Nitrite- and N2O-reducing bacteria respond differently to ecological factors in saline lakes.

The distribution of nitrite- and N2O-reducing bacteria is key to potential N2O emission from lakes. However, such information in highland saline lakes remains unknown. Here, we investigated the abundance and community composition of nitrite- and N2O-reducing bacteria in the sediments of six saline lakes on the Qing-Tibetan Plateau. The studied lakes covered a wide range of salinity (1.0-340.0 g/L). Results showed that in the studied saline lake sediments, nitrite-reducing bacteria were significantly more abundant than N2O-reducing bacteria, and their abundances ranged 7.14 × 103-8.26 × 108 and 1.18 × 106-6.51 × 107 copies per gram sediment (dry weight), respectively. Nitrite-reducing bacteria were mainly affiliated with α-, ß- and γ-Proteobacteria, with ß- and α-Proteobacteria being dominant in low- and high-salinity lakes, respectively; N2O-reducing bacterial communities mainly consisted of Proteobacteria (α-, ß-, γ- and δ-subgroups), Bacteroidetes, Verrucomicrobia, Actinobacteria, Chloroflexi, Gemmatimonadetes and Balneolaeota, with Proteobacteria and Bacteroidetes/Verrucomicrobia dominating in low- and high-salinity lakes, respectively. The nitrite- and N2O-reducing bacterial communities showed distinct responses to ecological factors, and they were mainly regulated by mineralogical and physicochemical factors, respectively. In response to salinity change, the community composition of nitrite-reducing bacteria was more stable than that of N2O-reducing bacteria. These findings suggest that nitrite- and N2O-reducing bacteria may prefer niches with different salinity.

Aquiflexum lacus sp. nov., isolated from a lake sediment sample.

A novel Gram-staining negative, crescent-like or rod-shaped, non-motile bacterium, designated strain CUG 91378 T, was isolated from a sediment sample of Qinghai Lake, Qinghai Province, China. The strain was red-colored, and catalase- and oxidase-positive. Strain CUG 91378 T was able to grow at 15-37 °C (optimum, 28 °C), pH 7-9 (pH 7.0) and in the presence of up to 3.0% (w/v) NaCl (0-2%). Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain CUG 91378 T formed a well-supported monophyletic clade with Aquiflexum balticum DSM 16537 T (95.4%) and Aquiflexum aquatile Z0201T (93.2%). The DNA G + C content of CUG 91378 T was 39.0%. Low (< 87%) average nucleotide identity (ANI) and (< 26%) digital DNA-DNA hybridization (dDDH) values were observed between strain CUG 91378 T and its closest species on the phylogenetic trees. The sole respiratory quinone of strain CUG 91378 T was MK-7. The predominant fatty acids (> 5.0%) were iso-C15:0 (19.1%), iso-C16:0 (12.0%), iso-C16:1 H (10.9%), iso-C16:0 3OH (9.2%), iso-C17:0 3OH (7.7%), C17:1ω6c (6.1%) and anteiso-C15:0 (5.8%). Strain CUG 91378 T contained as phosphatidylethanolamine (PE), phosphatidylglycerol (PG) and four unidentified lipids (L1, L2, L3 and L4). Based on the data from the current polyphasic study, the isolate represents a novel species of the genus Aquiflexum for which the name Aquiflexum lacus is proposed. The type strain of the proposed new taxon is CUG 91378 T (= KCTC 62637 T = CGMCC 1.13988 T).

Bicarbonate uptake rates and diversity of RuBisCO genes in saline lake sediments.

