Bingsong Yu

Microbial diversity in water and sediment of Lake Chaka, an athalassohaline lake in northwestern China

ABSTRACT We employed culture-dependent and -independent techniques to study microbial diversity in Lake Chaka, a unique hypersaline lake (32.5% salinity) in northwest China. It is situated at 3,214 m above sea level in a dry climate. The average water depth is 2 to 3 cm. Halophilic isolates were obtained from the lake water, and halotolerant isolates were obtained from the shallow sediment. The isolates exhibited resistance to UV and gamma radiation. Microbial abundance in the sediments ranged from 108 cells/g at the water-sediment interface to 107 cells/g at a sediment depth of 42 cm. A major change in the bacterial community composition was observed across the interface. In the lake water, clone sequences affiliated with the Bacteroidetes were the most abundant, whereas in the sediments, sequences related to low G+C gram-positive bacteria were predominant. A similar change was also present in the archaeal community. While all archaeal clone sequences in the lake water belonged to the Halobacteriales, the majority of the sequences in the sediments were related to those previously obtained from methanogenic soils and sediments. The observed changes in the microbial community structure across the water-sediment interface were correlated with a decrease in salinity from the lake water (32.5%) to the sediments (approximately 4%). Across the interface, the redox state also changed from oxic to anoxic and may also have contributed to the observed shift in the microbial community.

Diversity and Abundance of Ammonia-Oxidizing Archaea and Bacteria in Qinghai Lake, Northwestern China

The abundance and diversity of amoA gene, a gene putatively encoding ammonia monooxygenase subunit A, was evaluated in oxic lake water and anoxic sediments from two adjacent sites of Qinghai Lake, China. An integrated approach was employed including geochemistry, clone library construction, and reverse-transcription quantitative polymerase chain reaction. Whereas ammonia-oxidizing archaea (AOA) were more abundant than ammonia-oxidizing bacteria (AOB) in oxic lake water, the opposite was true in anoxic sediments. The AOB community was the same in the water and sediments and consisted of Nitrosomonas- and Nitrosospira-like sequences, suggesting that the sediment-associated AOB may be derived from the water column, but metabolically inactive. In contrast, the AOA community was distinct between the water column and the sediments, and clone sequences were related to those from aquatic environments and soils, respectively, suggesting that AOA in the sediments may be native and metabolically active. Our data have important implications for a better understanding of the potential role of ammonia oxidizers in lake ecosystems.

Dominance of putative marine benthic Archaea in Qinghai Lake, north-western China

Recent studies have revealed important and versatile roles that Archaea play in a wide variety of environmental processes on Earth. In this study, we investigated the abundance and diversity of archaeal communities in lake water and a 5 m sediment core collected from Qinghai Lake on the Tibetan Plateau, north-western China. An integrated approach was employed including geochemistry, quantitative polymerase chain reaction (Q-PCR) and 16S rRNA gene analysis. Here, we show that Archaea dominated the prokaryotic community in the lake sediments. Members of putative marine benthic groups [Marine Benthic Group (MBG)-B, -C and -D] and Miscellaneous Crenarchaeotic Group (MCG) were dominant, many of which were previously reported to be predominantly present in deep-sea environments. These results demonstrate that these groups are not limited to marine sediments. Despite their ubiquitous presence in aquatic environments, metabolic functions of these important groups largely remain unknown. Whereas many of these groups (such as MBG-B and -D) have typically been found in methane-hydrate deposits in marine environments, our carbon isotopic and molecular results from Qinghai Lake sediments indicate a lacustrine origin.

Abundance and diversity of aerobic anoxygenic phototrophic bacteria in saline lakes on the Tibetan plateau

Aerobic anoxygenic phototrophic (AAP) bacteria are heterotrophic prokaryotes that are capable of utilizing light as an energy source but are not capable of producing molecular oxygen. Recently, multiple studies have found that AAP bacteria are widely distributed in oceans and estuaries and may play an important role in carbon cycling. However, AAP bacteria in inland lake ecosystems have not been investigated in depth. In this study, the abundance and diversity of the pufL-M genes, encoding photosynthetic reaction centers of AAP bacteria, were determined in the oxic water column and anoxic sediments of saline lakes (Qinghai, Erhai, and Gahai Lakes) on the Tibetan Plateau, China. Our results indicated that AAP bacteria were abundant in inland lakes, with the proportion of AAP bacteria (in total bacteria) comparable to those in the oceans, but with a lower diversity. Salinity and pH were found to be potential factors controlling the AAP bacterial diversity and community composition. Our data have implications for a better understanding of the potential role of AAP bacteria in carbon cycling in inland lake ecosystems.

