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

Ecology of aerobic anoxygenic phototrophs on a fine-scale taxonomic resolution in Adriatic Sea unravelled by unsupervised neural network

BACKGROUND

Aerobic anoxygenic phototrophs are metabolically highly active, diverse and widespread polyphyletic members of bacterioplankton whose photoheterotrophic capabilities shifted the paradigm about simplicity of the microbial food chain. Despite their considerable contribution to the transformation of organic matter in marine environments, relatively little is still known about their community structure and ecology at fine-scale taxonomic resolution. Up to date, there is no comprehensive (i.e. qualitative and quantitative) analysis of their community composition in the Adriatic Sea.

RESULTS

Analysis was based on pufM gene metabarcoding and quantitative FISH-IR approach with the use of artificial neural network. Significant seasonality was observed with regards to absolute abundances (maximum average abundances in spring 2.136 ± 0.081 × 104 cells mL-1, minimum in summer 0.86 × 104 cells mL-1), FISH-IR groups (Roseobacter clade prevalent in autumn, other Alpha- and Gammaproteobacteria in summer) and pufM sequencing data agglomerated at genus-level. FISH-IR results revealed heterogeneity with the highest average relative contribution of AAPs assigned to Roseobacter clade (37.66%), followed by Gammaproteobacteria (35.25%) and general Alphaproteobacteria (31.15%). Community composition obtained via pufM sequencing was dominated by Gammaproteobacteria clade NOR5/OM60, specifically genus Luminiphilus, with numerous rare genera present in relative abundances below 1%. The use of artificial neural network connected this community to biotic (heterotrophic bacteria, HNA and LNA bacteria, Synechococcus, Prochlorococcus, picoeukaryotes, heterotrophic nanoflagellates, bacterial production) and abiotic environmental factors (temperature, salinity, chlorophyll a and nitrate, nitrite, ammonia, total nitrogen, silicate, and orthophosphate concentration). A type of neural network, neural gas analysis at order-, genus- and ASV-level, resulted in five distinct best matching units (representing particular environments) and revealed that high diversity was generally independent of temperature, salinity, and trophic status of the environment, indicating a potentially dissimilar behaviour of aerobic anoxygenic phototrophs compared to the general bacterioplankton.

CONCLUSION

This research represents the first comprehensive analysis of aerobic anoxygenic phototrophs in the Adriatic Sea on a trophic gradient during a year-round period. This study is also one of the first reports of their genus-level ecology linked to biotic and abiotic environmental factors revealed by unsupervised neural network algorithm, paving the way for further research of substantial contribution of this important bacterial functional group to marine ecosystems.

Analysis of Culturable Bacterial Diversity of Pangong Tso Lake via a 16S rRNA Tag Sequencing Approach

The Pangong Tso lake is a high-altitude freshwater habitat wherein the resident microbes experience unique selective pressures, i.e., high radiation, low nutrient content, desiccation, and temperature extremes. Our study attempts to analyze the diversity of culturable bacteria by applying a high-throughput amplicon sequencing approach based on long read technology to determine the spectrum of bacterial diversity supported by axenic media. The phyla Pseudomonadota, Bacteriodetes, and Actinomycetota were retrieved as the predominant taxa in both water and sediment samples. The genera Hydrogenophaga and Rheinheimera, Pseudomonas, Loktanella, Marinomonas, and Flavobacterium were abundantly present in the sediment and water samples, respectively. Low nutrient conditions supported the growth of taxa within the phyla Bacteriodetes, Actinomycetota, and Cyanobacteria and were biased towards the selection of Pseudomonas, Hydrogenophaga, Bacillus, and Enterococcus spp. Our study recommends that media formulations can be finalized after analyzing culturable diversity through a high-throughput sequencing effort to retrieve maximum species diversity targeting novel/relevant taxa.

