Microbial communities have numerous potential applications in biotechnology, agriculture, and medicine. Nevertheless, the limited accuracy with which we can predict interspecies interactions and environmental dependencies hinders efforts to rationally engineer beneficial consortia. Empirical screening is a complementary approach wherein synthetic communities are combinatorially constructed and assayed in high throughput. However, assembling many combinations of microbes is logistically complex and difficult to achieve on a timescale commensurate with microbial growth. Here, we introduce the kChip, a droplets-based platform that performs rapid, massively parallel, bottom-up construction and screening of synthetic microbial communities. We first show that the kChip enables phenotypic characterization of microbes across environmental conditions. Next, in a screen of ∼100,000 multispecies communities comprising up to 19 soil isolates, we identified sets that promote the growth of the model plant symbiontHerbaspirillum frisingensein a manner robust to carbon source variation and the presence of additional species. Broadly, kChip screening can identify multispecies consortia possessing any optically assayable function, including facilitation of biocontrol agents, suppression of pathogens, degradation of recalcitrant substrates, and robustness of these functions to perturbation, with many applications across basic and applied microbial ecology.

Constraints on phenotypic variation limit the capacity of organisms to adapt to the multiple selection pressures encountered in natural environments. To better understand evolutionary dynamics in this context, we select Escherichia coli for faster migration through a porous environment, a process which depends on both motility and growth. We find that a tradeoff between swimming speed and growth rate constrains the evolution of faster migration. Evolving faster migration in rich medium results in slow growth and fast swimming, while evolution in minimal medium results in fast growth and slow swimming. In each condition parallel genomic evolution drives adaptation through different mutations. We show that the trade-off is mediated by antagonistic pleiotropy through mutations that affect negative regulation. A model of the evolutionary process shows that the genetic capacity of an organism to vary traits can qualitatively depend on its environment, which in turn alters its evolutionary trajectory.

Highlights d Microbial communities can be invaded by new species changing their composition d Measurements of bacterial invasions in an algae-predator microbial community d Algae modify interaction between predator and bacteria changing invasion outcomes d Theory shows apparent three-species (higher-order) interaction depends on model detail

Dielectric fluctuations underlie a wide variety of physical phenomena, from ion mobility in electrolyte solutions and decoherence in quantum systems to dynamics in glass-forming materials and conformational changes in proteins. Here we show that dielectric fluctuations also lead to noncontact friction. Using high sensitivity, custom fabricated, single crystal silicon cantilevers we measure energy losses over poly(methyl methacrylate), poly(vinyl acetate), and polystyrene thin films. A new theoretical analysis, relating non-contact friction to the dielectric response of the film, is consistent with our experimental observations. This work constitutes the first direct, mechanical detection of friction due to dielectric fluctuations.The fundamental relationship between random forces and friction plays a pivotal role in physics, chemistry, and biology. Surprisingly, the origin of force fluctuations and friction between objects in close proximity, but not in physical contact, remains poorly understood. Such non-contact friction is important in a variety of seemingly disparate fields, including micro-and nanomechanics, trapped ions for quantum computation, and measurements of quantum gravitation at small length scales. The non-contact friction measurements reported here are motivated by recent advances in the mechanical detection of magnetic resonance [1,2]; the sensitivity in these measurements has so far been limited by non-contact friction between a cantilever tip and the sample surface [3].The advent of high sensitivity single crystal silicon cantilevers [4,5] provides a new opportunity to elucidate the mechanisms of non-contact friction. These cantilevers' combination of low spring constant and low intrinsic losses enable the detection of non-contact friction with unprecedented sensitivity. Initial work on non-contact friction using high sensitivity cantilevers by Stipe et. al measured dissipation using a conducting probe over metal and quartz substrates at tip-sample separations down to 2 nm and temperatures from 4 -300K [3]. Friction over Au (111) was found to be 7 orders of magnitude larger than predicted by Coulomb drag theories [6]; several alternative mechanisms have been suggested [7][8][9]. In this Letter we explore noncontact friction over polymer films.A single high-sensitivity silicon cantilever was used as shown in Fig. 1(a). The cantilever approached the surface in a perpendicular orientation to avoid snap-in to contact [4] and the motion of the cantilever was parallel to the surface. The cantilever was 250m long, 5m wide, and 340 nm thick, with a spring constant k = 7 × 10 −4 N/m, a fundamental resonance frequency ω c /2π = 7.385 kHz, and a quality factor Q = 31000 ( Fig. 1(b)) [5]. The tip region of the cantilever has been thinned from 340 nm to < 100 nm using a reactive ion etch. The cantilever tip has a radius of ~ 30nm and has been coated with a thin layer of platinum using a shadow mask technique [4].We measure the total friction Γ t by recording the cantilever ringdown time τ, Fig. 1(c...

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