Where less may be more: how the rare biosphere pulls ecosystems strings

Rare species are increasingly recognized as crucial, yet vulnerable components of Earth’s ecosystems. This is also true for microbial communities, which are typically composed of a high number of relatively rare species. Recent studies have demonstrated that rare species can have an over-proportional role in biogeochemical cycles and may be a hidden driver of microbiome function. In this review, we provide an ecological overview of the rare microbial biosphere, including causes of rarity and the impacts of rare species on ecosystem functioning. We discuss how rare species can have a preponderant role for local biodiversity and species turnover with rarity potentially bound to phylogenetically conserved features. Rare microbes may therefore be overlooked keystone species regulating the functioning of host-associated, terrestrial and aquatic environments. We conclude this review with recommendations to guide scientists interested in investigating this rapidly emerging research area

Dormancy dampens the microbial distance-decay relationship

ABSTRACTMuch of Earth’s biodiversity has the capacity to engage in dormancy whereby individuals enter a reversible state of reduced metabolic activity. By increasing resilience to unfavorable conditions, dormancy leads to the accumulation of “seed banks” that should diminish the influence of environmental filtering, while allowing passive dispersers to colonize new habitats. Although prevalent among single-celled organisms, evidence that dormancy influences patterns of microbial biodiversity and biogeography is lacking. We constructed geographical and environmental distance-decay relationships (DDRs) using 16S rRNA sequencing to characterize the total (DNA) and the active (RNA) bacterial communities in a regional survey of 49 forested ponds. As expected, the total community harbored greater diversity and exhibited weaker DDRs than the active portion of the community. These empirical observations were robust to different measures of community similarity and random resampling tests. Furthermore, findings from the field survey were reproduced by models that included aspects of dormancy along with the geographical coordinates and environmental characteristics of our study system. In addition to maintaining local diversity, our results support recent theoretical predictions that dormancy shapes geographical patterns of biodiversity.</jats:p

Role of nicotinic acetylcholine receptors in the effects of cocaine-paired contextual stimuli on impulsive decision making in rats

RATIONALE: Chronic cocaine exposure produces unconditioned enhancement in impulsive decision making; however, little is known about the effects of cocaine-paired conditioned stimuli on this behavior. Thus, this study explored the effects of cocaine-paired contextual stimuli on impulsive decision making and the contribution of nicotinic acetylcholine receptors (nAChRs) to this phenomenon. METHODS: Rats were trained to achieve stable performance on a delay discounting task, which involved lever press-based choice between a single food pellet (small reward) available immediately and three food pellets (large reward) available after a 10-, 20-, 40-, or 60-s time delay. Rats then received Pavlovian context-cocaine (15 mg/kg, i.p.) and context-saline (1 ml/kg, i.p.) pairings in two other, distinct contexts. Subsequently, delay discounting task performance was assessed in the previously cocaine-paired or saline-paired context following pretreatment with saline or cocaine (15 mg/kg, Experiment 1) or with saline or the nAChR antagonist, mecamylamine (0.2, 2 mg/kg, Experiment 2), using counterbalanced within-subjects testing designs. RESULTS: Independent of cocaine pretreatment, rats exhibited greater decrease in preference for the large reward as a function of delay duration in the cocaine-paired context, relative to the saline-paired context. Furthermore, systemic mecamylamine pretreatment dose-dependently attenuated the decrease in preference for the large reward in the cocaine-paired context, but not in the saline-paired context, as compared to saline. CONCLUSION: Cocaine-paired contextual stimuli evoke a state of impulsive decision making, which requires nAChR stimulation. Drug context-induced impulsivity likely increases the propensity for drug relapse in cocaine users, making the nAChR an interesting target for drug relapse prevention