Soil Chemistry and Nutrients Influence the Distribution of Aerobic Anoxygenic Phototrophic Bacteria and Eukaryotic Phototrophic Microorganisms of Physical Soil Crusts at Different Elevations on the Tibetan Plateau

Photosynthetic microorganisms are widely distributed in the soil and play an important role in plant-free soil crusts. However, the distribution and environmental drivers of phototrophic microbial communities in physical soil crusts, where the abundance of cyanobacteria is low, are scarcely understood. Here, we performed high-throughput sequencing of pufM and 18S rRNA genes in soil crusts at different elevations on the Tibetan Plateau and used the data combined with environmental variables to analyze the diversity and structure of phototrophic microbial communities. We found that the dominant taxa of aerobic anoxygenic phototrophic bacteria (AAPB) and eukaryotic phototrophic microorganisms (EPM) were shown to shift with elevation. The phototrophic microbial diversity showed a single-peak pattern, with the lowest diversity of AAPB and highest diversity of EPM at middle elevations. Moreover, the elevation and soil property determined the phototrophic microbial community. Soil salts, especially Cl^-, were the most important for AAPB. Likewise, soil nutrients, especially carbon, were the most important for EPM. The relationship between high-abundance taxa and environmental variables showed that Rhizobiales was significantly negatively correlated with salt ions and positively correlated with chlorophyll. Rhodobacterales showed the strongest and significant positive associations with Cl^-. Chlorophyceae and Bacillariophyceae were positively correlated with CO₃^2-. These results indicated that salinity and soil nutrients affected the diversity and structure of microbial communities. This study contributes to our understanding of the diversity, composition, and structure of photosynthetic microorganisms in physical soil crusts and helps in developing new approaches for controlling desertification and salinization and improving the desert ecological environment.

Bacterial Communities Associated with the Biofilms Formed in High-Altitude Brackish Water Pangong Tso Located in the Himalayan Plateau

Pangong Tso is a long and narrow lake situated at an altitude of ~ 4266 m amsl in the Himalayan Plateau on the side of the India/China border. Biofilm has been observed in a small area near the shore of Pangong Tso. Bacterial communities of the lake sediment, water and biofilms were studied using amplicon sequencing of V3-V4 region of the 16S rRNA gene. The standard QIIME pipeline was used for analysis. The metabolic potential of the community was predicted using functional prediction tool Tax4Fun. Bacterial phyla Proteobacteria, followed by Bacteroidetes, Acidobacteria, Planctomycetes, Actinobacteria, and Firmicutes, were found to be dominant across these samples. Shannon's and Simpson's alpha diversity analysis revealed that sediment communities are the most diverse, and water communities are the least diverse. Principal Coordinates based beta diversity analysis showed significant variation in the bacterial communities of the water, sediment and biofilm samples. Bacterial phyla Verrucomicrobia, Deinococcus-Thermus and Cyanobacteria were explicitly enriched in the biofilm samples. Predictive functional profiling of these bacterial communities showed a higher abundance of genes involved in photosynthesis, biosynthesis of secondary metabolites, carbon fixation in photosynthetic organisms and glyoxylate and dicarboxylate metabolism in the biofilm sample. In conclusion, the Pangong Tso bacterial communities are quite similar to other saline and low-temperature lakes in the Tibetan Plateau. Bacterial community structure of the biofilm samples was significantly different from that of the water and sediment samples and enrichment of saprophytic communities was observed in the biofilm samples, indicating an important succession event in this high-altitude lake.

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.

Surviving onshore soil microbial communities differ among the Qing-Tibetan lakes with different salinity

Little is known about the onshore microbial contribution to the microbial communities in nearby lakes and its response to salinity. In this study, transplanting experiments were established by caging onshore soils with dialysis bags followed by in situ 50-day incubation in nearby lakes with different salinity on the Qinghai-Tibetan Plateau. At the end of the experiment, geochemical and microbial analyses were performed on the original soils, caged soils and lake waters and sediments at the incubation sites. The results showed that the salinity increased significantly (P < 0.05) in the caged soils and such salinity increases showed significant (P < 0.05) positive correlation with the salinity of the studied lakes. The microbial community composition and predicted functions in the caged soils were significantly (P < 0.05) changed in comparison with their corresponding original soils, and such variation could be mainly explained by the succession of members of the Proteobacteria, Bacteroidetes and Actinobacteria from the original soils to their corresponding caged soils. The onshore microbial contribution appeared to be limited (up to 11.2% for sediment and negligible for water, respectively) to nearby lake microbial communities. Nevertheless, the survival of onshore soil microbial communities was mainly limited by the salinity of the receiving lakes.

