Crossing Treeline: Bacterioplankton Communities of Alpine and Subalpine Rocky Mountain Lakes

From the aboveground vegetation to the belowground microbes, terrestrial communities differ between the highly divergent alpine (above treeline) and subalpine (below treeline) ecosystems. Yet, much less is known about the partitioning of microbial communities between alpine and subalpine lakes. Our goal was to determine whether the composition of bacterioplankton communities of high-elevation mountain lakes differed across treeline, identify key players in driving the community composition, and identify potential environmental factors that may be driving differences. To do so, we compared bacterial community composition (using 16S rDNA sequencing) of alpine and subalpine lakes in the Southern Rocky Mountain ecoregion at two time points: once in the early summer and once in the late summer. In the early summer (July), shortly after peak runoff, bacterial communities of alpine lakes were distinct from subalpine lakes. Interestingly, by the end of the summer (approximately 5 weeks after the first visit in August), bacterial communities of alpine and subalpine lakes were no longer distinct. Several bacterial amplicon sequence variants (ASVs) were also identified as key players by significantly contributing to the community dissimilarity. The community divergence across treeline found in the early summer was correlated with several environmental factors, including dissolved organic carbon (DOC), pH, chlorophyll-a (chl-a), and total dissolved nitrogen (TDN). In this paper, we offer several potential scenarios driven by both biotic and abiotic factors that could lead to the observed patterns. While the mechanisms for these patterns are yet to be determined, the community dissimilarity in the early summer correlates with the timing of increased hydrologic connections with the terrestrial environment. Springtime snowmelt brings the flushing of mountain watersheds that connects terrestrial and aquatic ecosystems. This connectivity declines precipitously throughout the summer after snowmelt is complete. Regional climate change is predicted to bring alterations to precipitation and snowpack, which can modify the flushing of solutes, nutrients, and terrestrial microbes into lakes. Future preservation of the unique alpine lake ecosystem is dependent on a better understanding of ecosystem partitioning across treeline and careful consideration of terrestrial-aquatic connections in mountain watersheds.

Tropical bee species abundance differs within a narrow elevational gradient

Insect pollination is among the most essential ecosystem services for humanity. Globally, bees are the most effective pollinators, and tropical bees are also important for maintaining tropical biodiversity. Despite their invaluable pollination service, basic distributional patterns of tropical bees along elevation gradients are globally scarce. Here, we surveyed bees at 100 m elevation intervals from 800 to 1100 m elevation in Costa Rica to test if bee abundance, community composition and crop visitor assemblages differed by elevation. We found that 18 of 24 bee species spanning three tribes that represented the most abundantly collected bee species showed abundance differences by elevation, even within this narrow elevational gradient. Bee assemblages at the two crop species tested, avocado and squash, showed community dissimilarity between high and low elevations, and elevation was a significant factor in explaining bee community composition along the gradient. Stingless bees (Tribe Meliponini) were important visitors to both crop species, but there was a more diverse assemblage of bees visiting avocado compared to squash. Our findings suggest that successful conservation of tropical montane bee communities and pollination services will require knowledge of which elevations support the highest numbers of each species, rather than species full altitudinal ranges.

Diversity and Metabolic Potential of Microbial Communities in a Serpentinite-Hosted Spring at the Tablelands

