Affect Community Composition Research Articles

Do disturbed environments affect density of the tunnel-web spider Acanthogonatus centralis (Mygalomorphae: Nemesiidae) from native grasslands in Argentina?

Disturbance is an important factor affecting community composition and biodiversity in natural ecosystems. The Ventania hill system in central Argentina presents several ecosystem disturbances. Spiders are good candidates for studies related to human impact and conservation, as they can be good biological indicators of certain ecosystems. Therefore, we tried to elucidate if the abundance and spatial distribution of the mygalomorph spider Acanthogonatus centralis (Nemesiidae) are affected by different degrees of disturbance in these hilly grasslands. We studied 3 sites with different levels of disturbance located in the Ventania system in southwestern Buenos Aires Province, Argentina. We did not find differences in the mean density of A. centralis among sites. The highest density of this spider species was found in the site with the highest level of disturbance. In addition, the spatial arrangement indicated an aggregate distribution. In conclusion, we found that A. centralis populations are not affected by different levels of disturbances.

Lactulose Crystals Beneficially Affect Community Composition Along Entire Human Colon in Vitro, Resulting in Donor-Dependent Prebiotic Effects at Metabolic Level

A validated <i>in vitro</i> gut model (i.e. SHIME®) was used to assess the effect of repeated daily administration of lactulose on microbial metabolic activity and community composition in different colonic areas, with the focus on inter-individual differences among three human subjects. An initial rise in acetate and lactate levels was observed in the proximal colon after lactulose administration, which could be linked to an overall strong bifidogenic effect as well as higher Lactobacilli levels in donors 2 and 3. Particularly two operational taxonomic units (OTUs) related to <i>Bifidobacterium adolescentis</i> and <i>Bifidobacterium longum</i> increased with lactulose addition. The enhanced acetate and lactate production subsequently stimulated microbial species involved in cross-feeding interactions, resulting in the donor-dependent production of propionate and/or butyrate. Additionally, a reduction in markers of proteolytic fermentation was detected upon lactulose supplementation. A wide spectrum of propionate- and especially butyrate-producing microbes, such as the next-generation probiotics <i>Faecalibacterium prausnitzii</i> and <i>Akkermansia muciniphila</i>, were donor-dependently enhanced in the distal colon, which is of specific interest as many colonic diseases originate in the distal part of the colon. For the first time beneficial effects of lactulose on the microbiota as well as metabolic activity could be demonstrated over the entire colon <i>in vitro</i>.

Assessing the effects of hydrological and chemical stressors on macroinvertebrate community in an Alpine river: The Adige River as a case study

AbstractIn this study, the combined effects of hydrological and chemical stressors on benthic macroinvertebrates were evaluated in order to explore the response of the biological community to multiple stressors. The Adige River, located in the south‐eastern Alps, was selected as a case study because representative of the situation of a large river in which the variety of stressors present in the Alpine region act simultaneously. As expected, streamflow showed a seasonal pattern, with high flows in the spring–summer period; however, locally, the natural hydrological regime was altered by the presence of hydropower systems, which chiefly affected low flows. Multivariate analysis showed seasonal and spatial patterns in both chemical and hydrological parameters with a clear gradient in the concentration of nitrate, personal care, and pharmaceutical products moving from headwaters to the main stem of the river. The macroinvertebrate community composition was significantly different in summer and winter and between up and downstream sites. Streamflow alteration chiefly due to water use by hydropower affected community composition but not richness or diversity. Gammarus sp., Hirudinea, and Psychomyia sp., were positively correlated with flow variability, increasing their densities in the sites with higher streamflow variability because of hydropeaking. The results obtained in this study show that the composition of the macroinvertebrate community responded to seasonality and to changes in the main stressors along the river and highlights the importance of the spatial and temporal variability of stressors in this Alpine river. Taking into account, this variability will help the decision‐making process for improving basin management.