Diel changes and diversity of pufM expression in freshwater communities of anoxygenic phototrophic bacteria

The anoxygenic phototrophic bacteria (APB) are an active component of aquatic microbial communities. While DNA-based studies have delivered a detailed picture of APB diversity, they cannot provide any information on the activity of individual species. Therefore, we focused on the expression of a photosynthetic gene by APB communities in two freshwater lakes (Cep lake and the Římov Reservoir) in the Czech Republic. First, we analyzed expression levels of pufM during the diel cycle using RT-qPCR. The transcription underwent a strong diel cycle and was inhibited during the day in both lakes. Then, we compared DNA- (total) and RNA-based (active) community composition by sequencing pufM amplicon libraries. We observed large differences in expression activity among different APB phylogroups. While the total APB community in the Římov Reservoir was dominated by Betaproteobacteria, Alphaproteobacteria prevailed in the active library. A different situation was encountered in the oligotrophic lake Cep where Betaproteobacteria (order Burkholderiales) dominated both the DNA and RNA libraries. Interestingly, in Cep lake we found smaller amounts of highly active uncultured phototrophic Chloroflexi, as well as phototrophic Gemmatimonadetes. Despite the large diversity of APB communities, light repression of pufM expression seems to be a common feature of all aerobic APB present in the studied lakes.

Aerobic Anoxygenic Photosynthesis Is Commonly Present within the Genus Limnohabitans

The genus Limnohabitans (Comamonadaceae, Betaproteobacteria) is a common and a highly active component of freshwater bacterioplanktonic communities. To date, the genus has been considered to contain only heterotrophic species. In this study, we detected the photosynthesis genes pufLM and bchY in 28 of 46 strains from three Limnohabitans lineages. The pufM sequences obtained are very closely related to environmental pufM sequences detected in various freshwater habitats, indicating the ubiquity and potential importance of photoheterotrophic Limnohabitans in nature. Additionally, we sequenced and analyzed the genomes of 5 potentially photoheterotrophic Limnohabitans strains, to gain further insights into their phototrophic capacity. The structure of the photosynthesis gene cluster turned out to be highly conserved within the genus Limnohabitans and also among all potentially photosynthetic Betaproteobacteria strains. The expression of photosynthetic complexes was detected in a culture of Limnohabitans planktonicus II-D5^T using spectroscopic and pigment analyses. This was further verified by a novel combination of infrared microscopy and fluorescent in situ hybridization.IMPORTANCE The data presented document that the capacity to perform anoxygenic photosynthesis is common among the members of the genus Limnohabitans, indicating that they may have a novel role in freshwater habitats.

Diversity and Distribution of Freshwater Aerobic Anoxygenic Phototrophic Bacteria across a Wide Latitudinal Gradient

Aerobic anoxygenic phototrophs (AAPs) have been shown to exist in numerous marine and brackish environments where they are hypothesized to play important ecological roles. Despite their potential significance, the study of freshwater AAPs is in its infancy and limited to local investigations. Here, we explore the occurrence, diversity and distribution of AAPs in lakes covering a wide latitudinal gradient: Mongolian and German lakes located in temperate regions of Eurasia, tropical Great East African lakes, and polar permanently ice-covered Antarctic lakes. Our results show a widespread distribution of AAPs in lakes with contrasting environmental conditions and confirm that this group is composed of different members of the Alpha- and Betaproteobacteria. While latitude does not seem to strongly influence AAP abundance, clear patterns of community structure and composition along geographic regions were observed as indicated by a strong macro-geographical signal in the taxonomical composition of AAPs. Overall, our results suggest that the distribution patterns of freshwater AAPs are likely driven by a combination of small-scale environmental conditions (specific of each lake and region) and large-scale geographic factors (climatic regions across a latitudinal gradient).

