapas highlands.

Physical Conditions?

What are the conditions in your niche? Temperature, pressure, pH, moisture, etc.

Up until microbial life was discovered below the Earth’s surface, people could not imagine or believe that life can be sustained without light energy harnessed from the sun. However, not only do cave dwelling microbes exist, but they also thrive in rocky shelters or natural openings onto or under the earth. Besides on the cave itself, these microbes live in air currents and water, as well as on animals, especially bats and insects.

By exploring the great limestone cave, Cueva de Villa Luz, microbes were determined to be able to be sustained anywhere where there is water or moisture in a cave, such as flowing bodies of water, floors and surfaces of streams, deposit sites, and along water banks. Surfaces of running water leading into caves are a common place to find algae, whereas protozoa are likely to thrive in sediments. Fungal spores are abundant along the length of river banks. Ideal places for microbial growths are crossing points of air, cave surfaces, and water. Bacteria are ubiquitous in soil, sediments, and moisture films of limestone formations and of calcite speleothems. Chemoheterotrophs (such as fungi), organisms that rely on organic carbon, can be found in guano deposits, rotting wood, and water.

Microbes thrive in moist areas within a cave. Chemoheterotrophs benefit from the carbon-rich environment of streaming water, deposit sites, and organic matters, such as decomposing plants. Rainwater that has soaked through into the caves provides oxygen to the hydrogen sulfide, turning the hydrogen sulfide into sulfuric acid. The hydrogen sulfide gives off a distinctive odor; one that is similar to the smell of rotten eggs. The sulfuric acids in turns disintegrate the neighboring rocks and cave walls. This phenomenon helps enlarge the cave internally and aid in the formation of limestone. In limestone creation, the rainwater mixes with carbon dioxide, forming carbonic acid. This particularly weak acid wears away the walls of limestone caves.

Water can also diffuse into sulfidic caves from the ground, bringing in hydrogen sulfide for microbes of biofilms to harvest the sulfur as energy. In the process of harnessing that energy, sulfuric acid is produced, creating a highly acidic environment. Certain biofilms prefer extremely acidic conditions like the snottites microbes, which live in exceptionally acidic environments with a pH of zero to one. For Cueva de Villa Luz specifically, the walls have a pH of 0.0 to 0.3. The snottites’ name derived from its physical attribute – resembling snots. Bacteria that reside in red clay-like goo have a less acidic pH if 2.5 to 3.9. The outer layers of biofilms are comprised of bacteria that survive by turning oxygen in the air to hydrogen sulfide. However, oxygen is detrimental to certain microbes, causing those microbes to withdraw to bottom layers. The microbes residing in the middle and lower layers consume hydrogen sulfide and releases sulfuric acid. This sulfur cycle creates a living environment that is rich in reduced sulfur.

Temperatures in Cueva de Villa Luz, like most caves, do not show a wide range of change. The cave is slightly warmer in the winter with an average temperature of 30 degrees Celsius and an average temperature of 28 degrees Celsius in the spring. The average ground temperature is a few (four or five) degrees Celsius warmer than the air temperature because the ground has the ability to store more heat than the atmosphere. Water sources above ground in the cave are much colder at approximately three degrees Celsius. The average cave temperature is about seventeen degrees Celcius, which is lower compared to Cueva de Villa Luz.

Pressure in caves is dependant on temperature. Various ways that heat can enter are through the ceilings, grounds, and openings. The average air/wind flow rate is 1.90 m/s to 2.00 m/s. The average humidity is about 77, and the average air pressure is about 97 kPa to 100 kPa. Rocks within the cave itself serve as mediums for heat storage, while streams and pools cool down the cave, changing the pressure at those specific areas.

In complete darkness, green slime glazes rock surfaces, meaning that there are microbial lives thriving in flowing water near those rocks. Although the cave itself is mostly devoid of light, there are areas pierced with the sun’s rays. These skylights are created through tiny openings to the cave and through cracks in the ceiling and walls. The sunlight provides a mean for photosynthesis, thus, leading to organic products.

Influence by Adjacent Communities (if any)

Is your niche close to another niche or influenced by another community of organisms? Caves are not usually connected, so microbes evolve independently among caves and do not communicate with those in other caves. Within a cave, however, microbes can be genetically linked and communicate with each other. Similarity in microbial genome can be found in microbes of the same cave or other caves with comparable chemical conditions and geological make-up.

Since cave microbes such as fungi and bacteria live in seclusion from the outside world and in undernourished environments, certain colonies need to mark their region. Some microbes even develop a way to produce and release toxic chemicals to protect their territory from adjacent communities. However, the microbes can associate with nearby spider webs and fungus gnats to form white filaments in the stream and microbial curtains hanging from gypsums.

One major way that cave-dwelling microbes can be affected is by human contact. Caves that serve as tourist attractions are especially exposed to changes, which can be harmful. Humans can affect cave microbes by bringing in organic materials that can disrupt the living conditions of the microbial communities. Some examples of such organic matter include human waste (urine and feces), human cells (hair and skin), clothing fibers, and food. In contrast to synthetic fibers, cotton fibers from clothes are promptly devoured by microbes. These outside resources cause the greatest damage to the microbial colonies that can only survive in nutrient-poor conditions. The fungi that need organic input already have guano deposits from the bats and other animals living in the caves. If enough organic compounds are added, these microbes may die out. Another way humans can infringe on the native cave microbes is to introduce outside microbes into the cave and cave water sources via shoes, dirt/mud from shoes, clothes, and equipment. In contrast to synthetic fibers, cotton fibers are promptly devoured by microbes. Therefore, non-native microbes, also called transient microbes, can out-compete the native microbes if more nutrients are added to its environment. Human visits should be limited to allow outside microbes to die out along with their food source.