Effects of past and current drought on the composition and diversity of soil microbial communities

Drought is well known to have strong effects on the composition and activity of soil microbial communities, and may be determined by drought history and drought duration, but the characterisation and prediction of these effects remains challenging. This is because soil microbial communities that have previously been exposed to drought may change less in response to subsequent drought events, due to the selection of drought-resistant taxa. We set up a 10-level drought experiment to test the effect of water stress on the composition and diversity of soil bacterial and fungal communities. We also investigated the effect of a previous long-term drought on communities in soils with different historical precipitation regimes. Saplings of the holm oak, Quercus ilex L., were included to assess the impact of plant presence on the effects of the drought treatment. The composition and diversity of the soil microbial communities were analysed using DNA amplicon sequencing of bacterial and fungal markers and the measurement of phospholipid fatty acids.The experimental drought affected the bacterial community much more than the fungal community, decreasing alpha diversity and proportion of total biomass, whereas fungal diversity tended to increase. The experimental drought altered the relative abundances of specific taxa of both bacteria and fungi, and in many cases these effects were modified by the presence of the plant and soil origin. Soils with a history of drought had higher overall bacterial alpha diversity at the end of the experimental drought, presumably because of adaptation of the bacterial community to drought conditions. However, some bacterial taxa (e.g. Chloroflexi) and fungal functional groups (plant pathogens and saprotrophic yeasts) decreased in abundance more in the pre-droughted soils.Our results suggest that soil communities will not necessarily be able to maintain the same functions during more extreme or more frequent future droughts, when functions are influenced by community composition. Drought is likely to continue to affect community composition, even in soils that are acclimated to it, tending to increase the proportion of fungi and reduce the proportion and diversity of bacteria.

Amphibians in agricultural landscapes: the habitat value of crop areas, linear plantings and remnant woodland patches

AbstractMitigating the negative impacts of agriculture on amphibians requires knowledge of how different land uses affect species distribution and community composition. In the case of frogs, there is currently insufficient information on their use of terrestrial habitats in cropping landscapes to inform conservation planning. We examined how four different farmland types (linear plantings, cereal crops, grazing paddocks and woody mulch) and crop harvesting influenced amphibian abundance, richness, body condition and movement. We found the abundance of frogs was significantly higher in linear plantings compared to grazing paddocks and adjacent patches of remnant woodland vegetation. However, species richness and abundance of three individual species did not vary significantly between farmland types. For the most common frog Uperoleia laevigata, body condition was higher at the edges of the woody debris treatment (coupled with higher abundance) and lower in farmland with debris and linear plantings. The body condition of Limnodynastes tasmaniensis and L. interioris was not influenced by farmland type. Frog abundance and condition was largely unaffected by crop harvesting. However, frogs were less common after harvesting at the edges of farmland and within remnant patches. Movement patterns did not suggest mass movement out of crops after harvest, where almost half of all individuals recaptured remained within the farmland. These results suggest that some generalist frog species may have an affinity for habitats within agricultural paddocks, particularly when key habitat features like plantings are present. However, we found overall frog richness was low and did not differ between remnant patches, edges and farmland which may be an indication of habitat degradation within terrestrial habitats across the landscape. Although protection of remnant native vegetation is important, conservation strategies for the protection of amphibians will be ineffective if they do not consider the variety of land uses and the relationships of different species and their microhabitats within and outside of patches.

Variation in Tree Community Composition and Carbon Stock under Natural and Human Disturbances in Andean Forests, Peru

Deforestation and forest degradation in Andean forests is influenced by natural and social environments including a wide elevation range and anthropogenic disturbance. Tree community composition is receiving attention as a key indicator of forest degradation. However, difference in factors affecting community composition at different elevation zones remains unclear. We aimed at elucidating factors (natural and human disturbances, and forest characteristics) that influence the variations in community composition in Andean forests. We conducted a ground-based survey setting 45 plots across a wide elevation range (ca. 600 to 3500 m a.s.l.) in Cusco region, Peru. Above ground biomass (AGB) decreased with increasing elevation. The generalized linear models for multivariate abundance data suggested that a factor affecting community composition was natural disturbance (erosion) at low elevation (&lt;1000 m), while human disturbance (infrastructure such as sheds and trails) at high elevation (≥2400 m). Within each of the different elevation zones, the AGB affected community composition only at mid elevation (1000–2400 m), whereas mean tree height showed a consistent effect on community composition across the three elevations. Our results suggest that the effects of human disturbance on community composition were more prominent at higher elevation. The results also suggest that mean tree height may have a potential to be a key measure for evaluating variations in community composition in Andean forests.