Evolution and adaptation of SAR11 and Cyanobium in a saline Tibetan lake

Lake Qinghai is a unique lacustrine ecosystem located on the Tibetan Plateau and exhibits oligotrophic, alkaline, and saline conditions. Previous studies have focused on the community phylogenetic diversity of bacterioplankton in the ecosystem. This study aimed to address the ecotype diversity of bacterioplankton populations in the unique microbial habitat, using metagenomic sequencing and analysis. Phylogenetic analysis revealed two major bacterial populations: SAR11 IIIa (14% of the total) and Cyanobium (14%). Although the two populations shared high 16S rRNA gene sequence identity (> 98% identity) with their closest marine counterparts, they displayed substantial genomic divergence (≤ 80% average amino acid sequence identity). Comparative genomic analysis identified conservation of carbon and energy storage metabolism (biosynthesis of polyphosphate and polyhydroxyalkanoate) gene operons in the SAR11 IIIa and a cyanate (potential nitrogen source in alkaline conditions) transporter gene operon in the Cyanobium. We further identified genetic signature of positive selection acting on an exodeoxyribonuclease gene of the SAR11 IIIa population, which is potentially associated with DNA repair responsive to strong UV radiation on the high altitude mountain. Taken together, our results revealed the ecosystem-specific gene content of the bacterioplankton populations and provided new insights into their adaptations unique to the Tibetan lake.

Spatial and temporal variability of aerobic anoxygenic photoheterotrophic bacteria along the east coast of Australia

Aerobic Anoxygenic Phototrophic Bacteria (AAnPB) are ecologically important microorganisms, widespread in oceanic photic zones. However, the key environmental drivers underpinning AAnPB abundance and diversity are still largely undefined. The temporal patterns in AAnPB dynamics at three oceanographic reference stations spanning at approximately 15° latitude along the Australian east coast were examined. AAnPB abundance was highly variable, with pufM gene copies ranging from 1.1 × 10² to 1.4 × 10⁵ ml^-1 and positively correlated with day length and solar radiation. pufM gene Miseq sequencing revealed that the majority of sequences were closely related to those obtained previously, suggesting that key AAnPB groups are widely distributed across similar environments globally. Temperature was a major structuring factor for AAnPB assemblages across large spatial scales, correlating positively with richness and Gammaproteobacteria (phylogroup K) abundance but negatively with Roseobacter-clade (phylogroup E) abundance, with temperatures between 16°C and 18°C identified as a potential transition zone between these groups. Network analysis revealed that discrete AAnPB populations exploit specific niches defined by varying temperature, light and nutrient conditions in the Tasman Sea system, with evidence for both niche sharing and partitioning amongst closely related operational taxonomic units.

Analysis of cbbL, nifH, and pufLM in Soils from the Sør Rondane Mountains, Antarctica, Reveals a Large Diversity of Autotrophic and Phototrophic Bacteria

Cyanobacteria are generally thought to be responsible for primary production and nitrogen fixation in the microbial communities that dominate Antarctic ecosystems. Recent studies of bacterial communities in terrestrial Antarctica, however, have shown that Cyanobacteria are sometimes only scarcely present, suggesting that other bacteria presumably take over their role as primary producers and diazotrophs. The diversity of key genes in these processes was studied in surface samples from the Sør Rondane Mountains, Dronning Maud Land, using clone libraries of the large subunit of ribulose-1,5-biphosphate carboxylase/oxygenase (RuBisCO) genes (cbbL, cbbM) and dinitrogenase-reductase (nifH) genes. We recovered a large diversity of non-cyanobacterial cbbL type IC in addition to cyanobacterial type IB, suggesting that non-cyanobacterial autotrophs may contribute to primary production. The nifH diversity recovered was predominantly related to Cyanobacteria, particularly members of the Nostocales. We also investigated the occurrence of proteorhodopsin and anoxygenic phototrophy as mechanisms for non-Cyanobacteria to exploit solar energy. While proteorhodopsin genes were not detected, a large diversity of genes coding for the light and medium subunits of the type 2 phototrophic reaction center (pufLM) was observed, suggesting for the first time, that the aerobic photoheterotrophic lifestyle may be important in oligotrophic high-altitude ice-free terrestrial Antarctic habitats.

