Anchialine pools and cenotes

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Introduction

Amazing photo of Yax Mul cenote on the Yucatan Peninsula, Mexico.

The Great Blue Hole at Lighthouse Reef in Belize.

Anchialine pools, also known as Cenotes in the Yucatan Peninsula region, are subterranean bodies of water that have a connection to the surface. Anchialine pools are circular and cliffed sinkholes that contain watertable lakes. They are filled with brackish water. hThey contain large passage ways which make them desirable for cave explorations. They are very common globally, with the most popular located in the Hawaiian Islands and the Yucatan Peninsula, Mexico. They are also located in the Florida peninsula, southeastern South Australia, South Africa, Turkey on the Anatolian Plateau, and the Bahamas Banks. In the Bahamas Banks, they occur onshore and offshore, where they occur as drowned anchialine pools called Blue Holes.

Anchialine pools are home to different types of grazers. Thick and intricate microbial mats also form in these areas due to nutrients provided by hydrothermal inputs. The microbes in the microbial mats undergo anaerobic respiration. Microbial processes that occur here include: methanogenesis, sulfur reduction, and anaerobic ammonia oxidation.

Physical environment

Vegetation

There are many species that are endemic to anchialine pools including flora, fauna, and microbial communities. Characteristic heterogeneous vegetation, including tall evergreen trees like Ficus species, surrounds the opening of the pools. Removal of vegetation by large herbivores is threatening the ecosystems that reside in the anchialine pools.

Formation

Anchialine pools are formed by the dissolution of limestone by carbonic acid in karst landforms. Underlying these landforms is calcerous rock. As the rock breaks off and falls into the pool, it is further dissolved. In the Hawaiian Islands, these pools occur in highly porous substrates like recent lavas or limestone that are near the ocean.

Hydrology

Anchialine pools experience tidal fluctations and lack surface connections that are near the ocean. They contain very clear water and large channels. The anchialine pools of the Yucatan Peninsula in Mexico are the only known underground aquatic system located there. They are stratified, with brackish water on the top layer. They experience a salinity gradient throughout the water column. Salinity decreases as depth increases.

Topography

Anchialine pools are common on coastal karst plains with low topography.

Animal Community

Anchialine pools serve as the main source of water for humans and animals in regions like the Yucatan Peninsula.

Microbial communities

Microbial Mats

Methanogens

Methanogens are a type of microbe that belongs to the domain Archaea. They undergo a type of anaerobic respiration called methanogenesis.

Sulfur reducing bacteria

Microbial processes

Anaerobic respiration

Anaerobic respiration occurs when microbes use other terminal electron acceptors other than oxygen. In environments, like anchialine pools and cenotes, microbes use terminal electron acceptors like sulfate (SO₄^-2), nitrate (NO₃^-), sulfur (S), and carbon dioxide (CO₂). This type of respiration occurs in the absence of oxygen. Although these terminal electron acceptors produce energy, they release less energy than oxygen. Therefore, anaerobic respiration produces less energy than aerobic respiration.

Methanogenesis

Methanogenesis is the formation of methane by microbes. Methanogens, a type of Archaea, specifically carry out this process. This is a type of anaerobic respiration that uses carbon dioxide (CO₂) and acetic acid as the terminal electron acceptors.

CO₂ 4 H₂ → CH₄ + 2H

CH₃COOH → CH₄ + 2H

Sulfate reduction

Sulfur Cycle Diagram

Sulfate reduction is part of the sulfur cycle. Sulfate reduction occurs when sulfate is reduced to sulfide. Sulfur-reducing microbes use sulfate as the terminal electron acceptor in the anaerobic conditions that occur in anchialine pools.