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See detailActive reconfiguration of cytoplasmic lipid droplets governs migration of nutrient-limited phytoplankton
Sengupta, Anupam UL; Dhar, Jayabrata UL; Danza, Francesco et al

E-print/Working paper (2021)

As open oceans continue to warm, modified currents and enhanced stratification exacerbate nitrogen and phosphorus limitation, constraining primary production. The ability to migrate vertically bestows ... [more ▼]

As open oceans continue to warm, modified currents and enhanced stratification exacerbate nitrogen and phosphorus limitation, constraining primary production. The ability to migrate vertically bestows motile phytoplankton a crucial–albeit energetically expensive–advantage toward vertically redistributing for optimal growth, uptake and resource storage in nutrient-limited water columns. However, this traditional view discounts the possibility that the phytoplankton migration strategy may be actively selected by the storage dynamics when nutrients turn limiting. Here we report that storage and migration in phytoplankton are coupled traits, whereby motile species harness energy storing lipid droplets (LDs) to biomechanically regulate migration in nutrient limited settings. LDs grow and translocate–directionally–within the cytoplasm to accumulate below the cell nucleus, tuning the speed, trajectory and stability of swimming cells. Nutrient reincorporation reverses the LD translocation, restoring the homeostatic migratory traits measured in population-scale millifluidic experiments. Combining intracellular LD tracking and quantitative morphological analysis of red-tide forming alga, Heterosigma akashiwo, along with a model of cell mechanics, we discover that the size and spatial localization of growing LDs govern the ballisticity and orientational stability of migration. The strain-specific shifts in migration which we identify here are amenable to a selective emergence of mixotrophy in nutrient-limited phytoplankton. We rationalize these distinct behavioral acclimatization in an ecological context, relying on concomitant tracking of the photophysiology and reactive oxygen species (ROS) levels, and propose a dissipative energy budget for motile phytoplankton alleviating nutrient limitation. The emergent resource acquisition strategies, enabled by distinct strain-specific migratory acclimatizing mechanisms, highlight the active role of the reconfigurable cytoplasmic LDs in guiding vertical movement. By uncovering the mechanistic coupling between dynamics of intracellular changes to physiologically-governed migration strategies, this work offers a tractable framework to delineate diverse strategies which phytoplankton may harness to maximize fitness and resource pool in nutrient-limited open oceans of the future. [less ▲]

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See detailTrade-offs in phenotypic noise synchronize emergent topology to actively enhance transport in microbial environments
Dhar, Jayabrata UL; Thai, Le Phuong Anh UL; Ghoshal, Arkajyoti UL et al

E-print/Working paper (2021)

Phenotypic noise underpins homeostasis and fitness of individual cells. Yet, the extent to which noise shapes cell-to-population properties in microbial active matter remains poorly understood. By ... [more ▼]

Phenotypic noise underpins homeostasis and fitness of individual cells. Yet, the extent to which noise shapes cell-to-population properties in microbial active matter remains poorly understood. By quantifying variability in confluent \textit{E.coli} strains, we catalogue noise across different phenotypic traits. The noise, measured over different temperatures serving as proxy for cellular activity, spanned more than two orders of magnitude. The maximum noise was associated with the cell geometry and the critical colony area at the onset of mono-to-multilayer transition (MTMT), while the lower bound was set by the critical time of the MTMT. Our results, supported by a hydrodynamic model, suggest that a trade-off between the noise in the cell geometry and the growth rate can lead to the self-regulation of the MTMT timing. The MTMT cascades synchronous emergence of hydrodynamic fields, actively enhancing the micro-environmental transport. Our results highlight how interplay of phenotypic noise triggers emergent deterministic properties, and reveal the role of multifield topology--of the colony structure and hydrodynamics--to insulate confluent systems from the inherent noise associated with natural cell-environment settings. [less ▲]

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