Salt-saturated lakes such as the North Arm of Great Salt Lake and saltern crystallizer ponds contain 10-10 and more red microorganisms ml-1. Even the Dead Sea occasionally turns red due to microbial blooms. Three types of organisms may contribute to the coloration: the alga Dunaliella salina rich in carotene, halophilic Archaea (family Halobacteriaceae) containing 50-carbon bacterioruberin carotenoids and sometimes also retinal proteins (bacteriorhodopsin, halorhodopsin), and the recently discovered Salinibacter (Bacteroidetes) which contains pigment salinixanthin (an unusual acylated C40-carotenoid glucoside) as well as different retinal pigments. Bacteriorhodopsin and halorhodopsin enable the cells to directly use light energy for respectively the outward pumping of protons driving ATP generation and for the inward transport of chloride ions. The carotenoid pigments (carotene, bacterioruberin and derivatives, salinixanthin) primarily appear to protect the cells against photooxidative damage. Salinixanthin also acts as a light harvesting antenna for xanthorhodopsin, the proton pumping retinal pigment of Salinibacter. Quantitative assessment of the relative importance of the different pigments in the coloration of red brines of natural salt lakes and solar saltern crystallizer ponds suggests that bacterioruberin and other carotenoids contributed by members of the Halobacteriaceae are generally responsible for most of the color of the waters. The quantity of carotene present in dunaliella cells often greatly exceeds that of the haloarchaeal bacterioruberin pigments. However, the large amounts of carotene contribute only little to the optical properties of the brines because of the dense packing of the pigment in little globules within the chloroplast. Presence of salinixanthin and of bacteriorhodopsin and derivatives in the biomass can often be demonstrated as well, but these pigments have never been shown to contribute greatly to the overall optical properties of the waters. Thus, carotenoids of the bacterioruberin group appear to be the main factor causing the characteristic red color of hypersaline brines worldwide.
"Microbial diversity and microbial abundance in salt-saturated brines: Why are the waters of hypersaline lakes red?,"
Natural Resources and Environmental Issues: Vol. 15
, Article 49.
Available at: http://digitalcommons.usu.edu/nrei/vol15/iss1/49