The geochemical process of serpentinization releases energy and organic carbon: two of the basic requirements needed to support life. Sites of active serpentinization in the deep subsurface provide the intriguing possibility of a non-photosynthetically-supported biosphere. However, serpentinization also creates conditions, such as high pH and limited electron acceptors, which may limit microbial growth and diversity. Gaining an understanding of the identity and metabolic potential of microbes that thrive in these environments may provide insight as to whether serpentinization is sufficient to independently support life. Tablelands Ophiolite in Gros Morne National Park, Newfoundland, Canada is a continental site of serpentinization where serpentinite springs form surface pools. These pools provide easy sampling access to subsurface fluids and may allow for sampling of the subsurface microbial community. However, identification of members of the subsurface community in these pools is complicated by both surface contamination and contamination by organisms that inhabit the transition zone where hydrogen-rich subsurface fluids meet oxygen-rich surface fluids. This study was designed to distinguish among these potential sources of microorganisms by using a sampling technique that more effectively samples subsurface fluids. Community dissimilarity comparisons using environmental 16S rRNA gene sequencing indicate that the sampling design led to more direct access to subsurface fluids. These results are supported by metagenomic analyses that show metabolic pathways consistent with non-photosynthetic carbon fixation in the samples expected to represent subsurface fluids and that show hydrogen oxidation pathways in samples associated with the surface sources. These results provide a clearer picture of the diversity and metabolic potential of microbial communities potentially inhabiting subsurface, serpentinite-hosted habitats.

Phylosymbiotic Structures of the Microbiota in Mollitrichosiphum tenuicorpus (Hemiptera: Aphididae: Greenideinae)

Aphids harbor an array of symbionts that provide hosts with ecological benefits. Microbial community assembly generally varies with respect to aphid species, geography, and host plants. However, the influence of host genetics and ecological factors on shaping intraspecific microbial community structures has not been fully understood. In the present study, using Illumina sequencing of the V3 − V4 hypervariable region of the 16S rRNA gene, we characterized the microbial compositions associated with Mollitrichosiphum tenuicorpus from different regions and plants in China. The primary symbiont Buchnera aphidicola and the secondary symbiont Arsenophonus dominated the microbial flora in M. tenuicorpus. Ordination analyses and statistical tests suggested that geography and aphid genetics primarily contributed to the variation in the microbiota of M. tenuicorpus. We further confirmed the combined effect of aphid genetics and geography on shaping the structures of symbiont and secondary symbiont communities. Moreover, the significant correlation between aphid genetic divergence and symbiont community dissimilarity provides evidence for intraspecific phylosymbiosis in natural systems. Our study helped to elucidate the eco-evolutionary relationship between symbiont communities and aphids within one given species.

Nitrogen Addition and Arbuscular Mycorrhizal Fungi Beta Diversity: Patterns and Mechanisms

Global nitrogen eutrophication, which is disrupting the intimate plant–arbuscular mycorrhizal fungi (AMF) symbiosis, can alter the diversity and physiological functions of soil AMF greatly. However, shifts of beta diversity and the intrinsic patterns of AMF community dissimilarities in response to nitrogen addition remain unclear. Based on a 7-year nitrogen addition experiment in a Qinghai–Tibet Plateau alpine meadow, we detected the changes in soil AMF alpha diversity (richness and genus abundance) and the community composition beta diversity by partitioning the two components of Simpson and nestedness dissimilarities along (turnover) and within (variation) nitrogen addition treatments, and fitted with environmental factor dissimilarities. We found that nitrogen addition decreased AMF richness by decreasing the most dominant AMF genus of Glomus but increasing the abundance of the rare genera. The turnover of the AMF community overall beta diversity along the nitrogen addition gradients was induced by the increased nestedness dissimilarity, while the variation within treatments was explained by both increased Simpson and nestedness dissimilarities, which was significantly correlated with soil pH. Our study found both Simpson and nestedness dissimilarities worked on the AMF community dissimilarity after nitrogen addition and the significant variation within the same treatment, which would be important in the future for predicting global AMF or microbial diversity changes in response to nitrogen eutrophication.