Responses of arbuscular mycorrhizal fungi to long-term inorganic and organic nutrient addition in a lowland tropical forest.

Improved understanding of the nutritional ecology of arbuscular mycorrhizal (AM) fungi is important in understanding how tropical forests maintain high productivity on low-fertility soils. Relatively little is known about how AM fungi will respond to changes in nutrient inputs in tropical forests, which hampers our ability to assess how forest productivity will be influenced by anthropogenic change. Here we assessed the influence of long-term inorganic and organic nutrient additions and nutrient depletion on AM fungi, using two adjacent experiments in a lowland tropical forest in Panama. We characterised AM fungal communities in soil and roots using 454-pyrosequencing, and quantified AM fungal abundance using microscopy and a lipid biomarker. Phosphorus and nitrogen addition reduced the abundance of AM fungi to a similar extent, but affected community composition in different ways. Nutrient depletion(removal of leaf litter) had a pronounced effect on AM fungal community composition, affecting nearly as many OTUs as phosphorus addition. The addition of nutrients in organic form (leaf litter) had little effect on any AM fungal parameter. Soil AM fungal communities responded more strongly to changes in nutrient availability than communities in roots. This suggests that the 'dual niches' of AM fungi in soil versus roots are structured to different degrees by abiotic environmental filters, and biotic filters imposed by the plant host. Our findings indicate that AM fungal communities are fine-tuned to nutrient regimes, and support future studies aiming to link AM fungal community dynamics with ecosystem function.

Community consequences of foraging under fear

Non-consumptive effects of predators within ecosystems can alter the behavior of individual prey species, and have cascading effects on other trophic levels. In this context, an understanding of non-consumptive predator effects on the whole prey community is crucial for predicting community structure and composition, hence biodiversity patterns. We used an individual-based, spatially-explicit modelling approach to investigate the consequences of landscapes of fear on prey community metrics. The model spans multiple hierarchical levels from individual home range formation based on food availability and perceived predation risk to consequences on prey community structure and composition. This mechanistic approach allowed us to explore how important factors such as refuge availability and foraging strategy under fear affect prey community metrics. Fear of predators affected prey space use, such as home range formation. These adaptations had broader consequences for the community leading to changes in community structure and composition. The strength of community responses to perceived predation risk was driven by refuge availability in the landscape and the foraging strategy of prey animals. Low refuge availability in the landscape strongly decreased diversity and total biomass of prey communities. Additionally, body mass distributions in prey communities facing high predation risk were shifted towards small prey animals. With increasing refuge availability the consequences of non-consumptive predator effects were reduced, diversity and total biomass of the prey community increased. Prey foraging strategies affected community composition. Under medium refuge availability, risk-averse prey communities consisted of many small animals while risk-taking prey communities showed a more even body mass distribution. Our findings reveal that non-consumptive predator effects can have important implications for prey community diversity and should therefore be considered in the context of conservation and nature management.

Disentangling the effects of multiple environmental drivers on population changes within communities.