Taxonomic and functional diversity provides insight into microbial pathways and stress responses in the saline Qinghai Lake, China

Microbe-mediated biogeochemical cycles contribute to the global climate system and have sensitive responses and feedbacks to environmental stress caused by climate change. Yet, little is known about the effects of microbial biodiversity (i.e., taxonmic and functional diversity) on biogeochemical cycles in ecosytems that are highly sensitive to climate change. One such sensitive ecosystem is Qinghai Lake, a high-elevation (3196 m) saline (1.4%) lake located on the Tibetan Plateau, China. This study provides baseline information on the microbial taxonomic and functional diversity as well as the associated stress response genes. Illumina metagenomic and metatranscriptomic datasets were generated from lake water samples collected at two sites (B and E). Autotrophic Cyanobacteria dominated the DNA samples, while heterotrophic Proteobacteria dominated the RNA samples at both sites. Photoheterotrophic Loktanella was also present at both sites. Photosystem II was the most active pathway at site B; while, oxidative phosphorylation was most active at site E. Organisms that expressed photosystem II or oxidative phosphorylation also expressed genes involved in photoprotection and oxidative stress, respectively. Assimilatory pathways associated with the nitrogen cycle were dominant at both sites. Results also indicate a positive relationship between functional diversity and the number of stress response genes. This study provides insight into the stress resilience of microbial metabolic pathways supported by greater taxonomic diversity, which may affect the microbial community response to climate change.

Environmental controls and composition of anoxygenic photoheterotrophs in ultraoligotrophic high-altitude lakes (Central Pyrenees)

The phylogenetic composition of freshwater anoxygenic photoheterotrophs (APs) has been poorly investigated as compared with their marine counterparts. In this study, we explored a set of ultraoligotrophic cold high mountain lakes (Central Pyrenees, Spain) by both pufM gene denaturing gradient gel electrophoresis fingerprinting, and cloning and sequencing of selected lakes samples. Different ranges of limnological and physico-chemical values were explored as environmental drivers of APs richness and composition. We did not observe significant relationships between richness/diversity of pufM and any of the limnological characteristics measured or trophic status, but a negative correlation with ammonia concentration. Conductivity, pH and nitrate concentration were significantly related to changes in APs community composition, whereas lake area, altitude, temperature and trophic status did not. Most of the sequences (> 85%) had the pufM sequences of Limnohabitans (Betaproteobacteria) as the closest relative in databases, whereas less abundant clones were more closely related to Rhodobacter, Sulfitobacter and Brevundimonas (Alphaproteobacteria), in agreement with 16S rRNA gene sequences previously found in the area. Congregibacter-like Gammaproteobacteria were not detected. Comparison with available studies in inland waters showed taxonomic partitioning along salinity gradients, and Congregibacter-like sequences restricted to high saline conditions in continental water bodies.

Light enhances survival of Dinoroseobacter shibae during long-term starvation

Aerobic anoxygenic phototrophs (AAPs) as being photoheterotrophs require organic substrates for growth and use light as a supplementary energy source under oxic conditions. We hypothesized that AAPs benefit from light particularly under carbon and electron donor limitation. The effect of light was determined in long-term starvation experiments with Dinoroseobacter shibae DFL 12^T in both complex marine broth and defined minimal medium with succinate as the sole carbon source. The cells were starved over six months under three conditions: continuous darkness (DD), continuous light (LL), and light/dark cycle (LD, 12 h/12 h, 12 µmol photons m⁻² s⁻¹). LD starvation at low light intensity resulted in 10-fold higher total cell and viable counts, and higher bacteriochlorophyll a and polyhydroxyalkanoate contents. This coincided with better physiological fitness as determined by respiration rates, proton translocation and ATP concentrations. In contrast, LD starvation at high light intensity (>22 µmol photons m⁻² s⁻¹, LD conditions) resulted in decreasing cell survival rates but increasing carotenoid concentrations, indicating a photo-protective response. Cells grown in complex medium survived longer starvation (more than 20 weeks) than those grown in minimal medium. Our experiments show that D. shibae benefits from the light and dark cycle, particularly during starvation.

Recent advances and future perspectives in microbial phototrophy in antarctic sea ice

Bacteria that utilize sunlight to supplement metabolic activity are now being described in a range of ecosystems. While it is likely that phototrophy provides an important competitive advantage, the contribution that these microorganisms make to the bioenergetics of polar marine ecosystems is unknown. In this minireview, we discuss recent advances in our understanding of phototrophic bacteria and highlight the need for future research.

