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Microbes of the Pinnacled Mats of Lake Vanda, Antarctica

出版	University of California, Davis, 2018
ISBN	043892990X9780438929906
URL	http://books.google.com.hk/books?id=H8z7xwEACAAJ&hl=&source=gbs_api

註釋Ancient microbial mats provide some of the earliest evidence for life on Earth. Modern analogues of ancient microbial mats are rare and confined to extreme habitats, such as hot springs, hypersaline ponds, and acid mine drainage. The persistence of microbial mats in these environments is thought to be due to the absence of grazing microorganisms. Extensive microbial mats have been found to exist in Antarctic ice-covered lakes, which lack macroscopic grazing organisms due to isolation. In Lake Vanda, these microbial mats form unusual pinnacled structures that display a vertical pigmented zonation, in addition to fine laminae that approximately correspond to yearly growth layers. Previous research has shown that the Lake Vanda pinnacles are dominated by filamentous Cyanobacteria, with Cyanobacterial types determined by morphology and Cyanobacterial specific primers to be primarily Phormidium Pseudanabaena, and Leptolygbya. More extensive research on the pinnacles was undertaken in order to further characterize the microbial content of the pinnacled mats (Chapter 1). Sampling was accomplished by diving under the ice and collecting mat samples. The collected samples were then subjected to 16S rRNA gene sequencing. The mats were found to contain a diverse assemblage of microorganisms, which consisted of almost equal relative sequence abundances of Cyanobacteria and Proteobacteria, as well as many other taxa that are typical of Cyanobacterially dominated microbial mats. Statistical analyses of the differentially pigmented areas and structural components of the pinnacles were found to comprise different microbial communities. The pinnacled mats were found to contain members of a group of non-photosynthetic Cyanobacteria, the Melainabacteria. Originally discovered in a study of mouse gut microbiota, the Melainabacteria were subsequently found to be non-photosynthetic. The gut-type Melainabacteria, the Gastranaerophilales, have been found to be obligate fermenting organisms, while the other, free-living types have more diverse metabolisms that include cellular respiration. The free-living Melainabacteria tend to occur in very low abundances, while the microbial mats of Lake Vanda harbor abundances that are several orders of magnitude greater than those previously reported for free-living Melainabacterial clades (Chapter 2). The Lake Vanda mats contain members of four of the seven classes of Melainabacteria, the Gastranaerophilales, the Vampirovibrionales, the Sericytochromatia, and the Obscuribacterales. This diversity of Melainabacteria has not previously been reported in any environment, particularly a well lit, oxic area such as Lake Vanda. In addition to a remarkable assemblage of Melainabacteria, the Lake Vanda mats harbors a novel lineage of Cyanobacteria. This microorganism is a Gloeobacter, an early branching Cyanobacterial class that lacks thylakoids (Chapter 3). Gloeobacter consists of only two species, both lithophilic. The photosynthetic proteins of Gloeobacter are very divergent, thought to be due to the lack of thylakoids. This microbe found in Lake Vanda diverges from the two known Gloeobacter species, making it the most basal branching Cyanobacterium with genes for oxygenic photosynthesis yet known. The photosynthesizing proteins of the Lake Vanda Gloeobacter are also divergent from known Cyanobacterial proteins, making this organism's photosystem II D1 protein gene sequence the most divergent group 4 (active photosynthetic) D1 protein known.