The effects of different environmental drivers on the changes in species' population abundances can be difficult to disentangle as they often act simultaneously. Researchers have built statistical models that include environmental variables (such as annual temperature) or species attributes (such as a species' temperature preference), which are assumed to detect the impacts of specific drivers (such as climate change). However, these approaches are often applied separately or, if combined, not explicitly compared. We show the complementary insights gained by applying both these approaches to a community dataset on Danish terrestrial birds. We use our analysis to compare the relative importance of climate change and agricultural land-use change for the abundance changes within the community between 1983 and 2013. Population models were fitted to the community data of species' annual abundances with predictors comprising: species attributes (species' temperature and habitat preferences), environmental variables (climatic and agricultural land-use change variables) or both. Relationships between species' abundances and environmental variables were used to identify the drivers associated with average abundance changes of species in the community. Relationships between species' abundances and their attributes were used to understand the drivers causing interspecific variation in abundance changes. Warmer winters were positively associated with community-level abundances, and warm-adapted species had more positive abundance changes than cold-adapted ones. Agricultural land-use area was negatively associated with community-level abundances, and birds using a high proportion of meadow and habitat specialists had more negative abundance changes than birds using other habitats and habitat generalists. Effect sizes of environmental variables were larger for agricultural land-use change while those of species attributes were larger for climate change. The environmental data approach suggested that agricultural land-use change has decreased the average abundances of species in the community, affecting total community size while the species attribute-based approach suggested that climate change has caused variation in abundance among species, affecting community composition. Environmental variables and species attributes that are hypothesized to link to specific drivers can be used together to provide complementary information on the impacts of different drivers on communities.

Host species effects on bacterial communities associated with the fruiting bodies of Tuber species from the Sichuan Province in Southwest China

Tuber species produce highly sought-after truffles and host a wide diversity and high abundance of bacteria. It has been suggested that some of these bacteria contribute to the growth, maturity, and aromatic properties of truffles. Here, we characterized and compared the microbiomes of several species of truffles from the Sichuan Province in Southwest China using high-throughput sequencing of bacterial community 16S rRNA genes. Two T. pseudoexcavatum ascocarp samples had relatively similar bacterial communities, as indicated by PCoA analysis. In contrast, three T. indicum samples collected at different maturity stages did not contain similar communities, suggesting that the maturity stage of ascocarps affects community composition in addition to host phylogenetic background. Despite the variation seen among species and maturity stages, the Proteobacteria phylum dominated all communities, which is consistent with previous studies of Tuber-associated bacteria. Moreover, Bradyrhizobium was the dominant genus in most Tuber ascocarps, which is also consistent with previous studies, although the functional role of this genus within truffles is unclear. Notably, Serratia, which are essential producers of thiophene volatiles within T. borchii were dominant in all of our samples. This finding supports the hypothesis that the ability to produce thiophene volatiles is widespread among these bacteria.

Competition for nutrients and light: testing advances in resource competition with a natural phytoplankton community.

A key challenge in ecology is to understand how nutrients and light affect the biodiversity and community structure of phytoplankton and plant communities. According to resource competition models, ratios of limiting nutrients are major determinants of species composition. At high nutrient levels, however, species interactions may shift to competition for light, which might make nutrient ratios less relevant. The "nutrient-load hypothesis" merges these two perspectives, by extending the classic model of competition for two nutrients to include competition for light. Here, we test five key predictions of the nutrient-load hypothesis using multispecies competition experiments. A marine phytoplankton community sampled from the North Sea was inoculated in laboratory chemostats provided with different nitrogen (N) and phosphorus (P) loads to induce either single resource limitation or co-limitation of N, P, and light. Four of the five predictions were validated by the experiments. In particular, different resource limitations favored the dominance of different species. Increasing nutrient loads caused changes in phytoplankton species composition, even if the N:P ratio of the nutrient loads remained constant, by shifting the species interactions from competition for nutrients to competition for light. In all treatments, small species became dominant whereas larger species were competitively excluded, supporting the common view that small cell size provides a competitive advantage under resource-limited conditions. Contrary to expectation, all treatments led to coexistence of diatoms, cyanobacteria and green algae, resulting in a higher diversity of species than predicted by theory. Because the coexisting species comprised three phyla with different photosynthetic pigments, we speculate that niche differentiation in the light spectrum might play a role. Our results show that mechanistic resource competition models that integrate nutrient-based and light-based approaches provide an important step forward to understand and predict how changing nutrient loads affect community composition.

