Salt lakes have long been known as habitats for microbial mats. Despite the adverse conditions - high osmotic pressure, high insolation, fluctuations and extremes in temperature and hydration - salt lakes are biotically quite productive. At Lake Tyrrell, prokaryotes and eukaryotes inhabit the water column, and their carotenoid content causes the salt precipitated from the water column to be bright pink in color (see image at right). Mat-forming organisms colonize the salt-sediment interface; they are stratified by organism type. In most cases, oxygenic photoautotrophs, such as the cyanobacterial taxa Euhalothece, form the top layer. Besides generating organic carbon, these may also be involved in nitrogen-fixation. Beneath them are anoxygenic photoautotrophs - mostly green sulfur bacteria. The bottom layer of the mats is colonized by purple sulfur bacteria, another type of anoxygenic photoautotroph. Like the green sulfur bacteria, they use H2S as an electron donor to fix CO2. Beneath the mat is dark, sulfidic sediment, colonized by sulfate-reducing bacteria (SRB), which are mostly obligate anaerobes. As mats are buried by sedimentation, the organic carbon they contain is used as a food source by SRB. SRB oxidize organic compounds using sulfate as an electron acceptor.

    To determine whether community structure at Lake Tyrrell paralleled what had been reported from other saline environments, we collected samples of water, salt, mats and underlying sediment. Karla Heidelberg’s group at USC is characterizing the eukaryotic population; Eric Allen’s group at UCSD/Scripps is working with the Banfield group here at UC Berkeley to characterize the planktonic prokaryotic community.

    My project is to use genomic data generated by Sanger and 454 sequencing of DNA extracted from the mats to unravel the membership thereof, and to explore metabolic interactions between community members and groups. Preliminary findings indicate that the mats are primarily colonized by Salinibacter sp., a variety of halophilic archaea, and several species of cyanobacteria. Surprisingly, few sequences map back to green or purple sulfur bacteria.

    Stay tuned for updates...