Bacterial anoxygenic photosynthesis on plant leaf surfaces

The aerial surface of plants, the phyllosphere, is colonized by numerous bacteria displaying diverse metabolic properties that enable their survival in this specific habitat. Recently, we reported on the presence of microbial rhodopsin harbouring bacteria on the top of leaf surfaces. Here, we report on the presence of additional bacterial populations capable of harvesting light as a means of supplementing their metabolic requirements. An analysis of six phyllosphere metagenomes revealed the presence of a diverse community of anoxygenic phototrophic bacteria, including the previously reported methylobacteria, as well as other known and unknown phototrophs. The presence of anoxygenic phototrophic bacteria was also confirmed in situ by infrared epifluorescence microscopy. The microscopic enumeration correlated with estimates based on metagenomic analyses, confirming both the presence and high abundance of these microorganisms in the phyllosphere. Our data suggest that the phyllosphere contains a phylogenetically diverse assemblage of phototrophic species, including some yet undescribed bacterial clades that appear to be phyllosphere-unique.

Aerobic anoxygenic phototrophic bacteria in Antarctic sea ice and seawater

Aerobic anoxygenic phototrophs are obligate aerobes with unusually high concentrations of carotenoids, low cellular contents of bacteriochlorophyll-a and they lack light-harvesting complex II. In this study, sea ice and seawater samples were collected from six different sites in the Ross Sea, Antarctica. Using a combination of primers for pufM (which encodes a pigment-binding protein subunit of the reaction centre complex), clone libraries of DNA and cDNA were created and a total of 63 positive clones were obtained from three sites, all clustering within the α-Proteobacteria. Fifty-three of these clones were from seawater. The remaining clones were from sea ice and all were found in the middle and bottom sections of the ice. These sea ice bacteria may favour the lower part of the ice matrix where irradiance is low. This report highlights the first findings of AAnPs in antarctic sea ice and seawater within the Ross Sea Region.

A new extreme environment for aerobic anoxygenic phototrophs: biological soil crusts

Biological soil crusts improve the health of arid or semiarid soils by enhancing water content, nutrient relations and mechanical stability, facilitated largely by phototrophic microorganisms. Until recently, only oxygenic phototrophs were known from soil crusts. A recent study has demonstrated the presence of aerobic representatives of Earth's second major photosynthetic clade, the evolutionarily basal anoxygenic phototrophs. Three Canadian soil crust communities yielded pink and orange aerobic anoxygenic phototrophic strains possessing the light-harvesting pigment bacteriochlorophyll a. At relative abundances of 0.1-5.9% of the cultivable bacterial community, they were comparable in density to aerobic phototrophs in other documented habitats. 16S rDNA sequence analysis revealed the isolates to be related to Methylobacterium, Belnapia, Muricoccus and Sphingomonas. This result adds a new type of harsh habitat, dry soil environments, to the environments known to support aerobic anoxygenic phototrophs.

Planktonic actinobacterial diversity along a salinity gradient of a river and five lakes on the Tibetan Plateau

The diversity and community structure of planktonic Actinobacteria in a freshwater river and five fresh/saline/hypersaline lakes on the Tibetan Plateau, China were investigated with a combination of geochemical and 16S rRNA gene phylogenetic analyses. A total of 387 actinobacterial 16S rRNA gene clones were sequenced, and they could be classified into Actinobacteridae, Acidimicrobidae, and unclassified Actinobacteria. The Actinobacteridae sequences were distributed into five suborders (e.g., Corynebacterineae, Frankineae, Micrococcineae, Propionibacterineae, and Streptosporangineae) and unclassified Actinobacteridae. Some actinobacterial members (specifically Micrococcineae) were present in a wide range of salinities (from freshwater to NaCl saturation). Statistical analysis showed that salinity and salinity-related environmental variables (such as ions and total nitrogen) significantly (r > 0.5; P < 0.05) influenced the distribution of planktonic actinobacterial community in the investigated aquatic biotopes. Our data have implications for a better understanding of the distribution of Actinobacteria in high-elevation lakes.