Stable germination behavior but partly changing seed–seed interactions along a steep rainfall gradient

Germination marks a critical transition in plant life that is prone to high mortality. Strong selection pressure is therefore expected to finely tune it to environmental conditions. Our study on the common Mediterranean grass Brachypodium hybridum assessed whether germination behavior changes systematically along a steep natural rainfall gradient ranging from harsh desert to rather mild mesic-Mediterranean conditions. We specifically tested hypotheses that germination behavior confers greater risk-spreading in populations from drier, unpredictable environments, and that seeds from wetter populations are better competitors.In 14 populations (spanning 114–954mm annual rainfall) we assessed three alternative key parameters of germination in a greenhouse experiment: between-year dormancy, days to emergence within a season, and temporal spread. Addition of neighbor seeds accounted for competition as another crucial environmental factor. In six of the 14 populations, we also compared seeds originating from corresponding north (more mesic) and south (more arid) exposed hill slopes to test whether germination patterns along the large-scale rainfall gradient are paralleled at this smaller scale.B. hybridum exhibited generally high germination fractions and rapid emergence with very little temporal spread, indicating overall little risk-spreading germination. Surprisingly, none of the three parameters changed systematically with increasing aridity, neither at large scale along the rainfall gradient nor at small scale between north and south exposures. Neighbor seeds, however, mildly suppressed germination. Germination of neighbor seeds, in turn, was more strongly suppressed by B. hybridum seeds from drier populations, and this effect was stronger for forb than for grass neighbor species.Our results provide strong evidence that increased risk-spreading germination is not a universal, essential strategy to persist in increasingly dry, unpredictable environments. They also highlight that competition with neighbors occurs even at the earliest plant life stage. Since neighbor effects were species-specific, competition among seeds can affect community composition at later plant stages.

Inoculation history affects community composition in experimental freshwater bacterioplankton communities.

Priority effects occur when the arrival order of species or genotypes has a lasting effect on community or population structure. For freshwater bacteria, priority effects have been shown experimentally among individual species, but no experiments have been performed using complex natural communities. We investigated experimentally whether a foreign bacterioplankton community influences the community assembly trajectory when inoculated prior to the local community, whether inoculation time lag affects priority effects, and how the individual OTUs responded to time lag. Two bacterioplankton communities from dissimilar ponds were inoculated into one of the natural media with a time lag of 0, 12, 36 or 60 h, giving advantage in time to the foreign community. All three time lags resulted in priority effects, as the final community composition of these treatments differed significantly from that of the treatment with no time lag, but compositional shifts were not linear to inoculation time lag. The responses of individual OTUs to time lag were highly diverse and not predictable based on their immigration history or relative abundance in the inocula or control. The observed impact and complexity of priority effects in multispecies systems emphasize the importance of this process in structuring both natural and industrial bacterial communities.

Biochar affects community composition of nitrous oxide reducers in a field experiment

N2O is a major greenhouse gas and the majority of anthropogenic N2O emissions originate from agriculturally managed soils. Therefore, developing N2O mitigation strategies is a key challenge for the agricultural sector and biochar soil treatment is one reported option. Biochar's capacity to increase soil pH and to foster activity of specialized N2O reducers has been proposed as possible mechanisms for N2O mitigation. An experiment was undertaken to investigate whether changes in the community composition of N2O reducers was observed under field conditions after biochar application. The study objective was to assess the abundance and taxonomic composition of the functional marker genes nosZ and nosZ –II across a vegetation period of Zea mays L. after biochar or lime addition compared to an untreated control. After fertilization, biochar amendment resulted in a significant increase of nosZ gene copy numbers compared to the control and the lime treatment. Simultaneously a shift in community composition of nosZ-II bearing bacteria was observed in the biochar treatment that went beyond the sole liming effect. This study broadens our understanding of the functional impact of biochar on N2O emissions and emphasizes the possibility to shape the functioning of the N2O reducing microbial community through the addition of biochar at a field scale.