Response of Archaeal Community Structure to Environmental Changes in Lakes on the Tibetan Plateau, Northwestern China

To study how archaeal community responds to environmental changes, we investigated archaeal community structures in waters of three Tibetan saline lakes in northwestern China (Gahai, Xiaochaidan, and Charhan Lakes) with 16S rRNA gene phylogenetic analysis. Temperature, pH, and water chemistry (major anions and cations) of the lakes were measured. Three archaeal clone libraries were constructed with a total of 297 sequences. Incorporating our previous data obtained from other lakes on the Tibetan Plateau, we performed statistical analyses to identify dominant environmental parameters that could account for the observed variations in archaeal community structure. We concluded that salinity and water chemistry (Na and bicarbonate concentration in particular) played an important role in shaping archaeal community. In particular, the relative abundance of archaeal 16S rRNA genes affiliated with the Halobacteriales of the Euryarchaeota increased with salinity, whereas that of crenarchaeotal 16S rRNA gene sequences showed the opposite trend. Crenarchaeotal 16S rRNA gene sequences were retrieved from lake waters with salinity up to 28.3%. These results have important implications for our understanding of response of archaeal community to environmental changes in high-altitude lake ecosystems.

Impacts of environmental change and human activity on microbial ecosystems on the Tibetan Plateau, NW China

Microorganisms play important roles in maintaining ecosystem functions. It is poorly known, however, how microbial ecosystems respond to environmental changes and human activities. The purpose of this paper is to demonstrate that the microbial record in lake sediments and ice cores contains a wealth of paleoenvironmental and paleoclimatic information. Saline lakes on the Tibetan Plateau exhibit multiple environmental gradients and have accumulated thick sequences of sediments through time. Microbial abundance and species diversity vary considerably along environmental gradients across the plateau. Studies of lake sediments reveal that wet and warm climates are correlated with high bacterial abundance and diversity, whereas cold and dry climates result in low abundance and diversity. Recent human activities have enhanced sulfate reduction in lake sediments. Ice cores from the plateau reveal that bacterial abundance and diversity are positively correlated with dust particle concentration and

Response of Aerobic Anoxygenic Phototrophic Bacterial Diversity to Environment Conditions in Saline Lakes and Daotang River on the Tibetan Plateau, NW China

Variations of the pufM gene [encoding the M subunit of the photosynthetic reaction center in aerobic anoxygenic phototrophic (AAnP) bacteria] diversity in response to environmental changes were investigated in waters of six aqueous regimes (including Daotang River and five saline/hypersaline lakes on the Tibetan Plateau) representing a full salinity gradient from freshwater to NaCl-saturation. AAnP bacterial community structures responded to salinity change: Gamma-like AAnP community was predominant in freshwater Daotang River (0.01% salinity). AAnP community structure shifted from Loktanella-like sequences of the Alphaproteobacteria in saline Qinghai Lake to Roseobacter-like sequences in hypersaline lakes (Gahai, Xiaochaidan and Charhan). In addition to salinity, other environmental variables (e.g. N and P availability, TOC and/or DOC, and HCO− 3/CO2− 3 ions) were also important in affecting the pufM gene diversity in hypersaline lakes. These data have important implications for our understanding of the response of AAnP bacterial community to environmental variables in high-altitude aquatic ecosystems.

THE ROLE OF CLAY MINERALS IN THE PRESERVATION OF ORGANIC MATTER IN SEDIMENTS OF QINGHAI LAKE, NW CHINA

The role of saline lake sediments in preserving organic matter has long been recognized. In order to further understand the preservation mechanisms, the role of clay minerals was studied. Three sediment cores, 25, 57, and 500 cm long, were collected from Qinghai Lake, NW China, and dissected into multiple subsamples. Multiple techniques were employed, including density fractionation, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM), total organic carbon (TOC) and carbon compound analyses, and surface area determination. The sediments were oxic near the water-sediment interface, but became anoxic at depth. The clay mineral content was as much as 36.8%, consisting mostly of illite, chlorite, and halloysite. The TEM observations revealed that organic matter occurred primarily as organic matter-clay mineral aggregates. The TOC and clay mineral abundances are greatest in the mid-density fraction, with a positive correlation between the TOC and mineral surface area. The TOC of the bulk sediments ranges from 1 to 3% with the non-hydrocarbon fraction being predominant, followed by bitumen, saturated hydrocarbon, aromatic hydrocarbons, and chloroform-soluble bitumen. The bimodal distribution of carbon compounds of the saturated hydrocarbon fraction suggests that organic matter in the sediments was derived from two sources: terrestrial plants and microorganisms/algae. Depth-related systematic changes in the distribution patterns of the carbon compounds suggest that the oxidizing conditions and microbial abundance near the water-sediment interface promote degradation of labile organic matter, probably in adsorbed form. The reducing conditions and small microbial biomass deeper in the sediments favor preservation of organic matter, because of the less labile nature of organic matter, probably occurring within clay mineral-organic matter aggregates that are inaccessible to microorganisms. These results have important implications for our understanding of mechanisms of organic matter preservation in saline lake sediments.