麻豆淫院

Researchers at the Georgia Institute of Technology and India's National Center for Biological Sciences have found that yeast clusters, when grown beyond a certain size, spontaneously generate fluid flows powerful enough to ferry nutrients deep into their interior.

Evolutionarily, size offers distinct survival advantages to multicellular organisms. Even modest increases can allow escape from filter-feeding predators, improve access to nutrients, or support enhanced locomotion.

However, shifting from single cells to larger cluster assemblies imposes severe constraints. Once clusters grow past a certain diameter, simple diffusion becomes too slow to supply inner cells with the nutrients needed for growth. Many microbial colonies stall under this limit, growing only at their surface.

Some evolve cilia or branching networks to move nutrients internally. Others, like bacterial biofilms, reshape their architecture to improve access. Early evolutionary steps toward multicellularity likely occurred without such specialized structures, making it unclear how early lineages could have grown beyond the size where diffusion fails.

In the study, "Metabolically driven flows enable exponential growth in macroscopic multicellular yeast," in Science Advances, researchers used experimental evolution to determine whether non-genetic physical processes can enable nutrient transport in multicellular yeast lacking evolved transport adaptations.

Researchers cultured Saccharomyces cerevisiae yeast in liquid YEPD media at 30掳C and tracked cluster growth using time-lapse microscopy over 12-hour periods. Clusters were imaged from top and side views using a mirror-based setup. Fluid motion around clusters was visualized with fluorescent tracer beads and quantified using particle tracking and particle imaging velocimetry.

Imaging revealed spontaneous, three-dimensional fluid flows circulating through clusters in liquid, with inflow from the sides and outflow from the top. These flows matched the speed of cilia-driven currents in aquatic microorganisms such as Volvox and Stentor.

To determine the source of the flows, the team inverted imaging chambers and confirmed that flow direction flipped with gravity, suggesting a buoyancy-driven mechanism. Additional tests ruled out evaporation and surface tension as causes.

Tracer particles surrounding metabolically inactive or dead yeast moved diffusively, while particles near live yeast in glucose-rich media showed ballistic motion consistent with advective flow.

The flows ceased below a glucose concentration threshold and emerged only in clusters surpassing a critical size, as measured by tracking particle trajectories and calculating mean-squared displacements. Fragmenting large clusters suppressed flow; recombining smaller ones to exceed the size threshold restored it.

Each cluster acted as an independent "density pump," creating its own flow fields. Neighboring clusters produced distinct flows that interacted, forming stagnation points where the currents reversed direction.

The mechanism at work appears to be an emergent property of yeast metabolism, creating fluid density gradients through glucose uptake and ethanol and CO₂ production that in turn drive buoyant flows. These flows transport nutrients throughout the cluster, bypassing diffusion constraints and supporting growth at sizes previously considered inaccessible without specialized adaptations.

Researchers conclude that metabolically driven flows allow undifferentiated multicellular yeast to overcome diffusion-based nutrient limitations without evolved transport mechanisms.

The authors describe these flows as a "biophysical scaffold" that permits exponential growth at macroscopic sizes before the evolution of genetic adaptations.

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