Biophysics of aquatic microbes : randomness, energetics, and sticky polysaccharides Aquatic ecosystems are the world’s largest bioreactors, but they are not well mixed and contain strong spatiotemporal heterogeneities, giving rise to an environment where large outcomes are driven by small number of events, and stochastic effects are not averaged out despite the large scale. Specifically, I describe recent work on bacteria searching for rich but dilute nutrient hotspots, where we used microcalorimetry to quantify the cost of bacterial motility, that enhances bacterial encounters with such hotspots. Motility turns out to be the most expensive trait of a bacterial cell, consuming about 35% of total metabolism, and gives rise to a risk-reward strategy of motility endurance during energy limitation. Some bacterial species cease motility upon starvation to conserve resources, others convert daily 10% of their biomass into energy to remain motile for days. Most cells will be unsuccessful in finding a hotspot but the few lucky ones keep the population going. I also describe how sticky polysaccharides secreted by algae are potent chemoeffectors for bacteria to home in on such hotspots, but only in close proximity of the algae. Further away, bacteria need to search for hotspots without clear navigational cues. However, I show how the architecture of bacterial chemosensory arrays — large sensory protein complexes — exploits a phase transition to cause signalling fluctuations that are useful in exploring such information deserts. Finally, I argue that understanding these microscale interactions is paramount to a more predictive understanding of how microbes shape biogeochemistry and ultimately our climate.

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