There is limited knowledge of microbial carbon fixation rate, and carbon-fixing microbial abundance and diversity in saline lakes. In this study, the inorganic carbon uptake rates and carbon-fixing microbial populations were investigated in the surface sediments of lakes with a full range of salinity from freshwater to salt saturation. The results showed that in the studied lakes light-dependent bicarbonate uptake contributed substantially (>70%) to total bicarbonate uptake, while the contribution of dark bicarbonate uptake (1.35-25.17%) cannot be ignored. The light-dependent bicarbonate uptake rates were significantly correlated with pH and turbidity, while dark bicarbonate uptake rates were significantly influenced by dissolved inorganic carbon, pH, temperature and salinity. Carbon-fixing microbial populations using the Calvin-Benson-Bassham pathway were widespread in the studied lakes, and they were dominated by the cbbL and cbbM gene types affiliated with Cyanobacteria and Proteobacteria, respectively. The cbbL and cbbM gene abundance and population structures were significantly affected by different environmental variables, with the cbbL and cbbM genes being negatively correlated with salinity and organic carbon concentration, respectively. In summary, this study improves our knowledge of the abundance, diversity and function of carbon-fixing microbial populations in the lakes with a full range of salinity.

Microbial Responses to Simulated Salinization and Desalinization in the Sediments of the Qinghai-Tibetan Lakes.

Uncovering microbial response to salinization or desalinization is of great importance to understanding of the influence of global climate change on lacustrine microbial ecology. In this study, to simulate salinization and desalinization, sediments from Erhai Lake (salinity 0.3-0.8 g/L) and Chaka Lake (salinity 299.3-350.7 g/L) on the Qinghai-Tibetan Plateau were transplanted into different lakes with a range of salinity of 0.3-299.3 g/L, followed by in situ incubation for 50 days and subsequent geochemical and microbial analyses. Desalinization was faster than salinization in the transplanted sediments. The salinity of the transplanted sediment increased and decreased in the salinization and desalinization simulation experiments, respectively. The TOC contents of the transplanted sediments were lower than that of their undisturbed counterparts in the salinization experiments, whereas they had a strong negative linear relationship with salinity in the desalinization experiments. Microbial diversity decreased in response to salinization and desalinization, and microbial community dissimilarity significantly (P < 0.01) increased with salinity differences between the transplanted sediments and their undisturbed counterparts. Microbial groups belonging to Gammaproteobacteria and Actinobacteria became abundant in salinization whereas Bacteroidetes and Chloroflexi became dominant in desalinization. Among the predicted microbial functions, hydrogenotrophic methanogenesis, methanogenesis through CO2 reduction with H2, nitrate/nitrogen respiration, and nitrification increased in salinization; in desalinization, enhancement was observed for respiration of sulfur compounds, sulfate respiration, sulfur respiration, thiosulfate respiration, hydrocarbon degradation, chemoheterotrophy, and fermentation, whereas depressing was found for aerobic ammonia oxidation, nitrate/nitrogen respiration, nitrification, nitrite respiration, manganese oxidation, aerobic chemoheterotrophy, and phototrophy. Such microbial variations could be explained by changes of transplantation, salinity, and covarying variables. In summary, salinization and desalinization had profound influence on the geochemistry, microbial community, and function in lakes.

Identification of photosynthetic plankton communities using sedimentary ancient DNA and their response to late-Holocene climate change on the Tibetan Plateau.

Sediments from Tibetan lakes in NW China are potentially sensitive recorders of climate change and its impact on ecosystem function. However, the important plankton members in many Tibetan Lakes do not make and leave microscopically diagnostic features in the sedimentary record. Here we established a taxon-specific molecular approach to specifically identify and quantify sedimentary ancient DNA (sedaDNA) of non-fossilized planktonic organisms preserved in a 5-m sediment core from Kusai Lake spanning the last 3100 years. The reliability of the approach was validated with multiple independent genetic markers. Parallel analyses of the geochemistry of the core and paleo-climate proxies revealed that Monsoon strength-driven changes in nutrient availability, temperature, and salinity as well as orbitally-driven changes in light intensity were all responsible for the observed temporal changes in the abundance of two dominant phytoplankton groups in the lake, Synechococcus (cyanobacteria) and Isochrysis (haptophyte algae). Collectively our data show that global and regional climatic events exhibited a strong influence on the paleoecology of phototrophic plankton in Kusai Lake.