Heated communities: large inter- and intraspecific variation in heat tolerance across trophic levels of a soil arthropod community

Temperature extremes are predicted to increase in frequency, intensity and duration under global warming and are believed to significantly affect community composition and functioning. However, the effect of extreme climatic events on communities remains difficult to predict, especially because species can show dissimilar responses to abiotic changes, which may affect the outcome of species interactions. To anticipate community responses we need knowledge on within and among species variation in stress tolerance. We exposed a soil arthropod community to experimental heat waves in the field and measured heat tolerance of species of different trophic levels from heated and control plots. We measured the critical thermal maximum (CTmax) of individuals to estimate inter- and intraspecific variation in heat tolerance in this community, and how this was affected by experimental heat waves. We found interspecific variation in heat tolerance, with the most abundant prey species, the springtail Isotoma riparia, being more sensitive to high temperatures than its predators (various spider species). Moreover, intraspecific variation in CTmax was substantial, suggesting that individuals within a single species were unequally affected by heat extremes. However, heat tolerance of species did not increase after being exposed to an experimental heat wave. We conclude that interspecific variation in tolerance traits potentially causes trophic mismatches during extreme events, but that intraspecific variation could lessen these effects by enabling partial survival of populations. Therefore, ecophysiological traits can provide a better understanding of abiotic effects on communities, not only within taxonomic or functional groups, but also when comparing different trophic levels.

Effects of Late Stages of Emerald Ash Borer (Coleoptera: Buprestidae)-Induced Ash Mortality on Forest Floor Invertebrate Communities

Emerald ash borer (EAB; Agrilus planipennis Fairmaire) is an invasive wood-borer causing rapid, widespread ash tree mortality, formation of canopy gaps, and accumulation of coarse woody debris (CWD) in forest ecosystems. The objective of this study was to quantify the effects of canopy gaps and ash CWD on forest floor invertebrate communities during late stages of EAB-induced ash mortality, when the effects of gaps are predicted to be smallest and effects of CWD are predicted to be greatest, according to the model proposed by Perry and Herms 2016a. A 2-year study was conducted in forest stands that had experienced nearly 100% ash mortality in southeastern Michigan, USA, near where EAB first established in North America. In contrast to patterns documented during early stages of the EAB invasion, effects of gaps were minimal during late stages of ash mortality, but invertebrate communities were affected by accumulation and decomposition of CWD. Invertebrate activity-abundance, evenness, and diversity were highest near minimally decayed logs (decay class 1), but diverse taxon-specific responses to CWD affected community composition. Soil moisture class emerged as an important factor structuring invertebrate communities, often mediating the strength and direction of their responses to CWD and stages of decomposition. The results of this study were consistent with the predictions that the effects of CWD on invertebrate communities would be greater than those of canopy gaps during late stages of EAB-induced ash mortality. This research contributes to understanding of the cascading and long-term ecological impacts of invasive species on native forest ecosystems.

Comparative responses of early‐successional plants to charcoal soil amendments

AbstractCharcoal used as a soil amendment, or “biochar,” has received considerable recent research attention as a means to increase plant productivity while mitigating climate change through enhanced carbon sequestration. Interest in biochar for use in the restoration of disturbed sites is growing; however, biochar effects on wild plant species of the early phase of post‐disturbance succession have received almost no prior research attention. Physiological adaptations that enable rapid growth in early‐successional pioneers (e.g., high rates of photosynthesis) should be advantageous in soils with fresh charcoal since plants with a capacity for expeditious resource capture can capitalize on resource pulses from leachable mineral elements. In a glasshouse study, we tested the effects of biochar applied at two doses (10 and 20 t/ha) to brunisol/juvenile podzol soils, collected from a managed temperate mixed‐wood forest, on the growth and physiology of 13 herbaceous old‐field pioneers. We measured leaf‐level physiology and nutrient supply rates throughout the experiment, and biomass and reproductive performance at experiment completion. Overall, biochar treatments resulted in 30–37% increases in final average aboveground biomass, 13–17% increases in photosynthesis, and an average ~44% increase in leaf‐level water‐use efficiency (at 10 t/ha), but with a high species‐specific variation that included negative responses. We detected weak negative relationships between intrinsic photosynthetic rates (of non‐biochar controls) and some biomass responses: Species with high photosynthetic capacities tended to have low or negative biomass responses to biochar. Plants in biochar treatments flowered earlier and on average had double the reproductive biomass overall. Pulses of PO4− and K+ were supplied by biochar in the first four weeks of the experiment, while NO3− was significantly immobilized by biochar. These results suggest that by providing a pulse of P and base cations, biochar can improve the restoration of disturbed landscapes by enhancing the physiological and reproductive performance of a subset of pioneers that have moderate photosynthetic rates and nitrogen demand. Biochar has important potential applications to restoration; however, biochar is likely to affect community composition strongly, and careful consideration of the physiological rates and nitrogen requirements of target species will be necessary to maximize the success of biochar‐based restoration projects.

