Cave System in Mexico Is Filled With Creatures That Devour Methane

A diver explores a cave passage within a section of the Ox Bel Ha cave system in Mexico's Yucatan Peninsula. © HP Hartmann

Deep in the underground caves and rivers of the Yucatan Peninsula in Mexico, scientists have uncovered a fascinating ecosystem where methane is the key to life.

Once believed in Mayan lore to be the path to the underworld, the winding estuary system is filled with methane and the bacteria that feed off of it. The ecosystem is similar to those found in deep ocean cold seeps, or cold vents, which are found on the ocean floor and even some lakes, according to a team of researchers from the United States, Mexico, the Netherlands and Switzerland.

They describe their work as the most detailed ecological study of a coastal cave ecosystem that is always underwater. The scientists, who were trained in cave diving for the study, said they had to use techniques for their research that previously had been used by deep-sea submergence vehicles in order to be able to study the environment.

The study was conducted in the Ox Bel Ha, a subterranean estuary complex that's about the size of Galveston Bay. The area is unique because the flooded cave passages have two distinct water layers: freshwater fed by rainfall and salt water from the ocean.

A downward migration

In a study published in the journal Nature Communications, the researchers showed how bacteria and other microbes are the key to the cave ecosystem. Methane forms naturally beneath the jungle floor and migrates down into the water and caves. Typically, methane that forms in soil migrates upward, toward the atmosphere.

cross section of the Ox Bel Ha Cave
This cross-section of the Ox Bel Ha Cave shows how the area is filled with fresh, brackish and saline waters. Methane (CH4) and other forms of dissolved organic carbon (DOC) are created during the decomposition of soil from the overlying tropical forest. Brankovits et al

But here in the watery depths, methane sinks down and becomes part of the food chain. The microbes feast on the methane, as well as any other dissolved organic material brought in from the surface by the freshwater.

The researchers said they were surprised by their findings. Earlier studies had assumed that microbes mainly ate vegetation and other detritus that washed into the caves from the tropical forest.

"Finding that methane and other forms of mostly invisible dissolved organic matter are the foundation of the food web in these caves explains why cave-adapted animals are able to thrive in the water column in a habitat without visible evidence of food," lead author David Brankovits said in a statement. (Brankovits conducted the research during his Ph.D. studies at Texas A&M; University at Galveston, also known as TAMUG.)

However, deep within the caves, where the study was conducted, there's little of that surface debris, so the microbes depend on methane and the other dissolved organics percolating downward through the ceiling of the caves.

What the researchers found in this cave system is similar to what's happening in the oceans, particularly "oxygen minimum zones," where oxygen is at its lowest. This is critical because oxygen is necessary to sustain life.

Tom Iliffe is a professor in the Marine Biology Department at TAMUG. He's been studying the biodiversity, evolution and conservation of marine cave animals for nearly 40 years, and he explains why this work is so important:

"Providing a model for the basic function of this globally-distributed ecosystem is an important contribution to coastal groundwater ecology and establishes a baseline for evaluating how sea level rise, seaside touristic development and other stressors will impact the viability of these lightless, food-poor systems."