Black Holes of the Bahamas: what they are and why they are black

摘要

Black holes are vertical cave systems, which have no known lateral passages and form in the interior stable regions of the larger Bahamian islands. They are found where the land is transitional between submerged and dry land, not dissimilar to saltmarsh environments. Most Black Holes are located in the central to western side of the island of South Andros in the Bahamas, although one has been found on the northern transitional shore of Grand Bahama Island. The water in these holes appears black in colour due to the presence of a 1 m thick microbial layer located within the upper third of the water column (18-19 m) at the boundary between the oxic low salinity upper water mass and the denser anoxic saline water layer. The boundary between the two water masses is characterised by sharp discontinuities in physico-chemical gradients: salinity increased from 12 to 35 psu; temperature from 29℃ to 36℃; pH decreased from pH 8.6 to 6.45 and dissolved O_2 from 6 mg/l to < 1 mg/l. Anoxygenic phototrophic bacteria belonging to the genera Allochromatium and Thiocapsa have been identified as the dominant members of this warm (36℃), saline, sulphide rich layer and achieve population densities > 10~7 viable cells/ml. Calculations reveal that this layer of anoxygenic phototrophic bacteria, in the South Andros Black Hole may have a biomass content of approximately 5.06 ton dry weight.