Blue holes are hotspots of microbial biodiversity

Scientists discovered unique and diverse microbial communities during a cave diving expedition to blue holes in the Bahamas.

Blue holes are deep, circular columns of water that form when rainwater dissolves a sink hole through limestone rock. The Bahamas are thought to contain over 1000 blue holes, and only about 20% of these have been explored by scientists.

Blue holes in the Bahamas are one of the most unique environments on Earth. Many blue holes there are landlocked and rainwater creates a thin layer of freshwater that floats on top of denser, salty water. The density difference between the layers of fresh and salt water prevents them from mixing, and oxygen inputs from the atmosphere are unable to replenish the lower depths.

Life in these blue holes has adapted to the low oxygen environment and is fueled by the energy created from bacteria that use sulfate instead of oxygen during the process of photosynthesis. Scientists are keen on studying life in blue holes from the Bahamas because it may offer a glimpse of what life was like on Earth millions of years ago when the oceans were anoxic. Scientific research on these blue holes may also help to teach us what life may look like on other planets.

A group of scientists from Texas A&M University and Pennsylvania State University teamed up for a cave diving expedition with Bahamas Underground to collect samples of microorganisms from two geographically adjacent blue holes. They hypothesized that they would find comparable microbial communities in the blue holes given their close proximity.

The scientists discovered that microbial biofilms in both blue holes were dominated by green sulfur bacteria from the phylum Chlorobi and a diverse class of bacteria called deltaproteobacteria. These bacteria were present in colorful mats of slime that lined the cave walls.

Additionally, the scientists discovered numerous types of bacteria that were unique to each blue hole, and many of these may be new species as they were unable to identify the microorganisms through the use of known genetic libraries. The scientists speculate that different and diverse microbial communities developed in the blue holes despite their close proximity because the blue holes had different cave passage geometries that likely impacted the degree of sunlight penetration, water residence times and water chemistry.

The scientists published the results of their investigation of the microbial diversity present in blue holes in the November 2011 issue of Hydrobiologia.

Bottom line: Scientists discover unique and diverse microbial communities during a cave diving expedition to blue holes in the Bahamas. Learn more about them on EarthSky.

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