Seed-associated fungi in the alpine tundra: Both mutualists and pathogens could impact plant recruitment

Abstract Seed-borne microbes are important pathogens and mutualists in agricultural crops but are understudied in natural systems. To understand the diversity and function of seed-borne fungi in alpine tundra, we cultured fungi from seeds of six dominant plant species prior to seed dispersal and evaluated their function using germination experiments in Zea mays. A total of 55 fungal cultures (9 species) were isolated with up to 4 genera per plant species. Dominant orders included Pleosporales and Hypocreales. Sixty-six percent of the isolates showed pathogenic effects. The most common genus was Alternaria which had a negative effect on both seed germination and plant growth. Cladosporium was only isolated from the two dominant plant species and showed positive effects on germination and plant growth. The high number of pathogenic fungi found coupled with the variation in seed endophytic communities among plant species suggests that seed-associated fungi could affect community composition through differential seedling recruitment.

Direct and context‐dependent effects of light, temperature, and phytoplankton shape bacterial community composition

AbstractSpecies interactions, environmental conditions, and stochastic processes work in concert to bring about changes in community structure. However, the relative importance of specific factors and how their combined influence affects community composition remain largely unclear. We conducted a multi‐factorial experiment to (1) disentangle the direct and interaction‐mediated effects of environmental conditions and (2) augment our understanding of how environmental context modulates species interactions. We focus on a planktonic system where interactions with phytoplankton effect changes in the composition of bacterial communities. Further, light and temperature conditions can influence bacteria directly as well as through their interactions with phytoplankton. Epilimnetic bacteria from two humic lakes were combined with phytoplankton assemblages from each lake (home or away) or a no‐phytoplankton control and incubated for 5 d under all combinations of light (surface, ~25% surface irradiance) and temperature (five levels from 10° to 25°C). Observed light effects were primarily direct, while phytoplankton and temperature effects on bacterial community composition were highly interdependent. The influence of temperature on aquatic bacteria was consistently mediated by phytoplankton and most pronounced for bacteria incubated with “away” phytoplankton treatments, likely due to the availability of novel phytoplankton‐derived resources. The effects of phytoplankton on bacterial community composition were generally increased at higher temperatures. Incorporating mechanisms underlying the observed interdependent effects of species interactions and environmental conditions into modeling frameworks may improve our ability to forecast ecological responses to environmental change.

Microbial survival strategies in ancient permafrost: insights from metagenomics

In permafrost (perennially frozen ground) microbes survive oligotrophic conditions, sub-zero temperatures, low water availability and high salinity over millennia. Viable life exists in permafrost tens of thousands of years old but we know little about the metabolic and physiological adaptations to the challenges presented by life in frozen ground over geologic time. In this study we asked whether increasing age and the associated stressors drive adaptive changes in community composition and function. We conducted deep metagenomic and 16 S rRNA gene sequencing across a Pleistocene permafrost chronosequence from 19 000 to 33 000 years before present (kyr). We found that age markedly affected community composition and reduced diversity. Reconstruction of paleovegetation from metagenomic sequence suggests vegetation differences in the paleo record are not responsible for shifts in community composition and function. Rather, we observed shifts consistent with long-term survival strategies in extreme cryogenic environments. These include increased reliance on scavenging detrital biomass, horizontal gene transfer, chemotaxis, dormancy, environmental sensing and stress response. Our results identify traits that may enable survival in ancient cryoenvironments with no influx of energy or new materials.