Conditions under which the environment changes

Do any of the physical conditions change? Are there chemicals, other organisms, nutrients, etc. that might change the community of your niche.

Cave conditions remain pretty constant in darkness except near the cave openings because of variables such as light and air current. Temperature in caves like Cueva de Villa Luz is stable because of the natural protection that the cave offers from the harsh, changing weather and from the outside world. Since pressure is dependant upon temperature and cave temperature is constant, cave pressure is constant as well. Even so, most bacteria are dormant until sufficient nutrients are available. Outside factors such as human or other animal interactions and introduction of foreign microbes can greatly change and disturb native cave microbes by bringing in competition and the organic compounds that human visitors carry in changes the nutrient level of caves. Elevated nutrient levels can lead to death of microbes that can only survive in poor environments.

The physical size of caves can increase over time, although very slowly. Carbonic acid erodes approximately one third of an inch of cave walls each thousand years, whereas sulfuric acid wears away cave walls about two inches every thousand years.

Who Lives There?

Cave microorganisms

True caves are devoid of all light sources and therefore lack the most common source of energy supplied through photosynthesis. Cave microorganisms are consequently dependent upon alternative sources of energy derived from the surrounding atmosphere, minerals and rocks. Cave microorganisms are divided into two groups, heterotrophs and autotrophs. Although each group requires carbon compounds as a nutrient source, only the autotrophs have the ability to create the organic substances necessary for life directly from inorganic materials. While surface autotrophs usually gain their energy from the sun via photosynthesis, within the dark extremes of caves, certain autotrophic bacteria called chemoautotrophs have the ability to derive all of their energy needs from certain cave minerals.

Microbial life can be observed within all types of caves. Cueva de Villa Luz contains an extraordinarily diverse population of microbes. Along its walls of limestone can be found a dense layer of microbial mucus as great as half an inch thick. Sulfate reducing bacteria are present in very high numbers (105-106+) within the pores of rocks, which can run miles deep. These sulfur-eating bacteria form slender white mucus-like colonies on the cave ceilings and walls. These microbial veils have been nicknamed by researchers as “snot-tites”. Additionally, other sticky clusters of microbes, also facetiously named “phlegm balls” can be observed floating in subterranean streams of the cave.

Other forms of bacteria such as coliform, which are abundant in animal feces and aquatic environments, survive within the main stream passage of Cueva Villa Luz, but are no longer present in the springs that flow into the cave. Many of the unique and bizarre microbes encountered within the extreme environment of the cave are newly discovered and yet-to-be-identified microbes.

Other cave life

Cave animals comprise three main categories based on the amount of time they spend within the cave. Organisms that comprise the first group are called trogloxenes. These animals move freely in and out of the cave as temporary guests. Examples include bats, bears, skunks, moths and humans. The second group of cave inhabitants are called troglophiles. Troglophiles live their entire life cycle within a cave, but can also reside outside of the cave. Examples include cockroaches, beetles and millipedes. The final group, the troglobites, are considered the true cave dwellers, spending their entire lives within the dark zones of the cave. Examples include fish, shrimp, crayfish, salamanders, worms, snails, insects, bacteria, fungi and algae.

In Cueva de Villa Luz, scientists have observed at least five kinds of bats, including three leaf-nosed species, vampire bats, and Mexican free-tailed bats that flutter overhead near the entry and in fresh air pockets. The most widely found organisms are the midges, or tiny gnats, and the small fish. Predatory invertebrates such as spiders, fungus gnat larvae and amblypygids are also abundantly found throughout the cave.

Interactions amongst microorganisms and other cave life

The snot-tite biofilms that are present throughout the caverns of Cueva de Villa Luz are comprised of thin layers of distinct microbe species. This mode of growth provides the many species of bacteria found throughout the different layers of the colony with protection. Such protection allows certain bacteria that would otherwise perish, to flourish inspite of the surrounding hostile environment of the cave.

An added factor that makes Cueva de Villa Luz truly remarkable, is the abundance of life that flourishes within its dark interior. A mutualistic network amongst the caves inhabitants is established in which all life forms work together. Similar to an assembly line, one organism brings in the energy while another organism brings in the nutrients which then allows a third organism to supply the basic elements for still another organism to grow. In Cueva de Villa Luz, the mucus-like microbial communities that are so common to this cave, form the primary link in its dense food chain. These sulfur-eating bacteria use hydrogen sulfide to create the nutrients that sustain the cave’s extensive food web. Through the process of chemosynthesis these bacteria oxidize sulfur for energy much in the same way that surface plants perform photosynthesis. These bacteria have the ability to utilize carbon dioxide, water and sulfur to support life within Cueva de Villa Luz. Small invertebrates such as the midges feed on the microbes, who in turn provide food for spiders and small fish. Water bugs along with the occasional nocturnal rodents prey on the fish. Humans, the caves largest predators, complete the food web.