Host specific endophytic microbiome diversity and associated functions in three varieties of scented black rice are dependent on growth stage

The compositional and functional role of the endophytic bacterial community, associated with black scented rice, in correlation with its antioxidant property has been elucidated. Community dissimilarity analysis confirmed the overlapping of community in shoot and root tissues at the young stage, but not in mature plants. Proteobacteria was the most abundant phylum, in which Agrobacterium, Pleomorphomonas, Bradyrhizobium, Novasphingobium, Caulobacter were the most abundant genera, followed by Cyanobacteria and Planctomycetes in all three different varieties of the black rice. The antioxidant activity of mature plants was found to be higher in comparison to young plants. Intrinsically, the relative abundance of Pleomorphomonas and Streptomyces was positively correlated with total phenol content, while Gemmata, unclassified Pirellulaceae, unclassified Stramenopiles positively correlated with total flavonoid content and negatively correlated with Free radical scavenging activity. Accordingly, functional metagenome analysis of the endophytic microbiome revealed that naringenin -3-dioxygenase and anthocyanidin 3-O-glucosyltransferase for phenylpropanoid (flavonoid and anthocyanin) synthesis were abundant in the endophytic microbiome of mature plants. Specific enrichment of the antioxidant producing genes in the mature plant endophytic microbiome was assigned to some bacteria such as Streptomyces, Pantoea which might have contributed to the common pathway of flavonoid synthesis. The genomes of endophytic isolates Kluyvera sp.PO2S7, Bacillus subtilis AMR1 and Enterobacter sp. SES19 were sequenced and annotated, and were found to have genes for phenylpropanoid synthesis in their genomes.

Investigating variability in microbial community composition in replicate environmental DNA samples down lake sediment cores

Lake sediments are natural archives that accumulate information on biological communities and their surrounding catchments. Paleolimnology has traditionally focussed on identifying fossilized organisms to reconstruct past environments. In the last decade, the application of molecular methodologies has increased in paleolimnological studies, but further research investigating factors such as sample heterogeneity and DNA degradation are required. In the present study we investigated bacterial community heterogeneity (16S rRNA metabarcoding) within depth slices (1-cm width). Sediment cores were collected from three lakes with differing sediment compositions. Samples were collected from a variety of depths which represent a period of time of approximately 1,200 years. Triplicate samples were collected from each depth slice and bacterial 16S rRNA metabarcoding was undertaken on each sample. Accumulation curves demonstrated that except for the deepest (oldest) slices, the combination of three replicate samples were insufficient to characterise the entire bacterial diversity. However, shared Amplicon Sequence Variants (ASVs) accounted for the majority of the reads in each depth slice (max. shared proportional read abundance 96%, 86%, 65% in the three lakes). Replicates within a depth slice generally clustered together in the Non-metric multidimensional scaling analysis. There was high community dissimilarity in older sediment in one of the cores, which was likely due to the laminae in the sediment core not being horizontal. Given that most paleolimnology studies explore broad scale shifts in community structure rather than seeking to identify rare species, this study demonstrates that a single sample is adequate to characterise shifts in dominant bacterial ASVs.

Environmental DNA variability in lake sediment cores

Lake sediments are natural archives that accumulate information about biological communities and their surrounding catchments. Paleolimnology has traditionally focussed on identifying fossilized organisms to reconstruct past environments. In the last decade, the application of molecular methodologies has increased in paleolimnological studies, but further studies investigating factors such as sample heterogeneity and DNA degradation are required. Here we investigated bacterial community heterogeneity (16S rRNA metabarcoding) within depth slices. Sediment cores were collected from three lakes with differing sediment compositions. Samples were collected from a variety of depths (1-cm width) which represent a period of time of approximately 1,200 years. Triplicate samples were collected from each slice and bacterial 16S rRNA metabarcoding was undertaken on each sample. Rarefaction curves showed that except for the deepest (oldest) slices, the combination of three replicate samples were insufficient to characterise the entire bacterial diversity. However, shared Amplicon Sequence Variants (ASVs) accounted for the majority of the reads in each slice (max. shared proportional read abundance 96%, 86%, 65% in the three lakes). Within slice similarity was higher than between slice similarity. No general trend was observed in variability among replicates with depth amongst the lakes. In one core. there was a higher community dissimilarity in older sediment, which may be due to laminae not being horizontal. These results highlight the fact that microbial communities can be differentiated with depth however it is critical to interpret these results in the context of the stratigraphic data of the core.