Ocean carbon cycling is driven by the concerted action of marine microbes, but the fine-scale interactions between these microbes and their physical and chemical environments remains elusive. Experimental investigations of bacterial navigation (chemotaxis) have been limited to highly simplified nutrient profiles, such as linear gradients. Moreover, mathematical frameworks which model the ecological interactions between microbial communities and their environment routinely overlook heterogeneities, considering bulk averages of bacterial densities and environmental conditions.
I will present current work (ARC DECRA 2018-20) which utilises a novel experimental platform for delivering sub-millimetre scale nutrient pulses, quantitatively mimicking those found in the ocean. Advanced video-microscopy is used to characterise microbial motion at the single cell level, and reveals the precise conditions under which bacteria can detect and climb dynamic nutrient gradients. New mathematical theory, based on the counting of individual molecules of dissolved organic matter, is in striking agreement with the experimental findings. From these quantitative foundations, we have developed a mechanistic framework for microbial motion, which directly unifies individual behaviour (cell motility, chemotaxis) with population-scale phenomena (collective nutrient uptake, competition between species). This provides a new path towards predicting ocean carbon cycling which is firmly based on microscale observations.